| In order to meet the demand for food of increasing population and settle the problem of soil and water shortage, protected agriculture were widely and fast developed in China. However, the protected agriculture exists greenhouse effect because it is almost performed in a semi-closed system. Especially, high irradiation in summer sunny midday often results in over 40℃ in air temperature, which seriously affects the crop growth. Therefore, it is important to seek moderate shading measure and avoid the damage to crop by strong light and high temperature in summer midday. Although many studies on shading has been reported in some previous literatures, Combination effects of soil moisture and shading on crop water use at different growth stages are not yet reported. Thus, it is significantly important to enhance the crop yield and water use efficiency (WUE) by studying the effects of shading on photosynthetic and transpiration, dry matter accumulation, yield, nutrient status, and the relationship between WUE and environmental factors influenced by different shading levels under different soil water content and different growth stages. In this paper, based on theoretical analysis of shading, potted plants controlled water by manual work, PVC soil column experiment and computer simulation were used to study the effects of shading in summer sunny day on photosynthetic and transpiration, dry matter accumulation and yield, and WUE. The results are as follows.1．The effects of three soil moistures (high water, HW; medium water, MW; low water LW) and three shading levels (0% shading, CK; 75% shading, DN; 40% shading, SN) on cotton canopy microclimate, stomatal conductance(Gs), net photosynthetic rates(Pn), transpiration rates (Tr) and leaf water potential(LWP) of cotton (Gossypium hirsutum L. Zhongmain No.23)grown in pots from the bud to flowering stages in summer midday were investigated. Shading reduced net radiation above canopy, leaf temperature on daytime, and vapor pressure difference (VPD). The Gs of sunlit leaves in DN increased by 28.02% and 31.00% compared with CK at HW and MW, while the Pn of DN declined by 44.19% and 19.42%, respectively. The Gs of SN sunlit leaves enhanced by 19.12% and 25.44% compared to CK at HW and MW, respectively, the Pn of SN also increased by 20.96% and 33.98%. The Gs among CK, DN and SN did not exhibit significant differences at LW, but the Pn of DN and SN reduced by 18.82% and 9.48%. Significant differences in Gs did not exist between DN and SN at the same soil moisture, while there was significant difference in Pn between them (except at LW). It was suggested that Gs declined with the increasing of VPD and showed weak correlation with Pn by regress analysis at LW. Shading had no effect on LWP at the same soil moisture,while significantly increased the transpiration rate and intercellular CO2 concentration (except at LW).2．The method for estimating crop transpiration under different shading levels was studied systematically. The function relationship between the ratio of canopy resistance divided by aerodynamic resistance and the ratio of the introduced critical resistance divided by the aerodynamic resistance was established based on Penman-Monteith energy conservation formula, using available observed data for cotton grown in pots. A concise method for computing crop transpiration under shade conditions was attained only by simple meteorologic parameters. The theoretic analysis and example confirmed that the method was valid and applicable in estimating crop transpiration. The maximum absolute error and the maximum relative error between the computed and observed values were 0.0565mm/h and 26.62%, respectively. The precision of this model was over 70%. However, in 40%-shaded treatment, we observed only a slight underestimation transpiration of prediction of the model, while in 75%-shaded treatment the underestimation of transpiration fluxes reached almost 20%. The reason may be explained that the effect of lowed leaf temperature on canopy resistance was not considered in co...
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