High Day and Night Temperature Tolerance in Rice.

Rice combines true tolerance with some form of escape to withstand high temperature stress during critical developmental stages. A series of experiments on rice physiological responses to different combinations of ambient (30/24 °C day/night), high day temperature (HDT; 35 °C) and high night temperatures (HNT; 30 and 35 °C) applied for between 4 – 24 days beginning at microsporogenesis or anthesis were carried out using ten diverse rice cultivars. HDT and HNT of 35 °C reduced spikelet fertility to less than 20% in most of the cultivars except in an Oryza glaberrima (CG14) and an O. sativa aus (N22). The tolerance of N22 and CG14 were in part due to their having high optimum and maximum temperatures for pollen germination and tube growth, shorter duration of anthesis (in N22) and a relatively high flag leaf silicon content, a trait that is positively correlated with fertility (r=0.52; P<0.01). N22 also employed a form of heat escape by flowering 2 h earlier in the day after exposure to HNT. At HNT, N22 spikelet temperature was about 2 °C cooler than ambient and gas exchange data showed that changes in night-time flag leaf respiration, conductance and transpiration were not significant (P>0.05) whereas in the heat susceptible cultivar, WAB56-104, HNT increased gas exchange by 2 - 4 fold. In general, HNT during the second half of the night (0000-0530 h) had a more pronounced effect on rice than during the first half (1830-0000 h). Interestingly, cultivars with HDT tolerance had a relatively similar degree of HNT tolerance, suggesting these traits may be linked. Tolerance to heat stress in rice is a result of varied physiological adaptive mechanisms. Some of these physiological strategies will be discussed in the light of the impact of increasing day and night-time temperatures on rice yield and world food security.