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 I AA and ABA were determined in the same tissues. Ethylene production, w hich is believed to signal the ripening and senescence of mature fruit , was also measured. Ethylene production was highest in the outer tiss ue near the rind and gradually declined during maturation, except for a dramatic increase in all fruit tissues at the climacteric. In contra st 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 ra ther than the availability of ACC regulates ethylene production in dev eloping fruits. ABA and ABA esters generally declined during maturatio n, however an increase in ABA esters associated with the outer mesocar p tissue was observed in fully mature, climacteric fruit. IAA and IAA conjugates were only found in the outer tissue near the rind, and thei r levels remained low until the fruit was fully mature and entering th e climacteric. At that time, increased levels of conjugates were detec ted. The late burst of hormonal metabolism in the outer mesocarp tissu e appeared to signal its degeneration and the deterioration that typic ally occurs in ripening fruit. The tissue-specific conjugation of IAA and ABA, in addition to the production of climacteric ethylene, may re present part of the signaling mechanism initiating ripening and eventu al deterioration of tissues in muskmelon fruits.