Abstract
Cutting off carbohydrate supply to longan (Dimocarpus longan Lour.) fruit by girdling and defoliation or by detachment induced 100% abscission within a few days. We used these treatments to study the involvement of reactive oxygen species (ROS) in fruit abscission. Girdling plus defoliation decreased sugar concentrations in the fruit and pedicel and depleted starch grains in the chloroplasts in the cells of abscission zone. Prior to the occurrence of intensive fruit abscission, there was a burst in ROS in the pedicel, which peaked at 1 day after treatment (DAT), when H2O2 in the abscission zone was found to be chiefly located along the plasma membrane (PM). H2O2 was found exclusively in the cell walls 2 DAT, almost disappeared 3 DAT, and reappeared in the mitochondria and cell walls 4 DAT. Signs of cell death such as cytoplasm breakdown were apparent from 3 DAT. The burst of ROS coincided with a sharp increase in the activity of PM-bound NADPH oxidase in the pedicel. At the same time, activities of antioxidant enzymes including superoxide dismutase (SOD), catalase, and peroxidase (POD) were all increased by the treatment and maintained higher than those in the control. Accompanying the reduction in H2O2 abundance, there was a sharp decrease in PM-bound NADPH oxidase activity after 1 DAT in the treated fruit. H2O2 scavenger dimethylthiourea (DMTU, 1 g L–1) significantly inhibited fruit abscission in detached fruit clusters and suppressed the increase in cellulase activity in the abscission zone. These results suggest that fruit abscission induced by carbohydrate stress is mediated by ROS. Roles of ROS in regulating fruit abscission were discussed in relation to its subcellular distribution.
Highlights
Carbohydrates serve as the “hard currency” in plants, representing the costs for various biological functions including growth, maintenance, and defense
Effects of Girdling and Defoliation and Detachment Treatments on Fruit Abscission Girdling and defoliation increased fruit abscission compared with the rate in the control, and it increased with the rate of defoliation (Figure 2A)
Girdling is a common horticultural practice to promote flowering and fruit set, and serves as a useful tool for physiological study of shoot behavior when it is isolated from other plant parts in terms of carbohydrate exchange (Goren et al, 2004)
Summary
Carbohydrates serve as the “hard currency” in plants, representing the costs for various biological functions including growth, maintenance, and defense. Fruit are net importers of carbohydrates from the tree reserves or leaf photosynthesis (Mehouachi et al, 2000; Hieke et al, 2002; Iglesias et al, 2003). Fruit trees generally produce more fruitlets than they can support to harvest, and fruit abscission. Carbohydrate stress, H2O2, and abscission is a normal physiological event during fruit development due to a self-regulatory mechanism to reduce fruit load (Bangerth, 2000). According to Lakso et al (2006), environmental factors affect fruit abscission based on the carbohydrate supply–demand balance, and higher carbohydrate availability reduces sensitivities to abscission-inducing stresses or fruit-thinning chemicals. Mehouachi et al (1995) suggested the existence of a threshold carbohydrate concentration in citrus below which fruit shedding was intensified According to Lakso et al (2006), environmental factors affect fruit abscission based on the carbohydrate supply–demand balance, and higher carbohydrate availability reduces sensitivities to abscission-inducing stresses or fruit-thinning chemicals. Mehouachi et al (1995) suggested the existence of a threshold carbohydrate concentration in citrus below which fruit shedding was intensified
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