Effects of silicon on transpiration and leaf conductance in rice plants (Oryza saliva L.)

To characterize the protection effect of silicon against excessive transpiration in rice plants (Oryza saliva L.), we investigated the factors responsible for transpiration, such as cuticular conductance (gc), leaf surface wax content (WC), and stomatal movement in response to environmental stimuli, using leaves of plants grown in nutrient solution with silicon (+Si) and without silicon (-Si). The rate of transpiration of whole plants (E) under field conditions was clearly reduced by application of silica. The gc of -Si leaves under precisely regulated conditions was 33% higher than that of +Si leaves, reflecting higher adaxial gc. Silicon has little effect on WC, a factor responsible for regulation of cuticular resistance to water flux in rice leaves. These results suggest that silicon is one of the main factors responsible for regulation of gc, but that its effect was not due to an increase in the WG. Leaf conductance (gl) of -Si leaves tended to decrease more slowly than that of +Si leaves over time after changes from high to low light intensity. Moreover, it was always higher over the entire duration of the experiment, irrespective of irradiation intensity. The gl of -Si leaves was higher at lower relative humidity (RH) and the initial response to a subsequent increase in RH was slower than that of +Si leaves. A comparison of gl and gc between +Si and -Si leaves showed that the higher gl of -Si leaves was due to higher stomatal conductance. These results indicated that the reduction in E by application of silica was mainly attributable to reduction in the rate of transpiration through stomatal pores.