Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO2 under fully open‐air conditions

Understanding of leaf stomatal responses to the atmospheric CO₂ concentration, CO₂], is essential for accurate prediction of plant water use under future climates. However, limited information is available for the diurnal and seasonal changes in stomatal conductance (g_s) under elevated CO₂]. We examined the factors responsible for variations in g_s under elevated CO₂] with three rice cultivars grown in an open‐field environment under flooded conditions during two growing seasons (a total of 2140 individual measurements). Conductance of all cultivars was generally higher in the morning and around noon than in the afternoon, and elevated CO₂] decreased g_s by up to 64% over the 2 years (significantly on 26 out of 38 measurement days), with a mean g_s decrease of 23%. We plotted the g_s variations against three parameters from the Ball‐Berry model and two revised versions of the model, and all parameters explained the g_s variations well at each CO₂] in the morning and around noon (R² > 0.68), but could not explain these variations in the afternoon (R² < 0.33). The present results provide an important basis for modelling future water use in rice production.