Regulation of endogenous abscisic acid levels and transpiration in oilseed rape by plant growth retardants

Abstract

Plant growth retardants of the norbornanodiazetine (e.g. tetcyclacis) and triazole (e.g. BAS 111. . W, LAB 150 978) type considerably increased endogenous abscisic acid (ABA) levels in cell suspension cultures, detached leaves and hydroponically grown seedlings of oilseed rape ( Brassica napus ). In detached leaves treated via the vascular system, ABA accumulated in proportion to retardant concentration and was closely correlated with a reduction in transpiration. Thus, the growth retardants appear to influence transpiration by regulating ABA metabolism. In suspension cells and shoots of seedlings, maximal ABA content was achieved 4 and 5 days after treatment, respectively. ABA accumulation may be due to a retardant-caused inhibition of the metabolism of ABA, presumably to phaseic acid, as suggested by [ 14 C] ABA feeding experiments with cell cultures. The initial rise in ABA levels was followed by a sharp decline, an effect that might be explained by a stimulated ABA catabolism and/or by an inhibition of its biosynthesis. In contrast, foliar application of BAS 111. . W to plants at the 4th leaf stage did not change either ABA levels in shoots nor water consumption per shoot fresh or dry weight analysed during 10 days after treatment. In leaves of soil treated plants at the 5th leaf stage ABA levels also remained unaffected or dropped below those of controls, especially at higher BAS 111. . W concentration. It is suggested that the growth retardants regulate ABA metabolism in oilseed rape, dependent on the mode of application and intensity, and duration of action.