| It has been reported that there exists a significant difference in the population canopy temperature among different genotypes of wheat under the identical production conditions of climate, soil type and cultivation management. Based on the reality of the prerequisite to apply a new variety in any agro-ecological zone is that its main agronomic traits exceed those elite varieties used in local production and the findings of their research, Zhang songwu et al proposed that wheat genotypes could be divided into 2 types as low canopy temperature genotypes (abbreviated as LTG) and high canopy temperature genotypes (abbreviated as HTG) according to their canopy temperature and using those elite varieties as references, those genotypes of canopy temperature equivalent to or lower than that of references belong to LTG and those genotypes of canopy temperature higher than that of referees belong to HTG.The difference of canopy temperature in different genotypes can be observed in the whole growth period of wheat, but much more clearly during the kernel-plumping period, the key period for wheat to form kernel. As lots of studies showed that the cold or warm canopy temperature is a very important property of wheat genotype, which does not change along with the variation of climate and other production conditions. To investigate the inherent mechanism, some physiological and agronomic traits related to wheat yield-formation were investigated under normal meteorological condition and the results showed that there are more excellent performance were observed in LTG than in elite varieties and much more than in HTG, such as the duration of leaf function, content of leaf chlorophyll, content of leaf protein nitrogen, superoxide dismutase (SOD) activity, catalase (CAT) activity, transpiration rate and net photosynthesis rate. Therefore the property of LTG confers to high yield elite genotypes of wheat.However, in order to apply those findings in wheat breeding and production, it is badly needed to further the findings of those studies and to know that what LTG and HTG mean under drought or other unfavorable conditions. Therefore the main objective of this research is to answer one of those questions, i.e. what do LTG and HTG mean under drought?A series of experiments were conducted for over 4 consecutive wheat growth seasons (1999-2003) in the Agriculture station I, Northwest Sci-Tech University ofAgriculture and Forestry, Yangling, People's Republic of China. LTG and HTG as well as drought-resistant and drought susceptible wheat varieties were used in those experiments. To achieve drought condition, a drought simulating shelter was built, sunshine could go inside and no any rainfall could, all the wheat materials were planted under the shelter and no any water supply was applied after the heading stage of wheat. The population canopy temperature as well as some physiological and agronomic traits was investigated. Those traits were some traits in leaves, such as the duration of leaf function, content of chlorophyll, content of soluble protein, superoxide dismutase (SOD) activity, catalase (CAT) activity, transpiration rate, net photosynthesis rate and air conductive rate of stoma, and other traits as hormone ABA, root-sap amount of bleeding, and accumulation dynamics of dry matter in grain. The characteristics and the inherent mechanism of LTG and HTG wheat materials under drought as well as the relationship between canopy temperature and drought resistance were systematically investigated and the application of those disciplines revealed by this study to wheat breeding and production under drought also was discussed. The results are as followed:1. The canopy temperature of LTG materials is stably lower and that of HTG materials is stably higher under drought, no matter whenever the year and whatever the climate. The drought resistant wheat genotype share similar characteristics with LTG wheat and the drought susceptible genotype share similar characteristics with HTG materials.2. The excellent performance of...
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