Effect of high temperature on rice growth, yield & grain quality
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DESCRIPTIONEffect of high temperature on rice growth, spikelet sterility, yield & grain quality
- 1. Effect of High Temperature on RiceGrowth, Yield & Grain QualityByMEDIDA SUNIL KUMAR
2. Climate Change & Rice Rice is a globally important cereal crop and as a primary source of food accountsfor 35-75% of calories intake of more than 3 billion humans. Climate change will cause tremendous damage to rice production sector if notaddressed properly. Low rice supply along with increasing demand not onlyaffects food security but also the economy of the country. Reduce rice production in a sizable portion Drought affects all stages of rice growth and development. The strong effects ofdrought on grain yield are largely due to the reduction of spikelet fertility andpanicle exertion. Climate change may also affect weed ecology, the evolution of weed speciesover time, and the competitiveness of C3 vs C4 weed species. 3. Cardinal Temperatures of rice Critical low temperature -15oC Optimum Temperature -20-30oC Critical high temperature - 45oC 4. Role of Temperature on Growth & Development 5. Table: Effect of temperature on germination percentageTemperature Period Germination percent0-5o C >30 days 9025o C 6 days 9031-36 o C 2 days 90 6. Seedling growth Growth rate linearly increases between 22 & 31OC Temperatures of of 22oC or below are consideredsubnormal for seedling growth. Seedling growth may be reasonably good up to 35oC,above which declines sharply. Seedlings will die above 40oC. Optimum temperature for cell division of the radicle is25oC and cell enlargement is 30oC. Elongation of radicle stops below 15oC and above 40oC 7. Leaf emergence Temperature is a principle environmental determinant ofleaf appearance in rice. As per the temperature summation index thedevelopment of one leaf requires about 100 degree daysbefore initiation of panicle primordia. 8. Tillering High temperature increases tillers At 3-5 weeks after sowing temperature onlyslightly affected the tillering rate and RGR exceptlowest temperatures (Below 22oC). Higher temperatures may effect the synchronismand in the mobilization of assimilates andnutrients among he tillers 9. The optimum temperature for the normal development ofrice ranges from 27 to 32 C. A temperature increase of 1 C shortened the number ofdays from sowing to heading by 45 days for somegenotypes (Nakagawa et al 2001). Flowering and to a lesser extent the preceding stagebooting are considered to be the stages of developmentmost susceptible to temperature in rice (Satake &Yoshida 1978; Farrell et al. 2006) 10. HIGH TEMPERATURE AND SPIKELET STERILITY Temperatures higher than the optimum induced floret sterility and thus decreased riceyield (Nakagawa et al. 2003). Spikelet sterility was greatly increased at temperatureshigher than 35 C (Matsui et al 1997). In greenhouse experiments with both indica and japonica genotypes, Jagadish et al (2007)found that less than 1 h of exposure to temperatures above 337 C was sufficient toinduce sterility. Enhanced O2 levels may further aggravate this problem, possibly because of reducedtranspirational cooling (Matsui et al 1997a). Exposure to 41 C for 4 h at flowering caused irreversible damage and plants becamecompletely sterile (IRRI 1976) whereas this high temperature (41 C) had no effect onspikelet fertility at 1 day before or after flowering (Yoshida et al 1981). There is genotypic variation in spikelet sterility at high temperature in both the sub-speciesof O. sativa, indica and japonica (Matsui et al. 2001; Prasad et al. 2006), whichcan be defined by different temperature thresholds (Nakagawa et al 2003). However, in general, indica is more tolerant to higher temperatures than japonica (Satake& Yoshida 1978). 11. Mechanism of heat-induced floret sterility Male reproductive system in rice is known to be more sensitive to heatstress (Wassmann et al. 2009a). Prasad et al. (2006) reported that hightemperature stress during rice flowering led to decreased pollenproduction and pollen shed. The probable reasons were the inhibition of swelling of pollen grains,indehiscence of anthers and poor release of pollen grains (Matsui et al.2000, 2005), and thus fewer numbers of pollen grains were available to beintercepted by the stigma. This swelling of pollen grains in the locules is the driving force for antherdehiscence (Matsui et al. 1999) Exposure of pollen grains to high temperature resulted in a loss of pollenviability within 10 min (Song et al. 2001) The stigma is less sensitive to heat than the anther and pollination of the Stigma with unstressed pollen generally restores the spikelet fertility(Yoshida et al.1981). 