STOMATAL CONTROL BY BOTH [ABA] IN THE XYLEM SAP AND LEAF WATER STATUS- A TEST OF A MODEL FOR DROUGHTED OR ABA-FED FIELD-GROWN MAIZE

A model of maize stomatal behaviour has been developed, in which stoma tal conductance is linked to the concentration of abscisic acid ([ABA] ) in the xylem sap, with a sensitivity dependent upon the leaf water p otential (PSI(l)). It was tested against two alternative hypotheses, n amely that stomatal sensitivity to xylem [ABA] would be linked to the leaf-to-air vapour pressure difference (VPD), or to the flux of ABA in to the leaf. Stomatal conductance (g(s)) was studied: (1) in field-gro wn plants whose xylem [ABA] and PSI(l) depended on soil water status a nd evaporative demand; (2) in field-grown plants fed with ABA solution s such that xylem [ABA] was artificially raised, thereby decreasing g( s) and increasing PSI(l) and leaf-to-air VPD; and (3) in ABA-fed detac hed leaves exposed to varying evaporative demands, but with a constant and high PSI(l). The same relationships between g(s), xylem [ABA] and PSI(l), showing lower stomatal sensitivity to [ABA] at high PSI(l), a pplied whether variations in xylem [ABA] were due to natural increase or to feeding, and whether variations in PSI(l) were due to changes in evaporative demand or to the increased PSI(l) observed in ABA-fed pla nts. Conversely, neither the leaf-to-air VPD nor the ABA flux into the leaf accounted for the observed changes in stomatal sensitivity to xy lem [ABA]. The model, using parameters calculated from previous field data and the detached-leaf data, was tested against the observations o f both ABA-fed and droughted plants in the field. It accounted with re asonable accuracy for changes in g(s) (r2 ranging from 0.77 to 0.81). These results support the view that modelling of stomatal behaviour re quires consideration of both chemical and hydraulic aspects of root-to -shoot communication.