Browsing by Author "Bower, John Patrick."

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Aspects of improving cold hardiness of tomato (Lycopersicon esculentum) var. Rossol.
(2004) Ghebretinsae, Amanuel Ghebrehiwet.; Bertling, Isa.; Bower, John Patrick.
Tomatoes, particularly those of the determinate type, are one of the most popular vegetables in the East African country Eritrea. The crop is a source of income to small farm operators as well as commercial growers, and plays an important role in the nutritional supply of the population. Nonetheless, tomato production is limited during the cool season resulting - on the one hand - in a reduced nutritional supply during this period and - on the other hand - in an increased economic potential of the crop. Although performance of tomato plants under low temperature conditions is genetically influenced, prevailing temperatures as well as management practices also affect growth and development. In order to understand the long-term effects of a cold spell on tomato plants, experiments were carried out to determine the effect of a cold spell on plant vegetative and reproductive characteristics of determinate type ''Rossol'' tomatoes. Plants were moved for two to five subsequent nights from a tunnel to a cold room (4°C). The fruit set stage of "Rossol" tomatoes was found to be most sensitive to cold temperature, followed by the flowering stage. The juvenile stage was, of all the developmental stages examined, the least sensitive to cold. Furthermore, subjecting tomato plants for five subsequent nights (cumulative 60hrs) to 4°C resulted in a significant retardation of growth and development and in yield reduction. However, tomatoes exposed to a two-night cold spell during either the vegetative or the flowering stage recovered quickly and, ultimately, performed well. Furthermore, trials were carried out under tunnel and field conditions to investigate the impact of potassium application as well as mulching on cold tolerance of "Rossol" tomatoes. Under low temperature conditions, increasing the level of potassium to up to 150% of the recommended level (157.5kg*ha-1 ) resulted in quicker ontogenetic development and increased yields significantly. On the other hand, supplying tomato plants with 50% and 200% of the recommended potassium level reduced growth, delayed development and decreased yield and yield attributes. Using black plastic mulch also increased plant growth and speeded up plant development. Maize stover mulch, however, retarded plant growth and development so that certain stages ofthe phenological cycle were reached later than by non-mulched plants. However, yield and yield attributes increased significantly using stover mulch. Therefore, the management practises potassium fertilization and mulching provide excellent tools to increase the tolerance of tomato plants to low temperature conditions. Furthermore, organic mulches can be used to delay crop development and time maturity to achieve high retail process of the commodity in the cool winter months.
Biological control of Phytophthora root rot of citrus seedlings and cuttings.
(2005) Abraham, Abraha Okbasillasie.; Laing, Mark Delmege.; Bower, John Patrick.
With an increasing realization that many agrochemicals are hazardous to animals and humans, came the desire to replace these chemical agents with biological approaches that are more friendly to the environment and human health. Microorganisms play an important role in plant disease control, as naturally occurring antagonists. Microorganisms may also have beneficial effects on plant development when applied to plant roots. Research efforts worldwide have recorded successes in biological control and growth stimulation on many crops, particularly when using members of the genera Bacillus and Trichoderma. Their use on citrus rootstock could be advantageous to nurserymen and growers in reducing the incidence of seedling mortality and increasing production. To achieve these objectives, laboratory and tunnel experiments were conducted to develop effective biocontrol agents for citrus seedlings and cuttings. Nineteen 0 ut 0 f 23 Trichoderma isolates tested in vitro against Phytophthora p arasitica sp showed antagonistic activity by hyperparasitism and four out of eight Bacillus isolates resulted in antagonism by forming inhibition zones. The positive in vitro activity of Trichoderma and Bacillus isolates on Phytophthora provided motivation step for further trials in the greenhouse to evaluate their biological control activity on citrus seedlings and cuttings. A greenhouse trial was carried out to evaluate the biological control potential of 23 Trichoderma isolates (drenched at 5 x 105 spores / rnI) and two Bacillus isolates (drenched at 1 X 106 or 1 X 108 colony forming units (CFU) / rnI) to suppress Phytophthora parasitica sp. of rough lemon (Citrus jambhirini Lush.) seedlings. Five isolates ofTrichoderma (AA12, AA5, Trichoderma harzianum (AA16), SY3F and Eco-T~ were highly effective in suppressing Phytophthora root rot, with AA12 providing the best control. The Bacillus isolates also suppressed the pathogen but were not as effective as the Trichoderma isolates. This trial was used to test for growth stimulation activity by some of the biocontrol agents. To verify these results, a further trial was carried out to evaluate growth stimulation capabilities in the absence of any pathogen. Trichoderma Isolates AA13 and AA17 caused no 111 change in seedling growth, while other Trichoderma and Bacillus isolates had an inhibitory effect on the seedling growth. This trial indicated that the biocontrol activity was affected by inoculum densities, and as a result in vitro sporulation capacity was evaluated. TrichodermaIsolate AA16 was the largest spore producer, followed by Eco-T®. Spore production was lowest from Trichoderma isolates AA4 and AA12. Growth stimulation responses of Trichoderma Isolates AA4, AA16, Eco-TID and SYN6 were further studied at four different doses (1 X 103, 1 X 104, 5 X 105 or 1 X 106 spores / ml) on rough lemon and trifoliate orange seedlings. Trifoliate oranges responded positively to 1 X 104 and 5 X 105 spores / ml of Eco-TID, but rough lemon responded negatively to all dosages of the Trichoderma isolates applied. This indicates that the inoculum density responses may be host specific. Higher population density of 1 X 106 spores / ml of all tested Trichoderma isolates had a stunting effect on seedling growth of both species. Based on t he positive results 0 f individual applications of some Trichoderma and Bacillus isolates, of the biological control agents on rough lemon seedlings against Phytophthora parasitica in an earlier greenhouse trial, their combined effect in the control of the pathogen was performed. Before carrying out a greenhouse trial, activities of the isolates to be combined were evaluated in vitro. This trial showed that Trichoderma Isolates AA16 and Eco-T®were compatible. Trichoderma isolates AA16 and Eco-T®were also found to be compatible with Bacillus Isolates B77, B81 and PHP. As a result, further in vivo trials were conducted. The tunnel trials were carried out as two separate experiments: In the first experiment, a combination of two Trichoderma Isolates A A 16 and Eco-T®was conducted assayed at 5 X 105 or 1 X 106 spores / ml, on rough lemon seedling, and cuttings and trifoliate orange and sour orange seedlings. A combination of Trichoderma isolate AA16 and Eco-T®at 5 X 105 spore / ml increased significantly the new flush biomass of rough lemon cuttings compared to AA16 alone, but was not different from Eco-TID alone. The combination of AA16 and Eco-T® achieved no change of biomass of rough lemon and trifoliate orange seedlings. The combination of AA16 and Eco-TID did not increase the root biomass of sour orange compared to AA16 or Eco-r® alone. The combination of AA16 and Eco-r® at higher doses (1 x 106 spores / ml) showed significantly better suppression of Phytophthora root rot of rough lemon cuttings but did not show disease suppression in all seedling species verities tested. In a second experiment, individual and combined effects of Trichoderma isolates (drenched at 5 X 105 spores / ml) with Bacillus isolate (drenched at 1 X 106 colony forming units (CFU) / ml) for suppression of Phytophthora root rot on rough lemon and trifoliate orange seedlings was performed. The combination of Trichoderma Isolate AA16 and Bacillus Isolate B81 increased root biomass on rough lemon seedlings compared to the combination of Trichoderma AAI6 or Bacillus PHP but was not significantly different to Trichoderma AA16 alone. Bacillus PHP combined with Trichoderma AA16 or singly had no effect on rough lemon seedlings. Combining Trichoderma Eco--r® and with Bacillus B8I or PHP did not increase biomass of rough lemon seedlings compared to Trichoderma Isolate Eco--r® alone. There was no statistically significant differences in the effects of the combinations of the Trichoderma and Bacillus isolates compared to their individual applications on the biomass of trifoliate oranges. This study established the antagonistic potential of several South African isolates of Trichoderma and Bacillus as a viable alternative to agrochemicals for controlling Phytophthora parasitica. The growth stimulation capabilities of Trichoderma isolates in terms of seedling development was also demonstrated.
The cascade of physiological events leading to chilling injury : the effect of post-harvest hot water and molybdenum applications to lemon (citrus limon) fruit.
(2012) Mathaba, Nhlanhla.; Bertling, Isa.; Bower, John Patrick.
