Seasonal and diurnal changes in leaf water relations, stomatal behaviour and abscisic acid were studied in apricot (Prunus armeniaca L.) trees growing in Adelaide, Australia. Diurnal changes in stomatal conductance and foliar abscisic acid (ABA) were small and the ABA content of expressed xylem sap was low (0·3 to 0·9 × 10−4 mol m−3). This concentration of ABA was too low to influence stomatal conductance, although diurnal changes and some seasonal changes in leaf ABA could be accounted for by uptake from the xylem. Water potential fell throughout the growing season but high values of turgor were maintained by a corresponding fall in leaf osmotic potential. The fall in leaf osmotic potential was partly accounted for by a decrease in relative water content but measurements of osmotic potential at full turgor showed that there was a net accumulation of solute between December and March. Sorbitol was the solute chiefly responsible for this increase.
The ability of apricot leaves to synthesize ABA in response to zero turgor was greatly reduced between November and March, although this ability was partly regained in April. ABA appears to be relatively unimportant in the control of stomatal conductance in apricot.