| As one of the most important staple crops, wheat mainly planted in the North of China, where water shortage is a main environmental factor that constrains wheat production. Improving of wheat yield and water use efficiency is a great challenge and a hotspot area for the Chinese agricultural researches. In addition to the carbohydrate assimilation, crop yield is also related assimilates translocation and partition to grain. Study on the regulatory mechanisms of assimilates translocation could prove physiological basis on the high yielding and water use efficiency of wheat in the future dryland farming. In this paper, two winter wheat (Triticum aestivum. L) varieties, i.e. Changwu 134 (drought-resistant) and Shaan 253 (drought-sensitive), were cultivated in the field plots under a rain-proof shelter during a period from October, 2007 to June, 2008 at institute of soil and water conservation, YangLing. Two different water treatments were applied after greening and jointing stages, including optimal soil water treatment (the soil water content was 70 % - 80 % of the field water capacity) and soil drought treatment (the soil water content was 45 % - 55 % of the field water capacity). The post-anthesis dry matter accumulation and remobilization in vegetative organs were investigated. The photosynthetic characteristics and antioxidase activities of flag leaves, the carbohydrate metabolism and related enzymes'activities of leaf, stem, leaf sheath and grain were determined. Grain yield, harvest index and water use efficiency were also measured and calculated. Main results are as follows:1 Drought stress reduced grain weight of Changwu 134 and Shaan253 by 7.84 % and 23.36 % at 35 days after anthesis in the main spike. Drought stress significantly decreased the dry matter accumulation amount in the vegetative organs. Water stress no affect on the beginning time at which dry matter was remobilized from leaf, stem and leaf sheath, whereas increase the remobilization velocity during the late grain-filling in Changwu 134. In contrast, in Shaan 253, the commence of dry matter remobilization in leaf and leaf sheath was earlier in drought treatment than CK, while the commence of dry matter remobilization in stem internodes was not significantly affected by drought stress. A significant increase in remobilization rate was found in middle and late grain filling period. Under soil drought conditions, the remobilization rate of pre-anthesis C-reserves has been increased in Shaan 253 that grain weight was more dependent on the pre-anthesis reserves. While the grain weight of Changwu 134 was mainly related to the post-anthesis accumulation of photosynthate, which was particularly important for grain yield formation. The order of contribution rate of different vegetative organs to grain yield was: stem, leaf and leaf sheath. The effect of reserves accumulation changes in same vegetative organ of wheat on grain weight closely related to the distance to spike, and the farther away from ear the more effect of dry matter accumulation and remobilization on grain weight.2 Soil drought stress resulted in decrease less in the post-anthesis photosynthetic active duration and relative steady phase of chlorophyll content in flag leaf of Changwu 134 than those of Shaan 253. In Changwu 134, Soil drought stress significantly increased the activities of SOD, CAT in the early and middle periods of grain filling as well as the activities of POD in the early period of grain filling, and increased slightly the MDA content. In Shaan 253, because of the discordance in active oxygen scavenging system, an increase in POD activity could involve in the produce of active oxygen and MDA content of flag leaf had a sharp rise in post-anthesis period under the soil drought treatment. Soil drought promoted an increase of soluble protein to resist water stress in flag leaf of Changwu 134, whereas under similar conditions, Shaan 253 couldn't maintain a normal soluble protein concentration, which resulted in a vulnerable resistance to water stress. It is therefore, that under drought conditions, stronger ROS scavenging capacity of antioxidant enzyme system could lighten membrance lipid peroxdation, which maintain higher photosynthate supply ability and longer supply duration during the middle and later periods of grain filling in wheat.3 Soil drought stress led to a more significant increase of total soluble sugar and sucrose content in flag leaf of Changwu 134 than those of Shaan 253 in the early stage of grain filling. High sucrose supply ability of flag leaf was found in Changwu 134 in middle and late grain filling period, while this ability was decreased in Shaan 253. The sucrose phosphate synthase (SPS) in flag leaf was the key enzyme determining the sucrose synthesis for Changwu 134. For Shaan 253, the increase of sucrose content in flag leaf resulted from the joint action of SPS and sucrose synthase (SS), with SS playing a greater role. Soil drought stress increased the total soluble sugar and sucrose content of flag leaf sheath in two wheat cultivars. Drought delay the time of carbohydrate remobilization in flag sheath of Changwu 134 but that was promoted