New emerging markets such as Japan and the United States require cold sterilisation of South African citrus fruit as a phytosanitary standard against fruit fly. However, citrus fruit are chilling susceptible, with lemons being the second-most chilling susceptible after grapefruit. Chilling injury is a physiological rind disorder; the occurrence of which is despite its prevalence in horticultural commodities, not well understood. Therefore, the aim of this study was to investigate physiological compounds regulating chilling susceptibility or resistance in citrus fruit, with special emphasis on lemons. Furthermore, the potential of hot water dips or “molybdenum soaks” to maintain a certain level of physiological compounds which determine manifestation of chilling injury symptoms in citrus fruit was investigated. Moreover, it was attempted to create an understanding of the order in which physiological compounds mitigate chilling injury. Lemon fruit from different farms known to be chilling susceptible or resistant were obtained during the 2007 and 2008 harvest season. Thereafter, fruit were treated by soaking for 30 min in 1μM NaMo04.2H20 solution followed by a 2 min HWD 47 or 53°C. Treated fruit were waxed, weighed and stored at -0.5°C for up to 28 days and sampled for chilling injury evaluation 7, 14, 21, or 28 days into cold storage. A second evaluation was carried out five days after withdrawal from cold storage to allow development of chilling injury symptoms as a shelf-life simulation. After the second evaluation fruit were peeled, peel freeze-dried, milled using mortar and pestle and stored at -21°C for further physiological analysis. Freeze-dried peel was analysed for soluble sugars (glucose, fructose, sucrose), vitamin C (ascorbic acid), vitamin E (α-tocopherol), β-carotene, polyamines (putrescine, spermine, spermidine), specific flavanones (naringin and hesperidin) using HPLC-UV-Vis detector and proline, total antioxidant assays (FRAP, ABTS, DPPH), total phenolics, total flavonoids, lipid peroxidation using spectrophotometry, as well as for the heat shock protein (HSP70) using electrophoresis and silver-staining. Chilling susceptibility of lemon fruit varied with fruit source; those sourced from Ukulinga and Eston Estates were chilling resistant, while fruit from Sun Valley Estates showed chilling injury symptoms after 28 days of cold storage plus five days shelf-life. Furthermore, hot water dips (HW) 53°C, 1 μM Molybdenum (Mo) and 10 μM Mo plus HW 53°C significantly reduced chilling injury symptoms compared with the control and HW 47°C. In addition, Sun Valley Estates fruit also showed higher fruit weight loss compared with non-chilling resistant lemons. The alignment of higher fruit weight loss during storage with chilling susceptibility ascertains the use of weight loss as a non-destructive parameter for chilling susceptibility. With respect to flavedo sugars, glucose was found to be the dominant soluble sugar with multi-functional roles during cold storage. This plays a significant role in mitigating cellular stress. Chilling susceptible lemons from Sun Valley Estates had low flavedo glucose concentrations and, therefore, little conversion of glucose to ascorbic acid was possible resulting in a low antioxidant capacity. However, treatments with HW 53°C and Mo soaks seemed to enhance the enzymatic conversion of glucose to ascorbic acid leading to a higher antioxidant capacity in the flavedo of such treated fruit. Furthermore, glucose also feeds into the pentose phosphate pathway which is coupled with the shikimate pathway synthesizing secondary metabolites, especially of the phenolics group. The decrease in glucose was aligned to the levels of total phenolics, but not to that of β-carotene, naringin and hesperidin through 28 days into cold storage period. Moreover, as glucose also feeds into shikimate pathway, simultaneously an increase in proline flavedo concentration was observed. Proline is an antioxidant synthesized from glutamate; as cellular glucose decreases so does the total antioxidant capacity during cold storage. Ascorbic acid is a dominant and potent antioxidant in lemon flavedo as proven with the FRAP, ABTS and DPPH assays. Chilling resistant fruit have significantly higher ascorbic acid conversion. Furthermore, ascorbic acid also acts to generate the α-tocopheroxy radical to further important membrane-bound antioxidant, vitamin E (α-tocopherol equivalent). Furthermore, the DPPH assay was found to be effective in quantifying total antioxidants in lemon flavedo since it detects both lipophilic and hydrophilic antioxidants compared with the ABTS and FRAP assays which are bias to the estimation of liphophilic or hydrophilic antioxidants, respectively. The hot water and molybdenum treatments increased total antioxidants (DPPH assay) with reduced lipid peroxidation 7 days into cold storage and therefore, reduced chilling symptoms in fruit from Sun Valley Estates. The capacity of antioxidant to scavenge reactive oxygen species (ROS) was increased during cold storage and membrane stability significantly improved. Furthermore, putrescine as low valency polyamine was reduced as such compound acted as precursor to the synthesis of the high valency polyamines, spermine and spermidine. Chilling susceptible lemons from Sun Valley Estates showed increased soluble-conjugated polyamines as a response to stress. Furthermore, HW 53°C, 1 μM Mo and 10 μM Mo plus HW 53°C significantly increased the protein concentration and, therefore, likely also the occurrence of proteins with 70kDa (as estimator of HSP70). Additionally, the concentration of conjugated high valency polyamines was also increased, resulting in reduced chilling injury symptoms. The effect of ROS has only been viewed as damaging, while recently their role has also been viewed as stress acclamatory signalling compounds when produced concentrations below critical damaging threshold. Therefore, hot water dips seems to signals synthesis of total protein which include HSPs which then act throughout cold stress to protect other protein and channel other damaged proteins towards proteolysis. While molybdenum increased ROS production below damaging critical threshold, with ROS signalling stress acclimation by further signalling production of bioactive compound with antioxidant properties.
Development of a sulphur free litchi storage protocol using sealed polypropylene bags.
(2006) Archibald, Alison Joy.; Bower, John Patrick.; Bertling, Isa.
The use of sulphur as a method of postharvest disease control and colour retention in litchis is soon to be restricted by the European Union. It is therefore essential that new postharvest treatments and packaging techniques be developed in order to retain internal and external fruit qualities and thus allow for export. Good litchi quality is not only important for the export market but also for use on the local market. In this study, alternative methods for postharvest quality control were investigated with the aim of extending the litchi storage life to 40 days under modified storage. Packaging the fruit in polypropylene bags significantly decreased fruit water loss and resulted in an increase in shelf life, as determined by red colour and overall rind appearance. There was no distinct advantage of amodified atmosphere. The use of a punnet, lined with absorbent sheeting and placed within the sealed polypropylene bag, further improved the shelf life. The absorbent sheeting reduced the amount of free water and resulted in little pathogen infection, while the punnet was effective in protecting the fruit from damage. It was notable that most water loss occurred within the first 10 days of storage and that the majority would actually take place during the cooling phase. A hydrocooling technique was therefore investigated and was found to not significantly decrease water loss, possibly due to not hydrocooling the fruit for a long enough period of time. Temperature management was extremely important for both colour retention and pathogen control. It was found that treatments stored at 5.5QC showed better colour retention after the 40 days storage than the 1QC storage treatment. The higher storage temperature, however, enhances the potential for postharvest diseases. Three compounds, namely ISR 2000, 'Biosave' and F10, were tested for pathogen control. 'Biosave' showed the best results with the most effective concentration being 100 mill water and good pathogen control occurred when storage was at 10 C. Polyphenol oxidase (PPO) activity in the litchi rind was evaluated as it is thought to be closely related to browning of litchi fruit, probably due to the degradation of phenolics by PPO. Brown fruit had a high PPO activity whilst red fruit had much lower activity. It was also shown that PPO activity decrease over storage time, possibly due to product inhibition of the enzyme. The internal quality of the fruit was determined using the T88: acid ratio of the pulp, as it is well correlated to mean eating quality. For fruit to have excellent taste, it must have a T88: acid ratio of between 31:1 and 60:1. All the fruit had a ratio that met this criterion and would therefore ensure good eating quality.
Does sequential harvesting affect the quality of and income from organically grown potatoes?
(2009) Katundu, Mangani George Chilala.; Siwela, Muthulisi.; Bower, John Patrick.; Hendriks, Sheryl Lee.
