Diurnal changes in stomatal conductance and accompanying changes in the abscisic acid (ABA) content of expressed xylem sap were studied in unstressed grapevines (Vitis vinifera L. cvs Riesling and Silvaner) growing in the field in Adelaide, South Australia. Stomatal conductance increased rapidly during the early morning reaching a maximum in both cultivars at about 1000 h. Conductance during the latter part of the day was considerably reduced despite relatively mild environmental conditions.
Quantitative analysis of ABA in untreated sap was carried out by high performance liquid chromatography (HPLC). The concentration of ABA in xylem sap varied from 1.5 × 10−4 mol m−3in the morning to 5 × 10−4 mol m−3in the late afternoon. Uptake of ABA from the xylem adequately accounted for diurnal changes in leaf ABA.
Gas exchange analysis of detached field-grown leaves showed that ABA, supplied via the transpiration stream, caused substantial reductions in stomatal conductance and assimilation when supplied at a concentration equivalent to that in xylem sap [5 × 10−4 mol m−3 (+) ABA]. The reduction in assimilation was entirely stomatal in origin.
It is suggested that ABA uptake from the xylem causes partial stomatal closure during the afternoon and plays a key role in the control of stomatal conductance in field grown vines by optimizing the relationship between CO2 uptake and water loss.