Methyl jasmonate and salicylic acid enhance chilling tolerance in lemon (citrus limon) fruit.
South African ‘Eureka’ lemon fruit must be exposed to chilling temperatures (± 0.6°C) as a mandatory quarantine treatment against insect pests for all its overseas markets. Chilling lemon fruit at such temperatures may develop chilling injury (CI) symptoms on the flavedo. This negative effect on fruit quality reduces fruit marketability. This study evaluated postharvest factors influencing physiological, biochemical and ultra-structural mechanisms involved in alleviating CI in lemon fruit. It was hypothesised that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by maintaining cellular integrity and inducing synthesis of enzymatic and non-enzymatic antioxidants. Furthermore, fruit susceptibility to CI was associated with the source of fruit. Lemon fruit were harvested from three locations representative of moderate subtropical, warm temperate and cool subtropical environments. Harvested fruit were treated either with 10 μM MJ, 2 mM SA or 10 μM MJ plus 2 mM SA, stored either at -0.5, 2 or 4.5°C for 0, 7, 14, 21, or 28 days and afterwards transferred to 23°C for a week as shelf-life simulation. Thereafter, fruit were evaluated for alterations in physiological, biochemical and ultra-structural features involved in the manifestation of CI symptoms. Chilling damage was more severe in untreated lemon fruit than in treated lemon fruit. Storing lemon fruit at 4.5°C accelerated the manifestation of CI symptoms more so than at 2°C while storage at -0.5°C delayed the manifestation of CI symptoms. Lemon fruit of moderate subtropical origin were more chilling-tolerant than lemon fruit of warm temperate and cool subtropical origin. Treatment with 10 μM MJ plus 2 mM SA significantly (P < 0.05) improved chilling tolerance in lemon fruit. This treatment effectively maintained membrane integrity, thereby retarding electrolyte leakage and membrane lipid peroxidation as well as mass loss and respiration rate. Treatment with 10 μM MJ plus 2 mM SA was also effective in enhancing the antioxidant concentrations of vitamin E and carotenoids. The production of these antioxidants could have been part of a defence system against chilling damage, reducing CI and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA enhanced the concentration of compounds involved in chilling resistance, such as proline, soluble sugars, ascorbic acid and total phenolics as well as the enzyme phenylalanine ammonia-lyase (PAL). The enhancement of the defence mechanisms may have played a role in enhancing chilling tolerance in lemon fruit. The treatment also inhibited certain enzymes involved in tissue browning, such as peroxidase (POD) which might have contributed to delaying manifestation of symptoms. Polyphenol oxidase (PPO) was found to not be a good biochemical marker of the occurrence of CI. Treatment with 10 μM MJ plus 2 mM SA appeared to be able to enhance chilling tolerance in lemon fruit by maintaining the ultra-structure of the cuticle, cell wall integrity, cell membrane of parenchyma cells of the flavedo. This treatment also preserved the mineral nutrients of the flavedo (carbon, oxygen, phosphorus, potassium, calcium, magnesium, sulphur, sodium, silicon and aluminium) during cold storage. This could have played a role in protecting the fruit against chilling stress and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA reduced ROS production, while the activity of enzymatic antioxidants such as catalyse (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), and accumulation of essential proteins was enhanced. This increase in activity of enzymatic antioxidants and the presence of stress-responsive proteins in the lemon flavedo could have been directly involved in enhancing chilling tolerance. The CI symptoms were accompanied by an increase in membrane permeability, membrane lipid peroxidation as well as phospholipase D (PLD) and lipoxygenase (LOX) activity; however, treatment with 10 μM MJ plus 2 mM SA effectively reduced the membrane permeability, membrane lipid peroxidation, and PLD and LOX activity induced by the cold treatment. This could have contributed to the efficacy of 10 μM MJ plus 2 mM SA in inhibiting the manifestation of CI symptoms. Treatment with 10 μM MJ plus 2 mM SA enhanced flavedo total antioxidant capacity measured by ferric reducing ability of plasma; 2,2-diphenyl-1-picrylhydrazyl; 2,2-azinobis (3-ethyl-benzothiazoline-6-sulfonic acid) and the oxygen radical absorption capacity assays. The enhancement of antioxidant capacity in lemon flavedo could have contributed to the fruit’s chilling tolerance. Therefore, the effect of 10 μM MJ plus 2 mM SA treatment, enhancing chilling tolerance, may be attributed to its ability to enhance enzymatic and non-enzymatic antioxidants; activate essential proteins and mitigate the effect of ROS accumulation. With the use of 10 μM MJ plus 2 mM SA treatments, the South African citrus industry will be able to meet the quarantine temperature requirements for exportation of lemon fruit whilst reducing economic losses, depending on the preharvest conditions experienced by the fruit in each shipment.
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