Responses of leaf photosynthesis and plant water status in rice to low water temperature at different growth stages

Effects of low water temperature (T-w) on the rate of light-saturated leaf photosynthesis (P-max), plant water content and sap bleeding rate of rice (oryza sativa L.) were examined from field trials for 4 years. Cool irrigation water was applied at three temperature levels (16-25 degreesC in daily average and 20-30 degreesC in daily maximum) for 20-34 days firstly during the vegetative and secondly during the reproductive growth periods. P-max during the reproductive growth period had a significant positive correlation with T-w at the time of measurements in 3 of 4 years, where a 1degreesC decrease of T-w from 25 degreesC decreased P-max by 1.8-2.4%. On the other hand, no significant relation between P-max and T-w was observed during the vegetative growth period. Stomatal conductance (g(s)) followed a similar trend to that of P-max. Plant water content and sap bleeding rate were reduced by low T-w for the first 5-12 days after start of treatment, but the effects of low T-w became progressively smaller. Chlorophyll meter values (SPAD values) taken with the Minolta SPAD-502 chlorophyll meter, were significantly higher under low T-w than those under high T-w in the second half of each treatment period. Multiple regression analysis revealed that the negative effect of 9 decreased g(s) on P-max, caused by low T-w was alleviated by factors associated with increased SPAD values. These results supported our previous finding that radiation use efficiency was less affected by low T-w than canopy radiation capture [Field Crops Res. 73 (2002) 67]. Although complex interactions are involved, the limited responses Of P-max to the long-term exposure to low T-w also suggest that detailed physiological mechanisms that determine P-max can be simplified in a crop growth model that evaluates the effects of T-w on rice growth and yield under cool climates. (C) 2004 Elsevier B.V. All rights reserved.