Dynamic analysis of stomatal responses by an improved method of leaf heat balance

Dynamic stomatal responses to drastic changes in environment were analyzed in an on-line system using an improved heat balance method. The boundary layer resistance (r_AH) of a rough cucumber leaf was much smaller than that of a smooth artificial leaf (an aluminum plate similar in shape to the cucumber leaf). The effect of free convection on r_AH for mixed convection could be described exactly by a model in which the resistance for free convection was connected in parallel with the resistance for forced convection. The intact cucumber leaf was exposed to oscillating air temperatures (T_A) and water vapor densities (W_A) in a growth chamber, and the heat balance Method was used to calculate transpiration rate and leaf conductance using r_AH obtained from the parallel resistance model developed for the real leaf. The calculated transpiration rate agreed well with the value measured by weight change. Oscillations of T_A and W_A with periodicities of 6 and 12 min induced rapid stomatal oscillations which were synchronized with evaporative demand. The rapid stomatal oscillations were attributed to hydraulic interaction between guard and epidermal cells in the stomatal compelx exposed directly to the ambient air. Slower oscillations of T_A and W_A with a periodicity of 24 min perturbed the water balance of the whole leaf and induced slow and strong stomatal oscillations which were not synchronized with evaporative demand. Thus, the improved heat balance method was applicable for analysis of dynamic stomatal responses to the environment.