Abstract
The effect of abscisic acid on stomatal apertures of sunflower (Helianthus annuus (L.)) was investigated with a new method for feeding solutions into an attached leaf of an intact plant. Xylem sap was sampled with a Passioura-type pressure chamber. Then it was modified in its composition and fed back into a mature leaf of the plant from which it had been collected before the experiment. Simultaneously, unmodified xylem sap was fed into a control leaf at the same internode. The use of the Passioura-type pressure chamber during feeding, prevented embolisms and ensured minimum dilution of the feeding solution. The effect of feeding was measured by two gas exchange systems, located at the treatment and at the control leaf. During the feeding experiments up to 84% of the water volume transpired by the leaf was substituted by the supplied feeding sap. When feeding xylem sap, to which 2.5 mmol m−3 ABA (physiological range) was added, leaf conductance decreased to a similar value as in drought experiments. A log-linear relationship between the fed ABA-concentration and leaf conductance was observed. Low stomatal con-ductance was dependent on a continuous supply of ABA to the leaf. When total ABA-influx into the leaf was large, either due to long-term feeding of low concentrations or short-term feeding of high concentrations (i) recovery after feeding started later and (ii) the rate of recovery was decreased. Therefore, stomatal responses after short-term and long-term ABA-feeding were dependent on the loading of ABA into the leaf and not only on ABA-concentrations. The effectiveness of fed ABA was also dependent on the light intensity at the fed leaf.
