The driving force of plant transpiration is the concentration difference between leaf and air water vapor, usually expressed as leaf-to-air water vapor pressure deficit (VPDL). To regulate their water balance, plants restrict the stomatal conductance when air VPD is high, thereby regulating water loss. In our experiment, we were interested in stomatal response to changes in VPD. We observed the physiological response of plants to stepwise VPD changes at the leaf level and plant transpiration at different VPDs at the plant level of the two maize hybrids (drought tolerant, control) in the pot experiment. The experiment was conducted in two periods at the plant level and considered also different levels of substrate moisture. At the leaf level, stomatal conductance decreased more or less linear with increasing VPD in both hybrids. Although the tolerant hybrid had lower initial stomatal conductance (at 1 kPa) than the control hybrid, it began to limit the stomatal conductance more rapidly than control, at a lower VPD. The tolerant hybrid also had lower transpiration, due to the lower stomatal conductivity. On the other hand there were no differences in net photosynthesis between the hybrids. As a result the tolerant hybrid had higher water use efficiency and intrinsic water use efficiency. Transpiration at the plant level increased significantly with VPD and differed between substrate moisture levels in both periods of experiment. The tolerant hybrid is responding rapidly to environmental water changes even at lower VPD levels, implying that it uses water more efficiently even when it is relatively aboundant in the environment. We have gained insight into the dynamics of the stomatal response to changes in vapor pressure deficit in different maize hybrids and demonstrated the importance of this response for maize drought tolerance.
|
