Pre- and post-harvest response of selected indigenous leafy vegetables to water stress.

Abstract

South Africa has wide diversity of African leafy vegetables (ALVs) rich in nutrients and adapted to marginal production. However, there is limited availability of ALVs in South Africa due to lack of cultivation owing to limited agronomic and postharvest management information. The increase in population growth, malnutrition and climate change necessitates production of more food using limited water resources. The aim of this study was to evaluate pre and postharvest response of Amaranthus cruentus (pigweed), Vigna unguiculata (cowpea), Corchorus olitorius (Jute mallow) and a reference crop B. vulgaris (Swiss chard) to varying irrigation regimes. The current study consisted of a literature review and five experiments (two agronomic studies and three post-harvest studies. In literature, the performance of ALVs is drawn in comparison to exotic counterparts grown under different conditions; yet agronomic and nutritional factors are only valid when crops are grown under the same condition. Hence in the four experiments of this study, Swiss chard was used as a reference crop grown under same locality. Swiss chard was chosen because it is an alien leafy vegetable that has been indigenised in sub-Saharan Africa and is highly nutritious (contains high levels of Fe, Zn and β-carotene). Before conducting experiments there was need to identify potential gaps and research priorities for this study and even for future research. This was done by conducting a literature review study (Chapter 2) on the status of production and utilisation of ALVs in South Africa for the period 1994–2017. Results of the review indicated that there is a decline in consumption of ALVs partly as a result of limited availability and negative perception. In order to promote ALVs, further research on agronomy, post-harvest handling, storage and processing is required in South Africa. Field and rain shelter experiments were conducted at Roodeplaat, Pretoria, over two summer seasons, 2015/2016 and 2016/2017 to evaluate growth, yield and water-use of selected leafy vegetables under varying water regimes. A randomised complete block design with three replicates was used. The treatments evaluated were: three irrigation regimes (30%, 60% and 100% of crop water requirement (ETc) on three ALVs – Amaranthus cruentus, Corchorus olitorius and Vigna unguiculata and a reference crop, Beta vulgaris. Seeds of A. cruentus and C. olitorius were obtained from the seed bank of the Agricultural Research Council (ARC) - Vegetable and Ornamental Plants (VOP), Roodeplaat, Pretoria seed bank. Vigna unguiculata (Bechuana white, a runner type) and Swiss chard (B. vulgaris L.) cultivar ‘Ford Hook Giant’ seeds were obtained from Hygrotech Seed Pty. Ltd., South Africa. Soil samples were taken from the field prior to land preparation and soil fertility analyses done at the Agricultural Research Council–Institute for Soil, Climate and Water (ARC–ISCW). Nitrogen, phosphate and potassium were applied according to the results and recommendations of the soil fertility analysis for both seasons. Seedlings of A. cruentus, B. vulgaris and C. olitorius were raised in commercial growing medium and covered with vermiculate to minimize water losses from above surface. Vigna unguiculata was sown directly. Seedlings were transplanted at four weeks after sowing. Irrigation scheduling was based on reference evapotranspiration (ET) and a crop factor for each crop. Data collection in field and rain shelter trials included plant height, leaf number, chlorophyll content index (CCI), chlorophyll fluorescence (CF) and yield. In A. cruentus, drought stress (30% ETc) reduced yield consistently in both field and rain shelter trials. Plant height and chlorophyll content index (CCI) were significantly reduced by water stress under field conditions. For C. olitorius, drought stress (30% ETc) reduced yield under rain shelter conditions while all measured parameters were not affected under field conditions. In V. unguiculata, stem fresh mass increased with increase in water application from 30%-60 ETc with no further significant increase under field conditions while all measured parameters showed a similar trend under rain shelter although the results were not significant. In B. vulgaris leaf number, plant height, CCI, yield, Mg, Ca, Na, Zn, and Mn were reduced by water stress for rainshelter. Using 60% ETc proved to be suitable for production of A. cruentus and B. vulgaris var. cicla whereas 30% ETc would be recommended for V. unguiculata. For V. unguiculata and C. olitorius application of 30% ETc is recommended while application of 60% ETc can be used under to slightly improve yield. Amaranthus cruentus and B. vulgaris were comparable in their response to water regimes while C. olitorius and V. unguiculata performed better than B. vulgaris under water stress, an indication of an opportunity to use these