Diurnal and seasonal variation in bulk stomatal conductance of the rice canopy and its dependence on developmental stage

Bulk stomatal conductance (gS) is an important factor that expresses the effect of stomatal movements on water transfer between the plant and atmosphere at the canopy scale and is widely used as a parameter in many micrometeorological models. Diurnal and seasonal variations in gS of the rice canopy were determined using a heat transfer model based on heat flux measurements in irrigated rice fields. Season-long observations from transplanting to maturation of rice plants were conducted to obtain heat flux data in a humid temperate climate at three experimental sites with widely differing cropping seasons in Japan. A double source model was used as the heat transfer model to calculate gS. Seasonal variations in heat fluxes differed for sensible heat and latent heat. Sensible heat flux was smaller and relatively constant within the range −50 to 50 W m−2, whereas latent heat flux showed large variations from 0 to 250 W m−2 throughout the growth period. It was suggested that this would be a common pattern for paddy rice fields in all cropping seasons. Diurnal variation in gs showed a common trend in all growth periods with lower values in the morning and evening, and higher values during the midday because of its dependence on solar radiation. The relationship between absorbed solar radiation (Sabs) and gs was determined using a Jarvis-type model for each growth period. Maximum values of bulk stomatal conductance (gSmax) for saturated Sabs rapidly decreased from 0.06 to 0.02 m s−1 between the active tillering and panicle formation stages, and moderately decreased from 0.02 to 0.01 m s−1 during the ripening stage. This was considered to be due to the change in leaf chlorophyll concentration. Seasonal variation in gSmax can be commonly expressed for all cropping seasons using the function of developmental stage (PS). Using this function, the gS value can be obtained easily at a given developmental stage, which makes it possible to use micrometeorological models in relation to rice phenological development for evaluating important factors, such as water temperature and transpiration, that affect rice production.