Canopy temperature on clear and cloudy days can be used to estimate varietal differences in stomatal conductance in rice

Leaf photosynthetic potential, a major determinant of yield potential in rice, is mostly explained by stomatal conductance (gs) as well as leaf N content (Rubisco protein content). Therefore, the rapid and precise evaluation of gs as well as leaf N content would be a valuable addition to rice breeding programs aimed at improved yield potential. We established a simple method based on infrared thermography to estimate varietal differences in gs. Seven rice varieties were cultivated in two-row plots in 2007, and canopy temperatures in three varieties, including a control variety, Koshihikari, were simultaneously measured to determine canopy temperature difference (CTd) between Koshihikari and the other varieties on clear days. The varieties Takanari, Habataki, and TUAT1-5-6a displayed significantly (P < 0.05) lower leaf temperature and higher gs and leaf photosynthetic rate (Pn) than Koshihikari. CTd was closely correlated with gs and Pn. These results indicate that CTd may be useful as a relative index to estimate varietal differences in gs. A significant CTd was still observed even under cloudy conditions in 2008. Takanari and TUAT1-5-6a had lower leaf temperature than Koshihikari on cloudy days as well as on clear days. Subsequent investigation of photosynthetic light response curves revealed that Takanari and TUAT1-5-6a had higher gs and Pn than Koshihikari under lower irradiance. These results suggest that infrared thermography may be a simple method of evaluating varietal differences in gs through CTd, and that it may be practicable even under cloudy conditions.