The relationship between water uptake and transpiration rates in rice plants.

Abstract

It was clarified in the previous papers that leaf water potential, stomatal aperture and photosynthetic rate in rice plants decreased with increase in solar radiation and vapor pressure deficit even though rice plants grew under the condition where sufficient water was supplied to roots in submerged paddy field, and that the decrease in leaf water potential, stomatal aperture and photosynthetic rate were more remarkable in rice plants with low root activity or with poor root system. The present study was conducted to investigate the relationship between water uptake and transpiration rates and the effects of this relation on leaf water potential and stomatal aperture through their diurnal changes, and to discuss the characteristics for maintaining water balance in rice plants. Transpiration rate was higher than water uptake rate in the morning when transpiration was increasing rapidly with rapid increase in solar radiation and vapor pressure deficit. Both rates were practically the same in the midday and then transpiration rate was lower than water uptake rate in the evening when transpiration rate was decreasing rapidly with rapid decrease in solar radiation and vapor pressure deficit (Fig. 2 A). The difference between water uptake and transpiration rates was very small even when transpiration rate was changing rapidly (Fig. 2 B). In case of reduced water uptake due to low water potential of culture solution or NaN3 treatment to roots, transpiration rate decreased remarkably due to increase of stomatal closure in the daytime (Figs. 3 and 4). Therefore, the difference between water uptake and transpiration rates did not increase so much and leaf water potential decreased a very little even in the midday with high transpiration demand compared with decrease of water uptake rate and stomatal aperture (Figs. 3 and 4). These results suggested that water balance in rice plants was maintained by the process as follows: There is too much transpiration in the daytime with high solar radiation and valor pressure deficit, so water uptake could not overtake the transpiration, and leaf water potential decreased to a certain extent. As stomata in rice plants were very sensitive to change of leaf water potential compared with those of other plants, stomata closed very rapidly with response to the decrease of water potential, so that transpiration rate decreased to almost the same as water uptake. Therefore, the difference between water uptake and transpiration rates was very small, and decrease of leaf water potential was prevented. Futhermore, in case of rice plants with reduced water uptake due to low water potential of culture solution or NaN3 treatment to roots, all were the same as in the process of maintaining water balance. From these results and the high correlation between stomatal aperture and photosynthetic rate, it was considered that water uptake ability directly affected photosynthetic rate under sufficient solar radiation in rice plants with stomata responding very sensitively to change of leaf water potential. Futhermore, it was suggested that rapid wilting often observed in rice, soybean and cucumber under very large vapor pressure deficit or on water saturated soil in rainy season, baiu, could arise from both decrease of water uptake ablility and loss of sensitivity of stomata to decrease of leaf water potential.