Rapid and Specific Modulation of Stomatal Conductance by Blue Light in Ivy (Hedera helix) : An Approach to Assess the Stomatal Limitation of Carbon Assimilation.

Low intensity (0.015 millimole per square meter per second) blue light applied to leaves of Hedera helix under a high intensity red light background (0.50 millimole per square meter per second red light) induced a specific stomatal opening response, with rapid kinetics comparable to those previously reported for stomata with ;grass type' morphology. The response of stomatal conductance to blue light showed a transient ;overshoot' behavior at high vapor pressure difference (2.25 +/- 0.15 kiloPascals), but not at low vapor pressure difference (VPD) (0.90 +/- 0.10 kilo-Pascal). The blue light-induced conductance increase was accompanied by an increase in net photosynthetic carbon assimilation, mediated by an increase in the intercellular concentration of carbon dioxide. Values of assimilation once the blue light-stimulated conductance increase reached steady state were less than those at the peak of the overshoot, but the ratios of assimilation to transpiration (A/E) and blue light-stimulated DeltaA/DeltaE were greater during the steady-state response than during the overshoot. These results indicate that significant stomatal limitation of assimilation can occur, but that this limitation may improve water use efficiency under high VPD conditions. Under high intensity red light, the decline in A/E associated with an increase in VPD was minimized when conductance was stimulated by additional low intensity blue light. This effect indicates that the blue light response of stomata may be important in H. helix for the optimization of water use efficiency under natural conditions of high irradiance and VPD.