Developmental Responses to Drought and Abscisic Acid in Sunflower Roots

Abstract

Mitotic activity was studied in the root apices of aeroponically grown sunflower seedlings (Helianthus annum L. var. Russian Giant) which were draughted or treated with abscisic acid (ABA) over a 7 d period. Labelling index (LI) and mitotic index (MI) were scored from autoradiographs of median longitudinal sections of [³H] methyl-thymidine treated root apices. Both drought stress and ABA-treatment (at a concentration of 10⁻² mol m⁻³ inhibited DNA synthesis and mitosis within the first 6 h of treatment. The depression of mitotic activity was first evident in the proximal regions of the meristem (1000–1500 μm from the cap junction). This was followed by a general depression of mitotic activity throughout the meristem which was, in turn, followed by a partial recovery of mitotic activity in the distal regions of the meristem. The beginning of this partial recovery was concurrent with the activation of the quiescent centre (QC). Treatment with lower concentrations of ABA (10⁻³ mol m⁻³ and 10⁻⁴ mol m⁻³) also inhibited mitotic activity. Exogenous supplements of sucrose to the plant did not alleviate the inhibition of mitotic activity by drought or ABA. These findings support the hypothesis that ABA mediates drought-induced changes in the primary development of sunflower roots.