Hormonal Modification of Endogenous Abscisic Acid

Cytokinins stimulate ethylene production in wheat leaves (Loveys and Wareing 1971), pea stems (Fuchs and Liebermann 1968) and mungbean hypo-cotyls (Lau and Yang 1976, Imaseki etal. 1975). Since cytokinins increase the levels of free lAA, it is possible that the cytokinin effect on ethylene production is mediated by lAA. 

GA stimulates ethylene production in first internodes of Phaseolus vulgaris (Abeles and Rubinstein 1964), but is ineffective in enhancing ethylene production in pea seedlings, at least during the first 6 days of growth (Fuchs and Liebermann 1968). 

Since ethylene production is known to be autocatalytic (Abeles 1973), ethylene itself should be added to the list of hormonal factors capable of altering ethylene production. 

Eliasson (1969, 1975) has shown a correlation between promoter and inhibitor content in growing tissues. He noted that lAA maintains a high level of an ABA-like inhibitor in stem sections of aspen or pea and suggested that lAA 

Ethylene enhances ABA levels in citrus fruits (Goldschmidt et al. 1973) and in rose petals (Mayak and Halevy 1972). In the latter case the interaction between ethylene and ABA was viewed as an indication that ethylene may initiate senescence by enhancing ABA production and that ABA may, in turn, limit ethylene production by feedback inhibition of its biosynthesis. 

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