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

Abstract

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 °C in daily average and 20–30 °C 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 1 °C decrease of T w from 25 °C 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 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.