Physiological changes associated with senescence and abscission in mature citrus fruit induced by 5-chloro-3-methyl-4-nitro-1H-pyrazole and ethephon application

This research compares effects of the compound 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), a plant growth regulator that selectively promotes abscission in mature citrus fruit (Citrus sinensis), and the ethylene-releasing agent ethephon (2-chloroethylphosphonic acid). Application of CMNP and ethephon to mature citrus fruit reduced fruit detachment force and changed peel color from green to orange. More total chlorophyll was extracted from flavedo in early season (November) than late season (January), and both compounds caused a similar reduction in chlorophyll. In contrast, total carotenoid content was similar in November and January. Both abscission compounds increased total carotenoids, but induction was greater in January, and CMNP was more effective in both months. Phospholipase A(2) (PLA(2)) activity increased after CMNP but not ethephon application. Electrolyte leakage increased 2 h after CMNP treatment, and total protein content was reduced by 50% after 72 h. Ethephon caused only minor changes in electrolyte leakage and total protein content. Inhibition of PLA(2) activity with aristolochic acid did not reduce leakage but inhibited total protein loss and reduced visual peel damage associated with CMNP. Ultrastructural observations indicated decreased number, and length of starch grains 3 h after CMNP treatment. A transient increase in soluble sugars was measured 3 h after CMNP application. Ethephon had little effect on soluble sugar content and changes in starch grains. Collectively, the results indicate that CMNP and ethephon induced color change in peel and advanced mature fruit abscission. However, CMNP but not ethephon promoted other physiological changes associated with senescence.