in Shaan 253. The sucrose content of flag leaf sheath depends mainly on the change of SPS and SS activity. The accumulation of sucrose is resulted from hindering translocation during terminal grain filling in flag leaf sheath.4 Drought stress was benefit for increasing the total soluble sugar content of wheat during grain filling stage. At the middle and late grain filling stage, the export time of the total soluble sugar that reserved in stems remained unchanged but the export velocity increased. The changes in total soluble sugar content and remobilization in stems affected by soil drought were closely related to the changes of fructan and sucrose contents. Soil drought promoted the accumulation of fructan and sucrose in the early stage of grain-filling, which was related to the lower activity of fructan external hydrolase (FEH) and to the increase activity of SPS, SS that was induced by drought at the early grain filling stage. In Changwu 134, drought induced a significant increase of FEH activity at the grain filling stage, Comparately at the middle and late grain filling stage, great increased in the activity of FEH was observed in Shaan 253. The FEH activity of Shan 253 under stress was significantly less than that of CK at the terminal grain filling stage, which didn't resulted in the accumulation of fructan in stems. Changwu 134 was found to have a higher sucrose content and higher SPS and SS activities than CK at the middle and late grain filling stage. For Shaan 253 under drought treatment compared with CK, sucrose content was lower at the late grain filling stage but a slightly higher at the terminal grain filling stage; greater SPS and SS activity at the middle and late grain filling stage and lower SPS and SS activity at the terminal grain filling stage were observed. So, under drought condition, an enhancement of fructan decomposition could guarantee the sucrose synthesis in stems. Drought affected the remobilization of stem reserves to grain mainly by regulating SPS and SS activity at the middle and late grain filling stage. It was the retardation of sucrose translocation that directly resulted in the accumulation of sucrose in internodes at the mature stage.5 Soil drought stress had larger affect on the grain starch accumulation of Changwu 134, but not on that of Shaan 253. Soil drought promoted grain starch accumulation in the early stage of grain filling, which was related to increase of some enzymes activities, such as SS, adenosine diphosphate glucose pyrophosphorylase (AGPP), uridine diphosphate glucose pyrophosphorylase (UGPP), soluble starch synthase (SSS) and granule bound starch synthase (GBSS). In late grain filling period, SS and AGPP were the core enzymes that regulated the starch accumulation rate. AGPP played a main role for Changwu 134, while SS showed stronger regulated activity in Shaan 253. The decreasing amplitude of starch accumulation rate in Changwu 134 was greater than that of Shaan 253. In is concluded that regulating the activity of key starch synthases to accelerate grain starch accumulation in the early grain filling period could benefit to compensate the decrease in starch accumulation that resulted from insufficient supply of photosynthetic product in post grain filling period under soil drought conditions.6 Soil drought stress reduced grain yield of Changwu 134 and Shaan by 34.49 % and 45.42 %, respectively. The decrease of wheat yield was mainly attributed to the reduction of ear number and grain number per spike, while was largely independent of grain weight. Under soil drought conditions, Yield water use efficiency of Changwu134 was higher than that of Shaan 253, but harvest index of Shaan253 was higher than that Changwu134, which transformation efficiency of assimilates is higher too. It is highlight that improving water use efficiency of drought-resistant wheat could be realized by promote vegetative organ remobilization in the pre-anthesis stage to increase of harvest index.7 Soil drought stress significantly affect on the the remobilization of pre-anthesis C-reserves in vegetative organs and on the supply of post-anthesis photosynthetic product and the capacity inverting sucrose to starch in grain. Under drought conditions, the changes of photosynthate supply ability counld influence the remobilization of pre-anthesis C-reserves. The photosynthetic assimilation and supply in the flag leaf of Changwu 134 was stronger than that of Shaan 253, the former also enlongated supply duration, reducing the dependence of grain-filling on the pre-anthesis C-reserves. By contrast, there was a larger decrease in the supply of photosynthetic product in the flag leaf of Shaan 253 that grain-filling more depended on the pre-anthesis reserves in the vegetative organs. Under drought stress, the grain number per spike (sink capacity) of Changwu 134 reduced to less than that of Shaan 253, while the capacity inverting sucrose to starch in grain was affected greatly. The lower remobilization of pre-anthesis reserves of Changwu 134 than Shaan 253 may be attributed more to the invert capacity in sink than to the sink volume itself.
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