Accumulation of Abscisic Acid in Cell Suspension Cultures of Oilseed Rape Treated with the Growth Retardant BAS 111 .. W: Effects on Osmotic Potential and Potassium, Water and Sugar Contents

Abstract

Cell suspension cultures of oilseed rape ( Brassica napus L. cv. Kasan) showed a transient increase in cellular abscisic acid (ABA) content at the beginning of the exponential growth phase, with a maximum after 5 days of subculture. In proportion to the endogenous levels, ABA was excreted into the culture medium. Treatment with 10 -4 mol·L -1 fluridone, an inhibitor of ABA biosynthesis, suppressed the rise in ABA levels almost completely. Thus, ABA might be produced in response to osmotic stress resulting from subculturing the cells in fresh medium with higher sugar and mineral concentrations. This hypothesis was supported by the finding that endogenous ABA accumulation correlated with increasing osmotic stress induced by mannitol (from -0.39 to -0.74MPa) in the medium. Concomitantly, cellular water content and the osmotic potential of the cell sap decreased. Application of the triazole-type growth retardant BAS 111 .. W also increased the amount of endogenous ABA, but without changes in the osmotic potential of the medium. The compound is thought to inhibit ABA catabolism to phaseic acid. In contrast, the cyclohexanetrione-type retardants prohexadione calcium and LAB 198 999 did not alter ABA contents significantly. Dose response curves obtained 5 days after treatment with BAS 111 .. W revealed that ABA accumulation in the cells was accompanied by rising potassium and water levels and osmotic potential of the cell sap. The latter effect could be explained by a restricted uptake of sucrose and/or its hydrolysis products glucose and fructose, in BAS 111 .. W treated suspension cells. Calculated on a cell number and cell dry weight basis, ABA, potassium and water content increased to approximately 210, 160 and 140%, respectively, of the control values after treatment with 10 -5 to 10 -4 mol·L -1 BAS 111.. W. A significant increase was also found in magnesium while calcium was only slightly elevated and sodium levels remained unchanged. The changes in the potassium and water contents following the elevated endogenous ABA levels mediated by BAS 111 .. W were thus opposite to those observed when ABA was increased in response to osmotic stress. The specific effects of ABA on these parameters may thus be elucidated without interference from the effects of simultaneous water stress. It is speculated that retardant-caused ABA accumulation influences endogenous potassium, water and sucrose levels and the osmotic potential of the cell sap.