| In water-limited environments, plant productivity is determined jointly by the amount of available water and the efficiency by which the water is used by the plants. The important way to insure future food needs of the increasing world population should involve a better use of water by the development of crop varieties less demanding of water and more tolerant to drought. Water shortage is the major limiting factor for the sustainable development of agriculture in China, especially in North China. Winter wheat (Triticum aestivum L.) is the major crop in north China plain, which consumes the most water resource. Accounting about 70% water resource was consumed by winter wheat in NRP. Therefore it is urgently to breed high WUE cultivars of winter wheat, so as to save water and increase grain production. It has been reported that WUE is controlled by multi-genes, and regulated by many environmental factors such as light, water and fertilizer etc, it's difficult to choose the best trait of controlling WUE and physiological bases. But, many progress have been made about wheat WUE in recent years, and goes ahead quickly. Still, it's so absence about the physiological mechanism of drought tolerance and gene expression related to water use efficiency.In the study, three winter wheat varieties, Xifeng20 ( dry-field ) , Shi4185 (irrigated-field) and Hanyoul 18 (irrigated-dry-field), were used to investigate the differences of physiological fundamentals after osmotic stress with PEG6000, and the technique of cDNA-AFLP(amplified fragment length polymorphism) was used to isolate genes differently expressed in leaves of drought-stressed and non-stressed seedlings. The main results were as follows:Both the leaves WUE and per seedling WUE among three varieties were different. To the control, the leaves W UE of SW4185 was higher than that Xifeng20. The leaves WUE in Xifeng20 was not changed by drought stress, but it was decreased greatly in Shi4185 and Hanyoul 18 after 36h stress. However, short time stress (12h) didn't affect the WUE in Hanyoul 18. The reason is that the photosynthetic rate and the leaves WUE in irrigated-field variety SM4185 was higher than that in dry-field variety Xifeng20 under nowater stress condition, but the evaporation rate and stomatal conductivity were lower than that in Xifeng20. When subjected to osmotic stress, the photosynthetic rate and evaporation rate were all decreased greatly in both Shi4185 and Hanyoull8 than that in Xifeng20. Thus resulted the lower WUE in SW4185 at 36h after stress. The Xifeng20 kept higher leaves WUE when subjected to osmotic stress.The water lose rate of isolated leaves of different varieties were different, it's higher in dry-field Xifeng20 than that in irrigated-fleld and irrigated-dry-field variety before 24h after stress;The osmotic potentials between roots and leaves were different before stress, and all decreased from Oh to 36h after osmotic stress as a whole, but the regulation were different between varieties.The method of drought resistance was difference among varieties in which the WUE were different: at well water, the irrigated-field variety had higher production contributing to higher WUE, the dry-field variety absorbed limit water by evaporation pull;When subjected to drought stress, the irrigated-field variety protects themselves from hurt in maximum of stomatal close, but the dry-field variety strive to absorb little water, so as to maintain alive by higher evaporation rate. Comparatively, The irrigated-field variety had stronger resistance to drought, but the dry-field variety had better endurance to drought. From the changes of physiology traits, it could be calculated that irrigated-dry-field variety Hanyoul 18 was similar to irrigated-field variety Shi4185.The results of cDNA-AFLP showed that gene expression were different among three varieties under osmotic stress. From 180 primer combinations, 453 specific bands were screened. 12 cDNA fragments were cloned, and subjected to sequence analysis. Database searches indicated that 7 cDNA fragments were high homological to known genes, which mainly included proteins related to transposon which was included in No. 11 chromosome in rice, hydrolase, aspartate aminotransferase and glycosyl transferase etc. They participated in many processes of growth and development, metabolism regulation and abiotic stress (such as disease, salt and drought ect) respectively. The 2 cDNA were homological to known genes isolated from wheat, but the function was unknown. The other 3 cDNA were new genes which showed no detectable homology to known genes and registered in GeneBank. RT-PCR results indicated that the fragment of No. 11 and No.85 were expressed in root after osmotic stress too. The results of 5'-RACE and 3'-RACE in agarose gel appeared about 700bp in 5'-end of two fragments, the 3'-end was small.
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