Abscisic acid accumulation in leaves of two contrasting hybrid poplar clones affected by nitrogen fertilization plus cyclic flooding and soil drying.

Cuttings of hybrid Populus clones Tristis and Eugenei growing in pots in a greenhouse were treated with nitrogen fertilizer at two rates and subjected to repeated soil flooding or drying. Periodically, gas exchange measurements and radioimmunoassays, to determine abscisic acid (ABA) concentrations, were made on recently mature leaves.In both clones, photosynthesis and stomatal conductance were depressed five days after flooding, but leaf ABA concentrations remained relatively constant. In contrast, an initial, 9-day period of soil drying resulted in substantial ABA accumulation in leaves, which closely correlated with declines in photosynthesis and conductance. A second soil drying cycle of up to 9 days was less effective in modifying gas exchange and leaf ABA concentrations. High-N supply stimulated leaf ABA production as the soil dried. On the resumption of watering, gas exchange in Tristis recovered fully and rapidly and leaf ABA concentrations quickly returned to control values, whereas gas exchange in Eugenei recovered slowly and leaf ABA concentrations remained high for longer.Gas exchange in Eugenei was unaffected by soil drying until leaf ABA concentrations exceeded 100 ng g(dw) (-1), whereas Tristis showed a reduction in stomatal conductance and photosynthesis at leaf ABA concentrations of only 10 ng g(dw) (-1). A rise in internal CO(2) concentrations was associated with increased leaf ABA concentrations in Tristis, but not in Eugenei. Clonal differences in the relationship between gas exchange and leaf ABA concentration suggest contrasting physiological strategies for survival under prolonged drying conditions.