Diurnal changes in endogenous abscisic acid (ABA) in the leaves, together with stomatal conductance and water relations, were studied in grape vines (Vitis vinifera L. cvs Riesling and Silvaner) growing in the field in Adelaide, South Australia.
There was an increase in ABA content of the leaves which commenced in the morning and reached a peak of approximately twice the pre-dawn level at midday. Wilted excised leaves from the same vines accumulated ABA at a higher rate. The increase in attached leaves was sometimes unrelated to low leaf turgor. Import via the xylem may account for this increase during the early part of the day. Leaf conductance in Riesling was high in the early morning but fell as leaf ABA rose, and it is suggested that midday stomatal closure was due to ABA accumulation. Stomatal behaviour later in the day was more readily accounted for by environmental variables such as temperature and atmospheric vapour pressure deficit than by changes in ABA.
Abscisic acid did not consistently accumulate in attached leaves in response to zero turgor, even if this condition was maintained for the greater part of the day. It is suggested that loading into the phloem and transport out of the leaf may be important in controlling stress-induced accumulation of ABA.