Indole-3-acetic acid, ethylene, and abscisic acid metabolism in developing muskmelon (Cucumis melo L.) fruit

Hormonal metabolism associated with fruit development in muskmelon was investigated by measuring IAA, ABA, and ACC levels in several tissues at various stages of development. In addition, levels of conjugated IAA and ABA were determined in the same tissues. Ethylene production, which is believed to signal the ripening and senescence of mature fruit, was also measured. Ethylene production was highest in the outer tissue near the rind and gradually declined during maturation, except for a dramatic increase in all fruit tissues at the climacteric. In contrast to ethylene production, ACC levels increased during maturation and remained equal throughout the fruit until the climacteric, when levels in the outer tissues increased nearly 5- fold over levels in the inner tissues. The consistent presence of ACC indicates that ACC oxidase rather than the availability of ACC regulates ethylene production in developing fruits. ABA and ABA esters generally declined during maturation, however an increase in ABA esters associated with the outer mesocarp tissue was observed in fully mature, climacteric fruit. IAA and IAA conjugates were only found in the outer tissue near the rind, and their levels remained low until the fruit was fully mature and entering the climacteric. At that time, increased levels of conjugates were detected. The late burst of hormonal metabolism in the outer mesocarp tissue appeared to signal its degeneration and the deterioration that typically occurs in ripening fruit. The tissue-specific conjugation of IAA and ABA, in addition to the production of climacteric ethylene, may represent part of the signaling mechanism initiating ripening and eventual deterioration of tissues in muskmelon fruits.