vegetables under drought conditions. The evaluation of nutritional quality of A. cruentus, C. olitorius, V. unguiculata and B. vulgaris was motivated by recommendations made in most agronomic studies based on biomass yield with no follow-up on nutritional value. Samples from each crop were collected from each of the three irrigation regimes (30%, 60% and 100%ETc) during each harvest (6, 8 and 10 weeks after transplanting for both seasons) and analysed for macro and micronutrients. Results from A. cruentus indicates that Ca and Mg were significantly higher under drought stress (30% ETc) while Na, K and Zn increased with water application up to 60% ETc with no further increase thereafter. Similarly, Ca and Mg were higher under drought stress and Zn under medium stress in C. olitorius. Calcium was high under drought stress condition in B. vulgaris while Na and Zn where high in medium stress; with a further increase in water application resulting in diminishing returns. Phosphate and potassium were high in medium stress condition in V. unguiculata while in water application up to 100% ETc the two elements showed diminishing returns. The high nutrients alternated between the most severe water stress (30% ETc) and medium stress (60 ETc) treatments across all crops in this study, an indication that although the crops can be grown under drought conditions, slight irrigation can lead to improved production. Leaf Fe, Zn, Mn, Mg and Ca increased with time of harvesting that increased from 6 to 8 weeks, with no further change in nutritional yield when crops were harvested at 10 weeks in A. cruentus, V. unguiculata and B. vulgaris. In C. olitorius, Fe, Zn, Mn, Mg and Na were high when harvested at 6 weeks compared to late harvesting (8 and 10 weeks). The first postharvest study investigated the effect of three irrigation regimes (30%, 60% and 100% ETc) and three drying (sun, oven, shade) methods on phenolic, flavonoid and gallatannin content of the four vegetables. Fully irrigated C. olitorius and subjected to sun drying (100% ETc x sun drying) had higher total phenolic content followed by medium stress subjected to shade drying (60% ETc x shade drying). Furthermore, water stressed plants that were then shade- or sun-dried retained better gallotannin content than other treatment combinations. Amaranthus cruentus grown under drought then shade- or sun-dried (30% ETc x shade and sun drying) retained better quality in all phenolic components measured. In V. unguiculata, phenolic content was high in water-stressed plants subjected to sun-drying (30% ETc x sun drying) while sun drying retained flavonoid and gallotannin than shade and oven drying. In B. vulgaris, well irrigated plants and shade- or oven-dried (100% ETc x shade/oven drying) had better phenolic content. Shade dried leaves had better flavonoid while drought-stressed plants had better gallotannins content compared to other treatments in B. vulgaris. All three ALVs can be grown under drought stress and subjected to sun or shade drying to retain nutrient compared to B. vulgaris. The second experiment on postharvest investigated the interaction of packaging (non-perforated and perforated), temperature [room storage, refrigerated storage (4℃), retail storage, 10℃] and storage duration (2, 4, 6, 8, 10 days) on C. olitorius. Plants rarely experience a single stress factor but are simultaneously exposed to multitude stress factors in their growing environment. The results showed that treatment combination of 4℃ with perforated packaging retains higher phenolic content followed by perforated packaging at 10℃ while lower phenolics were in treatment combinations that were stored at room temperature. Total phenolic content was higher at 2 days and 4 days storage in non-perforated packaging compared to all other treatments combinations. Furthermore, phenolic content decreased disproportionately with storage duration in non-perforated packaging treatment combinations, performing better than perforated in every storage duration. Flavonoid content and total phenolics decreased with increase in storage duration while better retaining these in any treatment combination of 4℃/10℃ compared to room temperature. Phenolic content was significantly higher from 2 to 4 days then declined from 6 through to 10 days at 4℃. At room temperature, phenolic contents decreased from 2 to 4 days storage durations but were higher at 6 and 8 days storage durations before dropping at 10 days. Antioxidant activity and overall acceptance was improved in any treatment combinations kept at 4 and 10℃ compared to room temperature for both types of packaging as storage duration increased. Antioxidant activity and overall acceptance degradation was reduced in treatment combination kept at 4 and 10℃ compared to room temperature for both types of packaging as storage duration increased. Corchorus stored at room temperature had a shelf life of 2 days, but 8 days at 4℃ and 10 days at 10℃ for both types of packaging.