XYLEM-TRANSPORTED ABSCISIC-ACID - THE RELATIVE IMPORTANCE OF ITS MASSAND ITS CONCENTRATION IN THE CONTROL OF STOMATAL APERTURE

Abscisic acid (ABA) fed in pulses to the petioles of detached cherry l eaves in enclosed leaf chambers, caused a reduction in leaf conductanc e. The degree of inhibition was analysed with respect to the amount of ABA fed and to concentration of ABA in the feeding solution. Regressi on analysis of the data showed both variables to have a significant ef fect on leaf conductance. A hypothetical maximum ABA concentration occ urring in the leaf apoplast was calculated for each pulse from a simpl e model. This variable explained more of the variance within the data than either the amount or the applied concentration variable. A value for the rate at which ABA is removed from the apoplast is derived from the experimental data using the model. A second experiment attempted to evaluate this rate directly, by measuring the rate of catabolism of labelled ABA within the leaf. The results suggested a half-life of 36 min for the initial rate of decay. This figure is similar to that der ived from the model. the importance of ABA-metabolism for the control of leaf conductance is discussed in the context of root-to-shoot commu nication by ABA in the xylem stream.