Most of the rice is currently grown in those areas where the current temperatures are close to optimum. By the end of 21st century the rice yields have been estimated to be reduced by 41% (Ceccarelli et al., 2010). It has been evidenced that increase in night temperature has been the main cause of increase in global mean temperature resulting in decrease yields (Peng et al., 2004 and Sheehy et al., 2005).
Effect of high temperature on rice plant
The optimum temperature for the normal rice development ranges from 27 to 32oC (Yin et al., 1996). Flowering and the booting stage in rice is considered to be most susceptible to temperature. Temperature higher than the optimum induced floret sterility and decreased rice yield (Nakagawa et al., 2003). High temperature causes floret sterility and decreased ability of pollen grains to swell, resulting in poor pollen dehiscence. Temperature increase of 1oC shortened the number of days from sowing to heading in some genotypes. The symptoms of heat stress in rice has been shown in Table 1.
Mechanisms of heat tolerance and Breeding strategies
Heat tolerance is defined as the ability of the plant to grow and produce economic yield under high temperature (Wahid et al., 2007). Tolerance to heat comprises of escape or avoidance mechanisms like timing of panicle emergence, spikelet opening during stress period, anther dehiscence. Heat shock proteins are considered to be the stabilizing factors conferring tolerance to heat thereby protection of structural proteins, enzymes and membranes from heat damage is crucial in temperature tolerance. (Maestri et al., 2002).
Plant architecture is an important trait for tolerance to temperature stress. As an example in some genotypes the panicle is surrounded by many leaves the plant will be able to withstand high temperature stress due to increased transpirational cooling and prevention of evaporation from anther due to shading of leaves. The early morning flowering of rice plant is a useful phenomenon imparting heat tolerance to rice plant. These traits can easily be used in breeding programmes as it inherits in simple manner (Yoshida, 1981). Genotypes of Oryza glaberrima flower much earlier in the day with more than 90% of the spikelets reaching anthesis by 09.00 h. (Prasad et al., 2006). This trait can be used for introgression in O. sativa genotypes. It has been reported that cultivars with large anthers are tolerant to high temperatures at the flowering stage. (Matsui and Omasa, 2002).
For high temperature tolerance, traits such as spikelet fertility can be used as a screening tool during the reproductive stage. Selection of heat tolerance should be done for those materials which can tolerate temperatures higher than 38oC. Cultivars such as N22 has already been identified as high-temperature tolerant so these material can be used in breeding programmes as donors. Genetic modification of the male reproductive organs should be emphasized as it is more sensitive to high temperature. Candidates genes can be identified using QTL mapping, by studying the association of the phenotype and its associated markers. Identification and breeding of heat tolerant germplasm should be carried out for exploiting variation both in genotypic and morphological characters.