Stomatal response to abscisic acid fed into the xylem of intact Helianthus annuus (L.) plants

The effect of abscisic acid on stomatal apertures of sunflower(Helianthus annuus (L.)) was investigated with a new methodfor 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 amature leaf of the plant from which it had been collected beforethe experiment. Simultaneously, unmodified xylem sap was fedinto a control leaf at the same internode. The use of the Passioura-typepressure chamber during feeding, prevented embolisms and ensuredminimum dilution of the feeding solution. The effect of feedingwas measured by two gas exchange systems, located at the treatmentand at the control leaf. During the feeding experiments up to84% of the water volume transpired by the leaf was substitutedby the supplied feeding sap. When feeding xylem sap, to which2.5 mmol m^–3 ABA (physiological range) was added, leafconductance decreased to a similar value as in drought experiments.A log-linear relationship between the fed ABA-concentrationand leaf conductance was observed. Low stomatal con-ductancewas dependent on a continuous supply of ABA to the leaf. Whentotal ABA-influx into the leaf was large, either due to long-termfeeding of low concentrations or short-term feeding of highconcentrations (i) recovery after feeding started later and(ii) the rate of recovery was decreased. Therefore, stomatalresponses after short-term and long-term ABA-feeding were dependenton the loading of ABA into the leaf and not only on ABA-concentrations.The effectiveness of fed ABA was also dependent on the lightintensity at the fed leaf. Key words: Abscisic acid, feeding method, stomata, gas exchange, Helianthus annuus