The effects of partially drying part of the root system of Helianthus annuus on the abscisic acid content of the roots, xylem sap and leaves

Plants of Helianthus annuus were grown in soil in pots such that approximately 30% of the root system protruded through the base of the pot. After 7 d further growth in aerated nutrient solution, the attached, protruding roots were air-dried for 10-15 min and thereafter surrounded with moist still air, in the dark, for 49 h, whilst the soil was kept at field capacity. The roots of the control plants remained in the nutrient solution throughout the experiment.This treatment rapidly reduced the water content of protruding roots from 20.5 to 17.8 g g^-1 dry mass (DM), which remained less than that of the control roots for the rest of the experiment. This treatment also reduced root turgor and water potential. The abscisic acid (ABA) concentrations in the protruding roots, xylem sap and leaves of the treated plants increased significantly, compared to values recorded for control plants.In treated roots, the ABA concentration was significantly increased 4 h after treatment, with a maximum of 4.4+0.1 nmol g^-1 (DM) after 25 h. The ABA concentration in the xylem sap of the treated plants was significantly greater than in the controls 25 h, 30 h, and 49 h after the partial drying of the roots, with a maximum concentration of approximately 970 pmol ABA cm-3 at 49 h. Initially, the ABA concentration in the leaves was 0.45 nmol g^-1 (DM) which increased significantly to 1.1 ±0.1 nmol g^-1 at 25 h, to 1.7±0.3 nmol g^-1 at 49 h.Leaf conductance was significantly less in plants with air-dried roots than in the controls 8 h after the start of the treatment and thereafter. The water relations of the leaves of the treated plants did not differ from those of the control plants.These results confirm previous reports that ABA is rapidly generated in partially-dried and attached root systems and demonstrates a concomitant large increase in the ABA content of the xylem sap. It is suggested that partial dehydration of some of the roots of Helianthus annuus, increases ABA concentration in the transpiration stream and decreases leaf conductance in the absence of changes in leaf water status. As these responses were initiated in free-growing roots the stimulus is independent of any increases in soil shear strength that are associated with soil drying.