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Mechanisms of heat tolerance and Breeding strategies

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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. 

       Table1: Symptoms of heat stress in rice



Growth stage


Threshold temperature(oC)


Symptoms


Emergence


40


Delay and decrease in emergence


Seedling


35


Poor growth in the seedling


Tillering


32


Reduced tillering and height


Booting


----


Decrease number of pollen grains


Anthesis


33.7


Poor anther dehiscence and sterility


Flowering


35


Floret sterility


Grain formation


34


Yield reduction


Grain ripening


29


Reduced grain filling

 
 
File Courtesy: 
Chandan Kapoor, H. Kalita, R. Gopi, A.K. Mohanty and Pradeep Chettri ICAR Research Complex for NEH Region, Sikkim Centre, Tadong Gangtok TRAINING MANUAL ON RICE KNOWLEDGE MANAGEMENT FOR FOOD AND NUTRITIONAL SECURITY (28th Nov. – 04 th Dec., 2013)
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