Stomatal response to wind on abaxial and adaxial surfaces of cucumber leaf under different humidity conditions

In order to elucidate stomatal response to wind, leaf gas exchange and stomatal conductance in an intact leaf of cucumber plants (Cucumis sativus L.) were measured under different conditions of air current and humidity. A leaf gas exchange cuvette was improved for individual measurements on the abaxial and adaxial leaf surfaces and for adjustment of the leaf boundary air current, where leaf boundary layer conductance can be adjusted at a desired value within a range from 0.3 mol m^-2s^-1 to 1.0 mol m^-2s^-1. Rates of leaf gas exchange and stomatal conductance were higher on the abaxial surface than on the adaxial surface. Stomatal response to wind was appeared more sensitive on the abaxial surface, while leaf gas exchange and stomatal conductance on the adaxial surface was scarcely affected by wind. Under the humid air condition, increase in leaf boundary layer conductance significantly enhanced stomatal conductance as well as rates of transpiration and photosynthesis on the abaxial surface. On the other hand, under the dry air condition, stomatal conductance was depressed by increase in leaf boundary layer conductance with little effect on transpiration and photosynthesis. These opposite changes in stomatal conductance found under the humid and dry air conditions were proved visually as change in stomatal aperture by the microscopic observation of the abaxial leaf surface. This stomatal response to wind found on the abaxial surface was not related to feedback stomatal response to transpirational water loss and intercellular CO₂ concentration in the leaf. The mechanism of stomatal response to wind was explained on the basis of water balance within the stomatal complex and on the antagonism relationship between turgors in guard and epidermal cells.