DEVELOPMENT AND ACCUMULATION OF ABA IN FLURIDONE-TREATED AND DROUGHT-STRESSED VICIA-FABA PLANTS UNDER DIFFERENT LIGHT CONDITIONS

The effects of fluridone on guard cell morphology, chloroplast ultrast ructure and accumulation of drought stress-induced abscisic acid (ABA) were studied in Vicia faba L. plants grown under different light cond itions. Drought stress was induced by allowing the leaves to lose 12% of their fresh weight. The appearance of defective and undeveloped sto mata, and chloroplasts with a destroyed thylakoid membrane system was found in fluridone-treated plants grown at a photosynthetic photon flu x (PPF) of 600 mu mol m(-2) s(-1). Plants grown at a PPF of 40 mu mol m(-2) s(-1) had diminished levels of ABA after imposition of dehydrati on Fluridone treatment reduced the level of ABA in both unstressed and dehydrated leaves, Accumulation of ABA in the control plants was cons iderably reduced when they were exposed to dark periods of 24, 48 and 72 h just before imposition of the stress. Twenty-four hours after the dark treatment dehydration of the leaves resulted in a 3-fold decreas e in the level of stress-induced ABA, and 72 h after dark treatment th e amount of stress-induced ABA approximated the pre-stressed values. F luridone-treated plants failed to accumulate ABA under water stress. I n addition to functionally active chloroplasts, well-developed and fun ctional stomata are required for drought stress to elicit a rise in AB A.