Lack of effective storage facilities to mitigate post harvest losses threatens the profitability of organic farming. In rural KwaZulu-Natal, small scale farmers use traditional storage and sequential harvesting to keep potatoes post maturity while waiting to sell. The effect of these practices on potato quality has not been studied and documented. This study set out to investigate if traditional practices (sequential harvesting and traditional farmer’s store) of the Ezemvelo Farmers Organisation in Umbumbulu affect the quality and marketable quantity of organically grown potatoes. Specifically the study set to investigate the effect of traditional farmer’s store, in situ and controlled storage on the carbohydrate content and sensory quality of potatoes organically grown in Embo by EFO farmers; determine consumer quality expectations of organically and conventionally grown potatoes; investigate the effect of sequential harvesting on the potato quality expectations of consumers and to investigate the produce and income losses experienced by small scale organic farmers at harvest and during storage. Experiments were conducted to compare the effect of sequential harvesting, farmer’s store and controlled cold storage (7oC and 90% relative humidity) on the sensory, appearance and keeping quality of organically grown potatoes. A survey of 100 consumers (40 organic and 60 conventional consumers) was conducted to ascertain consumer appearance and keeping quality expectations of potatoes. In addition, a survey of 101 farmers investigated the storage practices of the EFO farmers who grew potatoes and the type of losses incurred in storage. Preference ranking was used to investigate if time of harvesting post maturity affected potato quality. Produce losses experienced by the farmers were quantified. A co-research group of three seasoned farmers of the EFO participated in the research. They produced potatoes used in the study and provided valuable input to ensure that the study adhered to storage practices of the farmers. The lowest and highest sugar levels were observed in potatoes stored in situ and under controlled conditions, respectively. Potatoes left in situ also recorded higher starch content. Potatoes stored in situ were significantly preferred by sensory panellists (p<0.05) over those stored in both farmer’s store and in controlled storage. Preference rank scores were negatively correlated to total sugar content and positively correlated to starch content. Consumers in the study highlighted five desirable appearance qualities in potatoes: absence of greening, absence of sprouting, smooth skin texture, absence of blemishes and light skin colour. No significant differences in the quality expectations between participating organic and conventional potato consumers were found. The majority of consumers expected potatoes to store for at least three weeks post purchase. Sequentially harvested potatoes met this expectation when potatoes were left in situ for a maximum of six weeks post maturity. Potatoes in situ also maintained good appearance and sensory quality. The highest produce losses were experienced in summer owing to soft rot problems. Production in the drier seasons (autumn and winter) increased the proportions of potatoes too small to be sold as table potatoes. With the exception of completely rotten potatoes, poor quality potatoes were consumed, used as seed potatoes and sold to the local market as seed and for food. Poor potato quality resulted in reduced income for the farmers. This investigation pioneered research into the effect of sequential harvesting on the quality of organically grown potatoes. The findings demonstrate that sequential harvesting provides resource-poor small scale organic farmers with an efficient storage option where other storage methods and technologies may be inappropriate, ineffective or unaffordable. It is recommended that government and other players in the agricultural sector plan initiatives to educate small scale potato farmers on the benefits of sequential harvesting as an effective short term method of potato storage. Research with other potato cultivars in different agro-ecological settings is needed to optimise sequential harvesting. Government policy aimed at training and developing farmer capacity in organic seed potatoes production is essential to ensure that farmers access disease and pest free seed. Farmers also need assistance to access to irrigation resources to improve production.
Effect of methyl jasmonate and salicyclic acid on chilling injury of 'eureka' lemons.
(2010) Siboza, Xolani Irvin.; Bower, John Patrick.; Bertling, Isa.
South Africa is the second largest exporter of citrus fruit in the world. There has recently emerged a strong demand for lemons in the world market due to their nutritional value, culinary and non-culinary uses. During exportation, fruit are subjected to low temperature (-0.5°C) for varying periods of time as an obligatory quarantine treatment. However, lemons are sensitive to low temperatures and easily develop chilling injury during this obligatory quarantine treatment. This has become a major limitation to the expansion of South Africa’s lemon industry. Postharvest treatments with methyl jasmonate (MJ) and / or salicylic acid (SA) have been successfully used in horticultural crops to reduce chilling injury. A similar treatment was applied to ‘Eureka’ lemons. During the 2008 harvest season, postharvest fruit were either dipped in 10 or 50 μM MJ or 2 or 2.5 mM SA solutions. A control or no dip treatment was also applied. Three replicates of 15 fruits per treatment were used. During the 2009 harvest season the following postharvest treatments were applied as dips: 10 μM MJ, 2 mM SA, 10 μM MJ & 2 mM SA, 1 μM MJ & 0.2 mM SA, or 0.1 μM MJ & 0.02 mM SA solutions. A control or no dip treatment was also applied. Three replicates of 15 fruits per treatment were used. Subsequently fruit were stored at -0.5ºC for 0, 7, 14, 21, 28, 35, and 42 days, before being transferred to room temperature (25°C) for 7 days where after chilling injury was rated. Treatments with 10 μM MJ and / or 2 mM SA reduced chilling injury symptoms in lemons harvested during the 2009 season. Although no visual symptoms of chilling injury were observed during the 2008 harvest season, treatments with 10 μM MJ and / or 2 mM SA reduced fruit mass loss, delayed the occurrence of stress symptoms such as lipid peroxidation and suppressed accumulation of ROS in the rind. Treatments with 10 μM MJ and / or 2 mM SA were more effective in inducing antioxidant capacity and other defence compounds such as phenolics, ascorbic acid, carbohydrates and chilling injury responses such as accumulation of proline in the rind. This may have increased the chilling tolerance of fruit during the cold storage. Therefore, this study revealed that MJ and SA have the potential to reduce and delay symptoms of chilling injury in lemons. This lead to the suggestion that both, MJ and SA dips should be further tested as treatments to mitigate chilling injury in lemons. Future studies should focus more on preventing the injury itself or preventing the primary event of chilling injury. This could probably reduce the chances of secondary events to take place.
Effects of potassium and mulching on Persea americana Mill. cv. Hass pheno/physiology, yield and fruit size.
(2001) Van Niekerk, Warren; Bower, John Patrick.; Johnston, M. A.
No abstract available.
Enhancement of 'Hass' avocado shelf life using ultra-low temperature shipping or 1-MCP treatment and cold chain management.
(2011) Kok, Richard Dean.; Bower, John Patrick.; Bertling, Isa.
Avocados are becoming an increasingly important crop in South Africa, where the main producing areas include Limpopo, Mpumalanga and KwaZulu-Natal provinces. The South African avocado industry faces considerable challenges including increasing competition exporting avocados, particularly to the European market. The processes involved to export avocados has markedly improved over the past two decades, however there is always room for improvement and it is necessary to remain competitive on a global scale. Issues such as fruit being partially soft on arrival, quality defects and cold chain management breakdown are still present. It is necessary to investigate new aspects of cold storage such as extending the storage period and understanding the physiological aspects involved. To improvement such issues, an investigation was conducted on ultra-low temperature shipping (1°C) as well as the use of 1-MCP; the implementation of deliberate cold chain breaks to achieve a better understanding as to the quality influences involved; an extended storage period of 56 days to assess the quality issues and benefits involved; as well as investigating the physiological aspects involved with all above treatments on 'Hass' avocados. An initial study saw early-, mid- and late-season 'Hass' avocados stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. Storage at 1°C was comparable with 1-MCP treatment for both fruit softening in storage and extending the ripening period. Storage at 5.5°C resulted in partial in-transit ripening, if 1-MCP was not used. Early-season fruit incurred the most external chilling injury but overall levels were minimal and not concerning. Mid-season fruit were the most sound in terms of quality. It is suggested that 1°C can be used as a viable economic alternative to 1-MCP for long distance shipping of 'Hass' up to 28 days. The cold chain break trial included a 24 hour delay before cold storage, a deliberate 8 hour break at day 14 of cold storage where fruit were removed from cold storage and a control of 28 days cold storage where no break was involved. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. It was found that cold chain breaks do influence the amount of water loss, fruit softening and days taken to ripen. Storage at 1°C did not entirely negate the effects of cold chain breaks compared with 5.5°C, but did result in fruit which were harder at the end of storage and took longer to ripen. The use of 1-MCP also had advantageous effects with respect to significantly lengthening the ripening period, even when a cold chain break occurred, compared with fruit not treated with 1-MCP. As results of the study differed in some respects to those of previous studies, it is recommended that further work be conducted to determine what fruit or pre-harvest factors affect the fruit physiological changes which take place when cold chain breaks occur. Having the option to make use of an extended storage period would be of benefit to the industry if delays occur and fruit have to be maintained under cold storage. Extended storage of South African avocados, especially at the end of the season would also allow for the option of strategically holding back fruit from the export market in order to extend the supply period. It would not only benefit export options, but would also be highly beneficial to local pre-packers, as it would reduce the need to import fruit from the Northern hemisphere production areas during the South African off-season. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 56 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. The combination of 1°C with the use of 1-MCP resulted in a good shelf life as well as maintenance of internal quality and integrity. External chilling injury is of concern for early-season fruit, however, mid- and late-season fruit did not incur extensive damage. It is, therefore, advised that fruit placed in extended storage are marketed through the 'Ready ripe' program to mask any chilling injury on the 'Hass' fruit. Avocados are renown as a "healthy food" due to their nutritional value as well as containing relatively high concentrations of antioxidants. The fruit also contain high amounts of C7 sugars which can act as antioxidants. Additionally, C7 sugars and other antioxidants play important roles in fruit quality. Therefore, it is important to understand how varying storage conditions and treatments affect the levels of these physiological parameters. Treatments of cold chain break/delay included a deliberate 8 hour break at day 14 of cold storage where fruit were removed from cold storage, a 24 hour delay before cold storage and a control of 28 days where no break was involved. A 56 day extended storage period was also used. Early-, mid- and late-season 'Hass' avocados were stored at 1°C or 5.5°C for 28 days. Additional treatments included fruit treated and not treated with 1-MCP as well as waxed and unwaxed fruit. The use of 1-MCP maintained higher levels of antioxidants, ascorbic acid and C7 sugars for both the 28 day and the 56 day storage periods. The 24 hour delay had a tendency to increase consumption of anti-oxidant and sugar reserves. The use of 1°C resulted in antioxidant and ascorbic acid levels decreasing while maintaining higher sugar levels. Overall, high stress imposed on fruit decreased reserves resulting in poor quality fruit. The use of 1°C and 1-MCP treatments maintained fruit quality.
