Cultivation methods
The agronomic countermeasures against high temperature damage include shifting cropping seasons, improving crop, soil, and nutrient management.
Escaping from high temperature from anthesis and early ripening stage by moving transplanting date is the first major way in cropping system. The most sensitive period to high temperature is first 20 days after heading for chalky grains. Lowering expected temperature in this critical period by delaying of transplanting date has been successful to improve grain quality to some extent.
Low N status magnifies the deterioration of grain quality, especially enhancing white-backed and white-base grains. Application rate of N has been decreasing since 1980s because of consumer’s preference for low protein content in the grain.
The low N application rate, in turn, induces the increased sensitivity to high temperature. Maintaining appropriate N status during ripening is important to maintain grain quality. Maintaining favorable soil conditions for root functions to ensure photosynthesis is important in soil management. Photosynthesis may be suppressed by high temperature due to reduced stomatal conductance, possibly related to the limitation of water uptake by root (Tsuno and Yamaguchi 1987).
High soil temperature suppresses root growth (San-oh and Kondo 2006).
Lowering soil temperature by cold water irrigation is effective to reduce fissured grain with the condition that water availability is not limited. The early drainage before harvest induces water stress that enhances deterioration of grain quality.
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Varietal improvement in grain quality
Genotypic variations have been reported in sensitivities to high temperature in chalkiness and grain size. Genetic analyses for white-based and white-backed grains have been performed using japonica genotypes and several candidate QTLs were identified (Kobayashi et al. 2007, Tabata et al. 2007). Identification of the physiological functions of these QTLs is now under way. The genotypic differences in susceptibility to milky-white grains and grain size are less clear at present. Putative physiological and morphological traits related to the milky-white grain and grain size include stable sink ability under various temperature ranges, high source ability, low panicle temperature, and morphologies of the panicle and grain.
Future research
In physiological aspects, detailed understanding is required to clarify key carbon metabolisms which confer stable grain development under high temperature and low radiation. Genotypic improvement in adaptability to high temperature in spikelet formation, sterility and grain filling under are desired using wider germplasms. Limited information is available in genetic diversity in optimum temperature in grain filling and photosynthesis. Understanding the genetic factor and agronomic measure to control stomatal conductance would be useful because it determines canopy temperature and also affect water use. Increasing temperature is anticipated to have not only negative impact, but also positive aspects such as expanding crop growing spell. Development of crop production system which maximizes beneficial effect of global warming should be also considered.
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International Symposium (2009)
Rice Research in the Era of Global Warming 10~16