CONFIRMATION THAT ABSCISIC-ACID ACCUMULATION IS REQUIRED FOR MAIZE PRIMARY ROOT ELONGATION AT LOW WATER POTENTIALS

Previous work showed that primary root elongation in maize (Zea mays L .) seedlings at low water potentials was severely inhibited when accum ulation of abscisic acid (ABA) was decreased using either fluridone or the vp5 mutant to inhibit carotenoid (and ABA) biosynthesis. The obje ctive of this study was to confirm that the inhibition of root elongat ion resulting from these treatments was indeed attributable to the dec rease in ABA levels. Seedlings were transplanted after germination to vermiculite at water potentials of -1.6 or -0.3 MPa. ABA was mixed at various concentrations with the vermiculite to test whether the effect s of fluridone and the vp5 mutation on root elongation could be overco me. In both treatments, restoration of ABA levels in the root apical 1 0 mm, which encompassed the elongation zone, resulted in recovery of r oot elongation rate at both -1.6 MPa and -0.3 MPa. Analysis of the spa tial distribution of elongation rate showed that the recovery of overa ll root elongation resulted from restoration of the profile of elongat ion, and did not involve over-promotion of local elongation rate at an y position. The recovery of root elongation was shown to be independen t of effects of ABA status on shoot growth. When ABA was applied at hi gh water potential, such that levels of ABA in the root tip reached th ose associated with maintenance of elongation at low water potentials, root elongation was inhibited. Thus, the response of root elongation to bulk tissue ABA content varied with the tissue water status, The re sults confirm that accumulation of ABA is required for the maintenance of maize primary root elongation at low water potentials.