Factors affecting mesocarp discolouration severity in 'Pinkerton' avocados (Persea americana MILL.)
(2005) Van Rooyen, Zelda.; Bower, John Patrick.
The susceptibility of the 'Pinkerton' avocado cultivar to mesocarp discolouration, after storage, has seriously threatened its export from South Africa. This disorder has proven to be complex, requiring a better understanding of the fruit's physiology. The purposes of this study were to identify the role of pre- and postharvest factors, or their interactions, in the development of the problem. This was done by obtaining fruit from several production areas of varying mesocarp discolouration histories (referred to as "high", "medium" or "Iow risk" areas) during the 2000 and 2001 seasons. Fruit were stored at 8, 5.5 and 2°C for 30 days, as well as ambient (20°C). Evaluations of fruit quality were made before and after storage, as well as after softening. Once removed from storage the weight loss (during storage) was determined, and fruit firmness and carbon dioxide (C02) production rates monitored daily. It was found that temperatures below the recommended shipping temperature of 5.5°C, i.e. 2°C, produced the best internal fruit quality. This was supported by the membrane integrity studies that showed less membrane stability at the warmer storage temperature of 8°C. Furthermore, remained hard during storage and subsequently had an extended shelf life. Fruit origin was also found to play a major role in browning potential, with discolouration being consistently more severe in fruit from "high risk" areas and increasing in severity as the season progressed. The rate of CO2 production was found to follow a similar trend, with rates increasing as the season progressed, and also being slightly higher in fruit from "high risk" areas. The higher CO2 production rates were thought to be related to a decrease in membrane integrity as the season progressed. While storage temperature was not found to have a significant effect on the rate of CO2 production after storage, it did affect the time taken to reach the maximum rate, with fruit stored at 2°C taking longer. Biochemical analyses to determine the concentration of total phenolics and the activity of the enzyme polyphenol oxidase (PPO) also showed that the potential for browning was initiated by preharvest conditions. While no significant differences were found between growers with regards to total phenol concentrations, the PPO activity was found to be higher in fruit from poor quality areas, and subsequently browning potential was expected to be higher in these fruit. It was, however, found that the potential for browning could be reduced by storing fruit at 2°C, as this decreased the total phenolics concentration. This evidence further emphasized the idea that storage at 2°C could be highly advantageous. Fruit mineral analysis showed that certain key elements played a significant role in the severity of mesocarp discolouration, with excessive fruit nitrogen and decreasing copper and manganese concentrations appearing to play major roles. The high fruit nitrogen concentrations were suspected to reflect fruit grown on very vigorous trees, resulting in shoots competing with fruit for available reserves. It is suggested that 'Pinkerton' of a quality acceptable to the market, can be produced by manipulating source:sink relationships, particularly through decreasing the availability of nitrogen, followed by low temperature (24° C) shipping. Future work should concentrate on manipulation of source:sink relationships, to take account of both climatic conditions and leaf to fruit ratios. The evaluation of chlorophyll fluorescence as a tool for predicting mesocarp discolouration potential in 'Pinkerton' proved to be unsuccessful in this study and future studies may require modifications to the current technique. It is suspected that differences in chlorophyll content, for example, between fruit from different origins, will have to be taken into account when interpreting results. The success of using 2°C storage to improve the internal quality on 'Pinkerton' fruit prompted further studies, during 2004, to ensure that the development of external chilling injury would not decrease the marketability of the cultivar. Low temperature conditioning treatments, prior to storage, proved to be highly successful in reducing the development of external chilling injury, thus further improving fruit quality as a whole. Preconditioning treatments consisted of fruit that were kept at either 10°C, 15°C or 20°C for 1 or 2 days before being placed into storage for 30 days at 2°C or 5.5°C. All preconditioning treatments were compared to fruit that were placed directly into storage. The effect of fruit packaging on moisture loss (as determined by weight loss) and chilling injury was also investigated using unwaxed fruit, commercially waxed and unwaxed fruit individually sealed in micro-perforated polypropylene bags with an anti-mist coating on the inside (polybags). Holding 'Pinkerton' fruit, regardless of packaging treatment, at 10°C for 2 days prior to storage at 2°C or 5.5°C significantly decreased the severity of external chilling injury. The use of polybags during preconditioning and storage showed potential in further reducing the development of external chilling injury, although the higher incidence of fungal infections in these fruit needs to be addressed. The determination of proline concentrations in fruit exocarp tissue after storage was helpful in determining the level of stress experienced by fruit that were subjected to different packaging and preconditioning treatments. In this study waxed fruit subjected to 1 d preconditioning at 10°C, 15°C or 20°C or placed directly into storage at 2°C showed very high proline concentrations and also displayed more severe external chilling injury, despite unwaxed fruit losing more weight during these treatments. The role of moisture loss thus needs further investigation. The thickness and method of wax application was thought to play an important role in the higher external chilling injury ratings in this study as waxed fruit often developed chilling injury symptoms around the lenticels and it was suspected that either the lenticels were damaged by the brushes used to apply the wax or that the lenticels became clogged thus resulting in reduced gaseous exchange. Nevertheless, the success of low temperature conditioning in reducing external chilling injury, while maintaining sound internal quality, may enable storage temperatures to be dropped even further, thus enabling South Africa to export avocados to countries that require a cold disinfestation period prior to entry to eliminate quarantine pests (e.g. fruit fly).
Gladiolus scabridus - the road to conservation and commercialisation.
(2005) Campbell, Tracey Barbara.; Bower, John Patrick.; Van Niekerk, Renate.
