The influence of predawn leaf water potential on stomatal responses to atmospheric water content at constant C-i and on stem hydraulic conductance and foliar ABA concentrations

Stomatal and photosynthetic responses to increasing leaf-to-air vapour pres sure difference (V) were investigated in watered and droughted Eucalyptus t etrodonta when either ambient CO2 (C-a) or internal CO2 concentration (C-i) were constant, Stem hydraulic conductance and xylem and foliar abscisic ac id levels were measured periodically during the drought period. As V increased, stomatal conductance (g(s)) declined. Maintaining C-i did n ot affect the response of g(s) to V or predawn leaf water potential (psi(pd )). In fully watered plants the decline in g(s) was insufficient to prevent increased transpiration rates (E-t) with increasing V, In contrast, in dro ughted plants, stomatal closure was sufficient to prevent increasing E-t. S tomatal sensitivity to increasing V was increased by drought. As drought de veloped, the three phases of stomatal responses to V progressively collapse d to one where feedforward mechanisms result in decreasing E-t with increas ing V. Thus as drought developed the feedforward response of stomata to V e ntirely dominated. Net photosynthesis (P-n) and g(s) responded in unison to changes in V and p si(pd). The decline in P-n as V increased, despite C-i being maintained con stant, was observed in fully watered plants and in severely droughted plant s. P-n was most dependent on g(s) at large values of V and in droughted pla nts. As drought progressed and psi(pd) declined, stem hydraulic conductance decr eased and foliar ABA concentrations increased. The decline in maximum g(s) was correlated with foliar ABA levels. It is concluded here that hydraulic signals, be they atmospheric water or soil water in origin, and possibly al so chemical signals regulate g(s), which in turn can limit assimilation rat es in seasonally dry savannas.