PROLINE ACCUMULATION IN MAIZE (ZEA-MAYS L) PRIMARY ROOTS AT LOW WATER POTENTIALS .1. REQUIREMENT FOR INCREASED LEVELS OF ABSCISIC-ACID

Previous work showed that the concentration of proline (Pro) increases greatly in the primary root tip of maize (Zee mays L.) at low water potentials (psi/w). It was also shown that the maintenance of root elongation at low psi w depends on increased levels of abscisic acid (ABA). In this study we have assessed whether ABA is required for the increase in Pro concentration. Seedlings were grown in vermiculite of various psi w, and endogenous ABA levels were decreased using either fluridone (FLU) or the vp5 mutant to inhibit carotenoid (and ABA) synthesis. In both treatments, Pro concentrations at low psi w were substantially decreased throughout the epical centimeter, which encompassed the elongation zone. Pro concentrations in FLU-treated roots were restored by addition of 7 mu M ABA to the vermiculite, which raised the internal ABA content to the level in untreated roots at the same psi w. Pro and water content profiles were combined with published growth-velocity distributions to calculate the distribution of net Pro and water deposition rates using the continuity equation. At a psi/w of -1.6 MPa, the rate of Pro deposition in the root tip was decreased by 75% in FLU-treated compared to untreated roots. FLU treatment increased root diameter and, therefore, water content per unit length, but water deposition rates decreased due to the dominant influence of reduced longitudinal expansion. Thus, the decrease in Pro concentration was attributable entirely to the decrease in Pro deposition. The results demonstrate that increased ABA is required for high rates of Pro deposition and, thereby, high Pro concentrations in the growing region of maize primary roots at low psi w.