12. Albinism is the absence of chlorphyll producing a white spikelet or panicle underheat stress Significant genotypic variations in high temperature induced floret sterility exist. The decrease in spikelet fertility can be termed a phenotypic character of rice plantunder high temperature, while the decrease in pollen germination and activity can beconsidered as the physiological factors responsible for this decrease (Tang et al.2008). Exogenous application of growth regulators has been shown recently to have somepositive effects on the spikelet fertility and pollination (Mohammed & Tarpley 2009). In fact, their exogenous application increased the level of endogenous antioxidantsand thus prevented the oxidative damage to the membranes in rice (Mohammed &Tarpley 2009). 13. Role of humidity in spikelet sterility at high temperature Higher relative humidity (RH) at the flowering stage underincreased temperature affects spikelet fertility negatively (Yan et al.2010). Relative humidity of 8590 % at the heading stage induced almostcomplete grain sterility in rice at a day / night temperature of 35/30C (Abeysiriwardena et al.2002). The temperature inside the spikelet decreases with a reduction inRH, possibly due to the enhancement of transpiration at low RH(Weerakoon et al.2008). 14. Table: Symptoms of heat stress in rice plantsGrowthThresholdstagetemperature(OC)Symptoms ReferencesEmergence 40 Delay and decrease inemergenceYashida (1978),Akman (2009)Seedling 35 Poor growth of the seedling Yashida (1981)Tillering 32 Reduced tillering andheightYashida (1978)Booting - Decresed no of pollengrainsShimazaki et al (1964)Anthesis 33.7 Poor anther dehiscence andsterilityJagadish et al 2007Flowering 35 Floret sterility Satake & Yashida(1978)Grain 34 Yield Morita et al 2004Ripening 29 Reduced grain filling Yashida (1981) 15. PHYSIOLOGICAL AND BIOCHEMICAL MECHANISMS OFHEAT TOLERANCE Heat tolerance is generally defined as the ability of the plant to grow andproduce economic yield under high temperature (Wahid et al. 2007). True heat tolerance at sensitive stages might be conferred by protectingstructural proteins, enzymes and membranes from heat damage. High temperatures generally induce the expression of HSPs and suppress, atleast in part, the synthesis of normal cellular proteins. These HSPs can help in coping with heat stress by improving photosynthesis,partitioning of assimilate, nutrient and water use efficiency and membranethermal stability (Wahid et al. 2007). Disruption of some plant growth hormones such as ethylene, salicylic acid,abscisic acid, calcium and hydrogen peroxide through mutation affected thethermo-tolerance capability of the plants, although the levels of accumulatedHSPs did not vary from their wild types in these mutated plants (Larkindale etal.2005). Another prominent alteration is the modification of antioxidant enzymes, andalteration of membrane composition and structure (Stone 2001). 16. MITIGATING STRATEGIES FOR THE FORTHCOMINGWARMER CLIMATE: Replacement of the sensitive cultivars by tolerant cultivars in the fields willincrease the global rice production. From the management point of view, it is essential to adjust the duration ofvarieties such that they avoid peak stress periods. And for this purpose, thedevelopment of varieties Site-specific adjustments in cropping systems may be needed because the effectof these changes in climatic factors varies with the region. Management practices such as saturated soil culture (SSC), alternate wetting anddrying (AWD) and aerobic rice cultivation in the case of drought stress, whilegrowing improved varieties containing thesub1 gene in flooded soils, offer someadaptive options for the indirect stresses related to temperature increase. The use of some growth regulators like MeJA (for advancing theflowering timeto early morning), salicylic acid and glycine betaine (for increasing the percentpollen germination and spikelet fertility) will be also useful in mitigating theyield reduction threats.