STOMATAL CONDUCTANCE IN RELATION TO XYLEM SAP ABSCISIC-ACID CONCENTRATIONS IN 2 TROPICAL TREES, ACACIA-CONFUSA AND LITSEA-GLUTINOSA

Two tropical trees, Acacia confusa and Litsea glutinosa, were grown un der controlled conditions,vith their roots subjected to soil drying an d soil compaction treatments. In both species, a decline in stomatal c onductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resul ted in a substantial decrease in stomatal conductance but had little e ffect on leaf water potential. A rapid and substantial increase in xyl em abscisic acid (ABA) concentration ([ABA]), rather than bulk leaf AB A, was closely related to soil drying and soil compaction. A significa nt relationship between stomatal conductance (g(s)) and xylem [ABA] wa s observed in both species. Artificially feeding ABA solutions to exci sed leaves of both species showed that the relationship between g(s) a nd [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between g(s) and xylem [ABA]. These results suggest t hat xylem ABA may act as a stress signal in the control of stomatal co nductance.