There is at present a growing concern and awareness of the endangered status of many indigenous South African plants in the wild, a number of which have potential for commercial production. One such example is Gladiolus scabridus, a vulnerable species endemic to the mountains of northern KwaZulu-Natal and southern Swaziland. It has considerable potential due to its floral characteristics. However, little is known about its horticultural requirements. Thus propagation and cultural practices were investigated with the aim of both conservation and commercialisation of the species. The ideal conditions for G. scabridus seed germination were determined. The presence or absence of light had no significant effect. Optimum germination was achieved at 20 QC of both winged and dewinged seeds. Higher temperatures appear to have a negative effect on germination and none of the winged seeds germinated at 30 QC. Under ideal conditions, fresh seed showed significantly higher vigour and viability than stored seed although the germination of stored seed was hampered by a higher internal fungal content. Successful tissue culture protocol was established for G. scabridus axillary bud and cormel halve explants. During the initial shoot initiation and proliferation stages (stage I and 11), the significantly higher shoot numbers occurred in the absence of growth regulators in both explant types. However, shoots with 1.0 mg 1 -1 6-benzyl-amino-purine (BAP) and 0.5 mg 1 -1 1-naphthalene-acetic-acid (NAA) were healthier in appearance. Higher levels of 5.0 mg 1 -1 NAA inhibited shoot production and encouraged root development in cormel halve explants. During stage 11, axillary bud explants showed root and cormlet development. More roots were initiated without growth regulators, whilst 5.0 mg 1 -1 NAA resulted in significantly better cormlet development. Shoot and cormlet growth of cormel halve explants during stage II was not significantly affected by the presence or absence of NAA and BAP. Significantly more roots were produced with 5.0 mg 1 -1 NAA. During stage III (rooting), the presence of activated charcoal (AC) was essential for the initiation and development of roots in vitro. Root and cormlet development in all explant types was significantly affected by the interaction between the previous treatments from stages I and 11 and the new treatments. During stage IV (hardening off) , most explants died down in the hardening off media leaving resting cormlets. There was a significant interaction between 1.0 mg 1 -1 NAA and 0.3 % AC from stage III, resulting in significantly more cormlets in both axillary bud and cormel halve explants. There was successful cormlet growth after cold storage which is advantageous in reducing the need for acclimatization. G. scabridus corms were successfully forced out of their normal flowering period. Temperatures of 10,5.5 and 2 QC showed successful corm initiation although the corms need to be stored for longer at warmer temperatures. It is suggested that a treatment of 2 QC for 6 weeks is the optimal condition for forcing G. scabridus corms. Although originating from a stressful environment, G. scabridus appears to show capacities for improved growth under controlled conditions. Fertilizer applications enhanced growth and reduced the time to flowering. Nitrogen (N) was found to be important for vegetative growth, flowering and daughter corm development, whereas potassium (K) influenced cormel production. Fertilizer with higher N and lower K is appropriate for the beginning and middle of the growing season and then adjusted to a lower N and higher K fertilizer to promote cormel formation. G. scabridus was found to produce prolific numbers of cormels which is an important source of plant material. Mineral leaf analysis showed that optimum levels for wild species are lower than those for hybrid gladioli with the optimum levels affected by physiological corm maturity and subsequent plant growth. Norms for postharvest handling of cut G. scabridus spikes have been developed. Spikes held in 2 % sucrose had a longer vase life and better floret opening and quality than those kept in distilled water, Prolong, Chrysal, 2 % sucrose and 2 % ethanol, 1 % sucrose, 4 % sucrose, 1 % sucrose and 0.5 % JIK, 2 % sucrose and 0.5 % JIK and 4 % sucrose and 0.5 % JIK, 2 % fructose and 2 % glucose. The use of commercially available solutions should be used with caution. Florets produced a climacteric-like CO2 peak, but levels of ethylene were unmeasurable. Cold storage and the use of polypropylene sleeves delays senescence. G. scabridus spikes secrete droplets of a sticky substance which was confirmed to be extra floral nectar through HPLC analysis. Market research revealed a positive response to the species from consumers and retailers alike with potential for cultivation as a cut flower and bedding plant. However, the cost will determine supply and demand. A field study conducted at Bivane Dam, northern KwaZulu-Natal, confirmed that G. scabridus colonies prevail in rocky, quartzite outcrops where they become wedged between the rocks. Plants were found at different stages of development with populations of not more than 108 plants per colony. Soil data of G. scabridus sites was compared to that of two sites nearby. It was found that G. scabridus soils are higher in phosphorus (P), zinc (Zn) and organic carbon. Leaf analysis confirmed that they have adapted their growth to low nutrient levels. The G. scabridus studies have clearly shown that the species can be successfully moved from a wild plant to a commercially viable one and in so doing its conservation status can also be improved.
Horticultural manipulation techniques to improve yield, fruit size and quality in 'Wai Chee' litchi (Litchi chinensis Sonn.)
(2010) Froneman, Izak J.; Bower, John Patrick.; Bertling, Isa.; Bijzet, Zelda.
In South Africa, a shortage of litchi cultivars to expand the very short harvesting period has seriously threatened the viability of the local export orientated Litchi Industry. Only two cultivars have dominated commercial plantings for more than a century, namely ‘HLH Mauritius’ and ‘McLean’s Red’. The marketing period of litchis from South Africa coincides with large export volumes from competitor countries such as Madagascar, resulting in lower returns for local growers. To address this situation, the late season cultivar ‘Wai Chee’ was imported amongst others from Australia. ‘Wai Chee’ is harvested at a time in South Africa when there is a gap in worldwide litchi production, making the cultivar potentially very profitable. However, its potential and subsequent use in the industry is affected by small fruit size and questionable internal quality. As the South African Litchi Industry is export orientated and the qualities of ‘HLH Mauritius’ fruit currently dictate export requirements, solutions need to be found to improve fruit size and fruit quality in ‘Wai Chee’ litchi. In this study, a number of horticultural manipulation techniques were investigated with the aim of enhancing yield, fruit size and quality in ‘Wai Chee’ litchi. The use of chemical applications of foliar nutrients and plant growth regulators were found to improve certain fruit characteristics in ‘Wai Chee’. Foliar nutrient applications of nitrogen, potassium and calcium during the early stages of fruit set and -development improved fruit set and subsequently yield, and also increased fruit mass, fruit size and flesh mass. Treatments with potassium nitrate (KNOз), calcium nitrate (CaNOз) and calcium metalosate proved to be the most enhancing nutrient applications. Applications of synthetic auxins and auxin-like substances during the 2-3g stage of fruit development improved fruit size, fruit mass and flesh mass in ‘Wai Chee’. The combination treatment of Tipimon® (2,4,5-TP), applied at the 2-3g stage, followed by Maxim® (3,5,6-TPA) a week later, yielded the best results in this regard. With biological practices, pollination was found to have an influence on litchi tree- and fruit characteristics. Pollen source proved to have an influence on fruit set and fruit retention at harvest in ‘Wai Chee’. Initial fruit set was lower when using cross-pollination compared to the use of self-pollination in female flowers of ‘Wai Chee’, whereas final fruit retention was higher with the use of cross-pollinators when compared to retention of fruit with self-pollination. Although some beneficial effects with different pollen donors on fruit characteristics were observed, these effects were not significant, and would therefore necessitate further investigation. Pollen donor effect on quality parameters such as Titratable acid (TA)- and Total Soluble Solid (TSS) content of fruit was not significant. Cultivar differences regarding fruit characteristics and maturation rate were detected with the use of cultural practices such as bunch covering materials in ‘HLH Mauritius’ and ‘Wai Chee’. Beneficial effects on fruit size were obtained with thicker covering materials with nominal mass of 70 and 80 g/m² respectively on ‘HLH Mauritius’, while with ‘Wai Chee’, thinner covering materials (60 and 65 g/m²) showed enhancing effects. Maturation rate was significantly delayed only on ‘Wai Chee’ with the use of thicker covering materials (70 and 80 g/m²). Differences in colour were detected amongst different covering materials, but these should be verified with chromameter technology. Covering of fruit bunches for better fruit size and a later harvest date would, especially for ‘Wai Chee’ as a late season cultivar, be beneficial, since better prices are realised towards the end of the season.
Integrated use of yeast, hot water and potassium silicate treatments for the control of postharvest green mould of citrus and litchi.
(2010) Abraham, Abraha Okbasillasie.; Laing, Mark Delmege.; Bower, John Patrick.
