| A field trial was conducted in the trial station of Agricultural university of Hebei during 2001-2002 winter wheat growing season. Eight varieties, which were divided as three groups, namely, drought resistant (Keyi 26, Xiaoyan 6, Henong 859), medium drought resistant (Beijing 8, Bima 4, Taishan 1), and drought susceptive variety (Baiquan 3039, Yumai 49), were used as experimental materials. Under the natural drought stress conditions around Baoding caused by the long dry spell during spring growing period of winter wheat, the effects of drought stress on photosynthetic physiology of the varieties with different drought resistance, and on the physiological indices related closely to drought resistance, were systematically determined. In addition, the physiological mechanism of wheat drought tolerance and the feasibility of evaluating wheat drought resistance by using some physiological indices were also examined. The main results were summarized as follows:1. It was feasible to determine the physiological procession related to drought stress and the differences among different groups of varieties by employing the drought stress caused by natural rainfall deficiency during wheat growing season around Baoding. Since the field drought stress caused by rainfall deficiency naturally was typical of the real conditions of wheat production in this area, it was of great significance to study the physiological changes in wheat under natural drought stress.2. Under drought stress, chlorophyll content (Chl), stomatal conductance (gs) and net photosynthetic rate (Pn) were decreased, while intercellular CO2 concentration (Ci) was increased. With the increasing of Q, the transduction ability of CO2 was reduced whichcaused great amount of CO2 to be accumulated in intercellular space and Pn to be decreaced. As a result, dry matter production was also reduced which led to the decrease of wheat yield. The physiological indices of flag leaf were higher than those of the third leaf from top under drought stress, however, drought stress had greater effects on the physiological indices of flag leaves. Among the different groups of varieties, the fluctuation range of the physiological indices in drought resistant varieties is much smaller than that in drought susceptive ones.3. Under drought stress, the content of soluble protein and the activities of superoxide dismutase (SOD) and peroxide dismutase (POD) were reduced and the accumulation of malondialdehyde (MDA) and proline (Pro) were increased in different leaves and different phenostages. The physiological indices of flag leaves were very sensitive to drought stress. The contents of soluble protein and Pro and the activities of SOD and POD were relatively higher and their fluctuations were smaller in drought resistant varieties than in drought susceptive ones.4. In some varieties the photosynthetic indices of the third leaves from top under drought stress were better than those under irrigation treatment, which suggested that moderate drought might be favorable to the photosynthetic procession. This phenomenon was named as "the effect of moderate drought stress". And this concept, like the concept of effects of moderate stress by other ecological conditions, is meaningful in explaining the short time or partial superiority in some biological indices caused by moderate stresses.5. With yield drought resistant index and yield component loss index as evaluation criteria, the differences of drought resistance for different wheat varieties were evaluated, and the results achieved by this evaluation were accorded to those used in wheat production.6. Applying 198 groups of data from physiological criteria, the drought resistance indices and yield component loss indices for these criteria were calculated. The results of correlation analysis between calculated values and measured values of drought resistance indices and yield component loss indices demonstrated that, the coefficients of correlation of 24 groups of data were statistically signi...
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