Developmental Responses to Drought and Abscisic Acid in Sunflower Roots: 2. MITOTIC ACTIVITY

Mitotic activity was studied in the root apices of aeroponicallygrown sunflower seedlings (Helianthus annum L. var. RussianGiant) 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 sectionsof [³H] methyl-thymidine treated root apices. Both drought stressand ABA-treatment (at a concentration of 10^–2 mol m^–3inhibited DNA synthesis and mitosis within the first 6 h oftreatment. The depression of mitotic activity was first evidentin the proximal regions of the meristem (1000–1500 µmfrom the cap junction). This was followed by a general depressionof mitotic activity throughout the meristem which was, in turn,followed by a partial recovery of mitotic activity in the distalregions of the meristem. The beginning of this partial recoverywas concurrent with the activation of the quiescent centre (QC).Treatment with lower concentrations of ABA (10^–3 mol m^–3and 10^–4 mol m^–3) also inhibited mitotic activity.Exogenous supplements of sucrose to the plant did not alleviatethe inhibition of mitotic activity by drought or ABA. Thesefindings support the hypothesis that ABA mediates drought-inducedchanges in the primary development of sunflower roots. Key words: Abscisic acid, drought, mitotic activity