There is a growing recognition globally that many agrochemicals are hazardous to humans, animals and the environment. Therefore, there is a need to substitute these chemical products with biological and physical treatments, and to change agronomic practices in order to control pests and diseases in agriculture. The primary objective of this thesis was to develop and test in laboratory, field and commercial packhouses trials as alternative control measures against green mould of citrus (caused by Penicillium digitatum Pers: Fr. Sacc) and Penicillium molds of litchi (caused by several Penicillium). A South African isolate of P. digitatum, isolated from an infected orange fruit, was found to be resistant to imazalil (the standard postharvest fungicide used in South Africa). Sixty yeast and 92 Bacillus strains were screened for their antagonistic activity against this isolate of P. digitatum. None of the yeasts or Bacillus isolates produced a curative action against P. digitatum on oranges. However, yeast Isolate B13 provided excellent preventative control of P. digitatum, superior to all the Bacillus isolates, when it was applied to citrus fruit prior to artificial inoculation with P. digitatum. Electron microscopy showed that yeast Isolate B13 inhibited conidial germination of P. digitatum. For the control of P. digitatum pre-harvest, trees were sprayed with a yeast, Isolate B13, a few months or a few days before harvest. However, this treatment alone proved to be ineffective in providing preventative control of green mould on Valencia oranges. For the control of P. digitatum preharvest, trees were treated with potassium silicate for a full season. Regular potassium silicate treatments resulted in a significant preventative control of P. digitatum infection on both navel and Valencia oranges. Treatment of Eureka lemons with potassium silicate as a postharvest treatment for the control of P. digitatum resulted in reduced disease lesion diameters when applied preventatively or curatively. Electron microscopy showed that potassium silicate inhibited germination of P. digitatum conidia and growth of its mycelium. Hot-water dip treatment at 50-58°C for 60-180 seconds (in increments of 15 seconds), significantly reduced infection development in inoculated wounds of Valencia oranges compared with control fruit treated with tap water, without causing any rind damage. The integration of the yeast, a hot water dip and potassium silicate pre-and postharvest applications provided control of P. digitatum control in multiple packhouse trials. The control achieved by the yeast Isolate B13 or hot-water, and potassium silicate in the packhouse alone was superior or equivalent to that provided by imazalil. A similar study was also carried out to determine possible control measures for Penicillium sp. on litchis. In this study, a total of 23 yeast and 13 Bacillus isolates were obtained from litchi fruit surfaces. Ten yeast and 10 Bacillus isolates that had shown good efficacy against P. digitatum of citrus were added to these for screening against Penicillium sp. of litchis. None of the yeasts or Bacillus isolates produced a curative action against Penicillium sp. infection on litchis. However, several yeast isolates (YL4, YL10, YLH and B13) resulted in reduced severity of the pathogen, when applied preventatively, compared with an untreated control. The yeast isolates were superior to all the Bacillus isolates, when applied to litchis prior to artificial inoculation by Penicillium infection on litchis. Potassium silicate as a postharvest treatment for the control of the pathogen caused reduced lesion diameters when applied preventatively or curatively to naturally infected litchis. The results presented in this thesis highlight the use of biological, physical and agronomic practices singly or in combination as an alternative control strategy against citrus postharvest green mould. This thesis also provides an insight into expanding these strategies, partly or fully, for the control of other postharvest Penicillium infections using litchi as an example.
Investigation into the mortality of potted Chromolaena odorata (L.) R. M. King & H. Robinson (Asteraceae)
(2007) Gareeb, Milly.; Pammenter, Norman William.; Bower, John Patrick.
The neotropical semi-woody perennial shrub Chromolaena odorata (L.) King and Robinson (Asteraceae: Eupatorieae) has become a serious invasive weed in the sub-tropical regions of southern Africa, resulting in the initiation of an insect biocontrol research programme at Cedara in the KwaZulu-Natal province. The programme has experienced difficulties in growing and maintaining healthy potted C. odorata plants for research, for almost ten years. In an attempt to identify possible causes of mortality, plants were grown on elevated grids in two trials; (winter-spring 2003) and (autumn-winter 2004) in Durban (subtropical region, within the natural range of C. odorata) and Cedara (temperate region, outside its natural range). Plants that established at all sites, in Trials 1 and 2, did not display any visual symptoms associated with mortality; however, results from Trial 1 indicated that plants grown in a tunnel at Cedara were physiologically stressed evidenced by lower transpiration rates despite higher vapour pressure deficits, compared with plants grown in a tunnel in Durban. Environmental stress was further indicated by the chlorophyll fluorescence ratio and a change in allocation of total non-structural carbohydrate (TNC) within Cedara-grown plants compared with Durbangrown plants. An attempt to relate mortality symptoms of unhealthy C. odorata plants with TNC content of the roots was unsuccessful but unhealthy plants had lower levels of N, P and K than healthy plants. Although mortality seem to be exacerbated by pathogenic fungi in saturated soil, infection was reduced by treating the soil with Previcur N and Benlate drench. Morphological adaptations of plants exposed to waterlogged soils include hypertrophied lenticels and new lateral roots; these were observed on the stem base of unhealthy C. odorata plants placed on the ground, that had reached a critical stage of die-back or had died back and re-sprouted. Examination of the root anatomy of C. odorata plants indicated an absence of cortical aerenchyma (interconnected system of air spaces), which may explain plant mortality in waterlogged pots wherein levels of oxygen in the soil may be reduced. C. odorata stock plants at Cedara have been placed on elevated grids since August 2004 and two years later plant mortality has been significantly reduced. Presently, the only feasible option is to grow potted C. odorata plants on elevated grids at Cedara because this allows for adequate drainage, aeration of soil and air-nipping of roots, and spacing plants on the grids also improves penetration of light.
Management of avocado postharvest physiology.
(2011) Blakey, Robert John.; Bower, John Patrick.; Bertling, Isa.
Avocados are an important horticultural crop in South Africa, especially in the provinces of KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets, phytosanitary restrictions to lucrative markets such as China, the USA and Japan and increased competition in the European market have challenged the South African avocado industry. The industry has responded with improved logistics and shipping, a co-ordinated market access program and a global system to co-ordinate exports of avocados to the European market. To remain competitive on the global market, further improvements and innovations are required to improve the efficiency of postharvest operations. These improvements and innovations should be guided by a greater understanding of postharvest physiology. Avocados are a relatively new export crop, so there is still much to be learnt about avocado postharvest physiology and the optimisation of postharvest management. In this regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion on ripening was examined and the effect of a cold chain break on fruit physiology and quality determined; near-infrared spectroscopy was also examined for its potential for its use in the avocado industry. As an initial study, the relationships between individual sugars, protein and oil were studied to understand the changes in avocado fruit during ripening. It was found that mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very low levels during the first 10 days postharvest. The concentrations of glucose and fructose increased, while sucrose declined slightly during ripening. The concentration of protein increased sigmoidally during ripening, reflecting the increase in the ripening enzymes, particularly cellulase and polygalacturonase. The oil content fluctuated slightly during ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy sources during ripening. Glucose concentration was also found to increase earlier in fast ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity and may be related to the ABA functioning. Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production, respiration, softening and heptose consumption, without appreciably affecting fungal rots, physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no significant difference in the peak activity of cellulase, polygalacturonase or pectin methylesterase. In a separate experiment, fruit ripening was significantly affected by the infusion of ABA in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced the time to ripening and the variation; it is suggested that water stress and ABA are intrinsically involved in the ripening processes and may act as a ripening trigger. The water concentration in fruit was measured non-destructively using reflectance NIR; this model was used to determine the maturity of fruit and the loss of water during cold storage. In the cold chain break experiment, it was found that although fruit recovered after a cold chain break, in terms of ethylene production and respiration, there was a loss in quality because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in avocado physiology over a 6h period. The respiration rate of fruit significantly increased after 4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short amount of time. Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA, ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to the understanding of avocado postharvest physiology and should aid in better management of avocados for improved fruit quality and consumer satisfaction.
Means of improving colour expression in pepper fruit.
(2003) Mosoeunyane, Molipa.; Bertling, Isa.; Bower, John Patrick.
Colour is a characteristic that makes fruit appealing to the consumer, hence, is a prime factor when (s)he chooses to buy certain horticultural commodities. If colour is unappealing, consumers tend to ignore other quality characteristics of the fruit. Although colour expression in Capsicum annuum L. is genetically influenced, it is also affected by environmental factors as well as crop management practices. In an attempt to identify which are the dominant red colour-imparting carotenoids in red pepper and how to influence their occurrence, extracts from crude and saponified mature-green, colour-break and red-ripe fruit were purified on TLC silica gel 60 GF(254) plates. TLC R(f) values of the identified intensive red bands from saponified red-ripe extract were compared to those of authentic capsorubin, capsanthin, 13-Z-capsanthin and 9-Z-capsanthin standards. Two dominant red carotenoids were isolated from red-ripe fruit, by separating either crude or saponified extract by TLC. The two bands were then subjected to reversed-phase HPLC analysis. Both bands were characterized by comparison of their absorption spectra to that of authentic capsanthin and capsorubin standards. Red colour imparting compounds of the red pepper cultivar Capistrano surfaced at colour-break stage and their intensity increased with the ripening process. HPLC results show that there was structural modification of ketocarotenoid, mainly capsorubin. Trials were carried out in a plastic tunnel and in a growth room to investigate the impact of various plant management practices (moisture regimes, macronutrient applications, thinning of reproductive structures, subjecting pepper plants to optimal growing temperature) on pepper fruit pigment concentration. This concentration was determined at the mature-green and red-ripe stage. Subjecting pepper plants to optimal growing temperature (23/18°C) from the vegetative stage onwards, providing them with recommended optimal amount of K fertilizer (25.2 Kg ha-1 ) and supplying Mg at supra-optimal level (455 Kg ha-1 ) as well as subjecting them to drought stress starting from 30 or 50 days after transplanting and water logging conditions from 30 days after first petal fall onwards increased pepper fruit carotenoid concentration significantly. On the other hand, other levels of the above-mentioned factors (subjecting pepper plants to the optimum temperature later than the vegetative stage, supplying 'very low', 'low' or 'high' K or, 'very low', 'low' or 'medium' Mg concentrations as well as thinning of reproductive structures of pepper plants early in the fruit developmental stage and subjecting plants to drought from 30 days after first petal fall onwards or to water-logging from 50 days after transplanting or 30 days after first petal fall onwards) resulted in a significant decrease in the carotenoid concentration. Therefore, management practices can be explored to improve red colour formation in red-ripe Capsicum fruit.
