The role of nocturnal transpiration in plant growth is debatable, presumably due to the soil moisture-dependent characteristics of transpiration. To address this issue, we analyzed the relationship between nocturnal and diurnal transpiration and plant growth across various soil moisture and atmospheric CO2 levels. We investigated the ratio of nocturnal to diurnal transpiration under eight soil moisture conditions (30 %, 35 %, 40 %, 50 %, 60 %, 80 %, 100 %, and 120 % [w/w]) and three CO2 concentrations (approximately 407, 610, and 843 ppm) in five rice genotypes (cv. Nipponbare, IRAT109, NERICA 1, Taichung 65 [T65], and DV85). The total plant biomass increased linearly with increase in diurnal transpiration but showed a saturated response to increase in nocturnal transpiration, suggesting that nocturnal transpiration did not contribute to growth when the soil water content was above 60 %. CO2 enrichment did not decrease the nocturnal transpiration rate (En) but decreased the diurnal transpiration rate (Ed), thereby causing an increase in the En /Ed value. The En /Ed value was the lowest when the soil water content was approximately 50 %. The graphical curves representing the change in En /Ed values plotted along with the soil water content showed a V-shaped pattern for all genotypes examined. Furthermore, the patterns of variation of En /Ed values with respect to changes in soil moisture contents were similar under different CO2 concentrations. Therefore, we concluded that suppression of nocturnal transpiration in the absence of drought would be an important target for reducing water consumption without compromising biomass pro-duction, and estimation of the En /Ed values will help in selecting individuals showing low nocturnal transpi-ration regardless of the CO2 concentration in the environment.
