XVI International Botanical Congess

Early studies of ABA-deficient wilty" mutants of tomato showed that shoot growth was impaired. This conflicted with the general finding that applied ABA inhibits shoot growth, and it was concluded that the impairment was due to adverse water relations. We have reassessed this question in flacca and notabilis. Controlled-humidity conditions were usred to maintain the same water status in the mutants and wild type throughout experiments. Shoot growth of mutants remained substantially impaired in all parameters measured, including leaf area, dry weight and height. Growth was restored by applied ABA or silver thiosulfate, an inhibitor of ethylene action. The results indicate that in well-watered tomato, endogenous ABA is required to maintain shoot growth by restricting ethylene synthesis and/or sensitivity. Implications for water stress will be discussed.