Optimisation of propagation methods in Prunus persica (L.) Batsch.
(2001) Lebese, Thabiso C.; Bertling, Isa.; Bower, John Patrick.
Propagating methods for peach (Prumis persica L. Batsch.) are currently limited to the use of seeds or cuttings. Most of the rootstocks commonly used for establishing peach trees commercially originate from a narrow genetic base. The most commonly used peach rootstock in South Africa is 'Kakamas', which has disadvantages such as slow growth, and a high susceptibility to certain pests and diseases. 'Kakamas' is classified as a mid to late ripening cultivar, hence, its use as a rootstock is restricted in early ripening cultivars. Optimising peach seedling and cutting production may on the other hand increase more selection material that will increase genetic variability and also serve as the basis for future production and selection of rootstocks for peach nursery establishments. Hence, this study was designed to improves success of peach cutting production by increasing the rooting percentage and the survival rate of cuttings. This was firstly achieved by the application of various concentrations of IBA and several rooting-cofactors. Secondly, rooting success of cuttings taken at different seasons was studied. The response of two cutting positions to various IBA and rooting-cofactor was evaluated. Attempts were also made to overcome the requirement of peaches for a seed stratification period by using plant growth regulators. Studies to optimise the establishment and growth of embryos of early to late ripening cultivars ('Klara', 'Oom Sarel' and' Summer Giant') in vitro were undertaken to overcome the problem of small underdeveloped embryos which are often obtained from early ripening peach cultivars. The early ripening peach cultivars 'DeWet', 'Earlibelle' and 'Florida Prince' were propagated by softwood, semi-hardwood and hardwood cuttings prepared from August 1999 to July 2000. It was found that softwood cuttings prepared in October and November responded well to an IBA application of 1000 mg 1(-1), resulting in 100 % rooting in all three cultivars. In 'Florida Prince' and 'DeWet' the rooting-cofactors chlorogenic acid and phloroglucinol at the concentration of 100ug 1(-1) yielded 77 % and 81 % rooting, respectively while quercitin and rutinin gave only 46 and 44 % rooting for all cultivars. The basal portion of the cutting rooted better than the terminal portion (78 % versus 58 %). Rooting percentage differed in all treatments in response to IBA application and rooting-cofactors 74 % for 'Florida Prince', 62 % for 'DeWet' and 54 % for 'Earlibelle'. The use of IBA and Ca-EDTA proved to be beneficial for rooting of 'Florida Prince' cuttings and resulted in a rooting percentage of 86 % in wounded hardwood cuttings of this cultivar. It was also shown that the simple sugars glucose, fructose and sucrose as well as the sugar alcohols sorbitol and mannitol accumulated at the base of the cutting during adventitious base root formation if the cutting bases were treated with 1000 mg 1(-1) IBA. The concentration of these sugars and sugar alcohols were lower in untreated cuttings compared to IBA treated cuttings during the adventitious root formation process. Studies on the origin of adventitious root formation in stem cuttings of peaches were conducted using light microscopy and transmission electron microscopy. These studies revealed that adventitious roots originate (in peaches) in the vicinity of the vascular bundle tissue and in the cells around them. It was found, however, that adventitious root formation in IBA treated cuttings is associated with the formation of root primordia. These may trigger root initiation and ultimately the development of adventitious roots. In embryo culture studies the highest number of roots and greatest length of roots per embryo as well as the highest number of embryos forming roots was achieved when either the medium of Murashige and Skoog (MS) (1962) or of Steward and Hsu (SH) (1978) were employed. The Woody Plant medium (Lloyd and McCowan, 1978) and the medium after Schenk and Hildebrandt (1972) were found to be less effective when compared to MS and SH media. Addition of GA(3) (0.01 mg 1(-1)), BAP (0.2 mg 1(-1)) or GA(3) (0.01 mg 1(-1))+ BAP (0.2 mg 1(-1))+IBA (0.5 mg 1(-1)) to the media gave the best results with respect to embryos forming roots (86 %), number of roots per embryo (8.0) and total length of roots per embryo (7.57 cm). Furthermore, the following conditions for optimal rooting of peach embryos were established: pH 5.2, 1.5 g 1(-1) agar, 60 g 1(-1) sucrose and 16h light/8h darkness photo period. Further studies on the influence of the stratification temperature on germination of embryos revealed the highest germination percentage after exposure to 40 C ±2 constantly for 30 to 60 days. However, this chilling requirement can be successfully substituted by the addition of GA(3) , kinetin, zeatin, BA and thiourea to the culture media.
Postharvest maintenance of the shelf life and quality attributes of banana and papaya.
(2003) Ghebreslassie, Biniam M.; Bower, John Patrick.; Bertling, Isa.
Banana and papaya are two commonly grown fruits of the tropical regions of the world. Like most tropical climacteric fruits, these fruits have a short shelf life and deteriorate rapidly after harvest. This becomes more pronounced in tropical areas where cold chain management practices are poor and the ability environmental control is limited. To extend the shelf life with maintained quality of the fruits and to investigate the effect of various postharvest treatments a study was conducted on banana cv. Williams and on papaya cv. Hortus Gold. The treatments included storage in different packaging materials such as micro perforated polypropylene bags (MPB), Micro perforated polypropylene bags with ethylene absorbent (MPB+K), macro perforated polypropylene bags coated with anti mist coating (PP), fruit waxing, gibberellic acid (GA3) and indole butyric acid (IBA) applications. Comparison was made against untreated fruits. Banana fruits were held at 12, 15 and 22°C, while papaya fruit were held at 5.5, 7, 10 and 22°C. Percentage weight loss (PWL), firmness change, visual colour development and respiration rate were evaluated on a weekly basis during the storage period. Selected quality attributes such as total soluble solids (TSS), pH, total chlorophyll, total carotenoids and organic acids for banana and TSS, pH, titratable acid (TA) and sugar:acid ratio for papaya were analyzed at the end of the storage period. PWL, softening, colour development and respiration rate increased significantly (P
The potential of post-harvest potassium silicate dips to mitigate chilling injury on citrus fruit.
(2012) Mditshwa, Asanda.; Bertling, Isa.; Bower, John Patrick.
The South African Citrus Industry is the second largest exporter of citrus, after Spain. The industry is under pressure to supply high quality fruit as well as to expand into new, high paying markets. However, high paying markets such as Japan and the USA require cold sterilised fruit as obligatory quarantine treatments against Mediterranean fruit fly (Ceratitis capitata) in order to reduce any possible spread of the pest. Citrus fruit originated from tropical climates and hence are chilling susceptible. Chilling injury symptoms appear as dark brown spots, pitting and/or decay when fruit are transferred to shelf temperatures; thus reducing the marketability of citrus fruit. Therefore, there is need for methods to mitigate chilling injury. Previous studies have shown silicon to mitigate many forms of stress without any hazardous effect on human health. Thus, the aim of the study was to investigate the potential of post-harvest silicon dips in mitigating chilling symptoms in citrus fruit. Briefly, fruit from two sources (Ukulinga Research Farm and Ithala Farm) were dipped in different silicon concentrations (0, 50, 150, and 250 mg ℓ-1) for 30 minutes and thereafter stored at -0.5 or 2⁰C for up to 28 days with weekly evaluation for chilling injury symptoms. Total antioxidants were determined using FRAP, ABTS, and DPPH assays under spectrophotometer. In addition, sugars, ascorbic acid, phenolics and flavonoids were analysed using High Performance Liquid Chromatography (HPLC). Fruit from Ukulinga Research Farm showed significantly higher total antioxidants (ascorbic acid total phenolics and specific flavonoids hesperidin and naringin) and sugars relative to fruit from Ithala Farm. Low concentrations of silicon dips significantly reduced the appearance of chilling injury symptoms by inducing an enzymatic conversion of glucose to ascorbic acid, thereby increasing the antioxidant capacity of chilling susceptible fruit. Moreover, silicon increased the concentration of total antioxidants, total phenolics and total flavonoids. High silicon concentrations had a negative effect on post-harvest quality of lemons by increasing fruit weight loss and electrolyte leakage, resulting in appearance of chilling symptoms. In conclusion, the study showed that silicon had potential to reduce chilling injury. However, high silicon concentrations raised concern, in particularly, on fruit appearance.
A preliminary study on the effect of climatic conditions and fruit mineral concentration on the development of lenticel damage in 'Tommy Atkins' and 'Keitt' mangos (Mangifera indica L.) and rind pitting in 'Benny Valencia' oranges (Citrus sinensis).
(2008) Magwaza, Lembe Samukelo.; Bower, John Patrick.; Kruger, Frans J.
The South African fresh fruit industry is a significant exporter, accounting for approximately 45% of the country’s agricultural exports. Of the total exported fruit in the 2005/06 season, 60% was subtropical fruit. However, certain physiological rind disorders such as mango lenticel damage and citrus rind pitting are frequently observed, reducing the commercial value of the fruit. This thesis deals with the epidemiology of these rind physiological disorders, in an attempt to obtain basic information that could serve as a guideline to predict and manage the fruit susceptibility to these disorders. The study further investigated the relative effects that certain pre-harvest factors have on the postharvest development of these disorders. Factors of particular interest were harvest maturity, climate and the mineral content of the fruit. Rind pitting is a physiological disorder of citrus that develops during storage. A study was conducted to investigate the relative effects that certain pre-harvest factors have on the post-harvest development of superficial rind pitting in ‘Benny Valencia’ oranges. Factors of particular interest were harvest maturity, climate and the mineral content of the fruit. In addition, trees were treated with two different formulations of nitrogen viz. limestone ammonium nitrate (LAN) and a slow release nitrogen fertilizer (Horticote®), during March 2006. The fruit were then sampled on a two-weekly basis over a period of three months. On each sampling date a set of quality related readings, such as juice sugar and titratable acid concentration were taken, after which the mineral concentration of the exocarp and mesocarp was measured. Fruit were also stored under export simulation conditions. The results indicate that fruit from trees that received additional N were more susceptible to rind pitting than those from control trees. Of the two N applications, fruit from trees that received slow release N were more susceptible to rind pitting than fruit from trees that received the LAN treatment. Another important observation made was that the nitrogen concentration of oranges from trees that received extra nitrogen fertilizer was lower than that from the controls. In addition, experimental fruit were smaller than control fruit. Fruit from trees that received the slow release nitrogen treatment were smallest. A sink/source hypothesis aimed at explaining this phenomenon has been formulated and is currently being investigated. A number of control mechanisms are also being explored. Mango lenticel damage is a serious defect that occurs on the rind of the fruit. This condition causes fruit to attain a speckled appearance and become unattractive to the buyer. Each season, the disorder reduces the packout of fresh fruit by about 16%. Several preharvest factors play a critical role in the postharvest development of lenticel damage. Preliminary studies have shown that the incidence of lenticel damage has some relationship to fruit moisture and fruit nutrient concentration, depending on which of these factors first achieve a critical threshold. A study was conducted to develop appropriate skin moisture content parameters to predict lenticel damage potential before harvest. The study further aimed to provide certain biological markers regarding appropriate nitrogen fertilization practices to reduce lenticel damage. To do this, two trials were conducted approximately a month before harvest. Firstly, a plastic ground cover to restrict water supply was laid under ‘Tommy Atkins’ trees at Numbi Estates in the Hazyview area of the Mpumalanga province, South Africa. Secondly, additional nitrogen was applied as three different formulations, viz. limestone ammonium nitrate (LAN), potassium nitrate (KNO3), and a slow release nitrogen fertilizer under trade name Horticote®, to two ‘Tommy Atkins’ and two ‘Keitt’ orchards at Bavaria Estates in the Hoedspruit area of the Limpopo province, South Africa. Fruit samples were harvested, packed and stored at different intervals after rain during January 2006. With regard to lenticel damage on ’Tommy Atkins’ fruit from Numbi, there was no significant difference between the control and plastic cover treatments. However, when compared to fruit from other localities, the Numbi fruit had the highest incidence of lenticel damage, followed by the conventional orchard at Bavaria. The Numbi fruit, which had the highest incidence of lenticel damage, also had the highest pulp and skin nitrogen concentration. Of the four treatments at Bavaria, the LAN treatment had the highest incidence of lenticel damage, but the pulp and skin nitrogen concentrations of this treatment were comparable with the other treatments. The organic fruit had significantly lower lenticel damage incidence and also had the lowest pulp and rind nitrogen concentrations. Similarly, ‘Keitt’ results showed that the intensity of lenticel damage was significantly higher in the orchard that received additional nitrogen in the form of LAN. From the results it was, however, not possible to formulate nitrogen-lenticel damage correlations. The study failed to prove the case for a direct relationship between the disorder and with nitrogen, as there were no significant or consistent correlations with nitrogen content. An interesting relationship was nevertheless observed between lenticel damage and the time of harvest before and after rainfall. The results indicated that both ‘Tommy Atkins’ and ‘Keitt’ fruit become more susceptible to lenticel damage when harvested a day after rainfall and this gradually reduces afterwards. The effect of rainfall in this regard and a sink-source hypothesis arising from these observations are also discussed.
Some aspects of water relations on avocado Persea americana (Mill.) tree and fruit physiology.
(1985) Bower, John Patrick.; Wolstenholme, B. Nigel.
The effect of long-term irrigation on water relations of 'Fuerte' avocado trees, and the consequences for fruit ripening and physiology, particularly physiological disorders, were studied. Four irrigation regimes were used, namely dryland relying on rainfall, occasional (irrigation when soil moisture tension reached 80 kPa), regular (soil moisture tension 55 kPa) and frequent (soil moisture 35 kPa). Seasonal tree leaf water potential was studied. This became more negative during the dry season (winter and spring) and less negative during the period of summer rainfall. During the dry periods, the dryland and 80 kPa treatments had considerably more negative leaf water potential than the 55 kPa and 35 kPa regimes. These differences decreased during the summer rainfall period. Acclimation was studied by measuring leaf osmotic pressure and osmotic pressure at zero turgor. A pattern similar to seasonal leaf water potential emerged. Further, the dryland treatment showed higher osmotic pressure, particularly at zero turgor, during winter and spring. It was concluded that these trees may have acclimated. Diurnal cycles of stomatal resistance, transpiration and leaf water potential on a summer, two winter and a spring day were monitored. Dry land trees showed acclimation, with delayed reaction to environmental water demand and decreasing soil moisture. Trees of the 80 kPa treatment showed greatest stress. Fruit water potential became more negative between April and July, with fruit softening becoming more rapid. Treatment differences were inconclusive. Polyphenol oxidase activity (PPO), soluble and total, was measured. For fruit picked in April and July 1983, the 55 kPa treatment showed the lowest activity and the 80 kPa the highest. Storage at 5,5⁰C for 30 days increased the activity, while fruit softening decreased it. July activity was higher than the April-harvested fruit. The same pattern emerged for fruit harvested in April 1985, although treatment differences were not significant. Rainfall was considerably higher during the fruit development period of 1985 fruit as compared with that of 1983. A significant interaction between restricted container ventilation during ripening and irrigation history was obtained, the 80 kPa fruit showing higher PPO activity than 55 kPa fruit. Ethylene evolution during ripening showed a normal climacteric pattern for 55 kPa and dryland fruit, but a delayed peak for 80 kPa fruit. Fruit calcium concentrations showed rapid changes between 7 and 16 weeks after fruit set thereafter remaining constant to harvest. There were no clear treatment differences. Fruit abscisic acid levels at 50% soft (100% is eating soft) were lowest in 55 kPa fruit, and highest in 80 kPa. A significant correlation between these values and soft fruit PPO activity was found. A preliminary fruit quality prediction model is suggested.