在不同土壤供水条件下，研究了高丹草水分利用效率（WUE）与叶片光合速率（Pn）、蒸腾速率（Tr）、气孔导度（Gs）、叶温（Tl）、叶片相对含水量（RWC）的关系。结果表明，WUE随RWC、Pn呈二次曲线变化，Pn在27 µmol CO2 m-2 s-1时，WUE值最大 (8.7 µmol CO2 mmol-1 H2O)；Tr在3.5~4 mmol H2O m-2 s-1时WUE达最大值(8.4 µmol CO2 mmol-1 H2O)；Pn与Tr的非线性关系可以用抛物线方程表述，其中Pn最高时的Tr为临界值，超出该值即为奢侈蒸腾。Gs在0.4 molH2O m-2 s-1时，WUE达到峰值8.39 µmol CO2 mmol-1 H2O。实施提高气孔阻力并抑制蒸腾的措施，既节约水分，又促进光合作用，增加产量。Pn和Tr随温度的增加而增加，在35~36℃时Pn达最高值，表明在一定温度范围内，温度升高对提高WUE有利。WUE随RWC的升高而上升，RWC在84%~86%时WUE最大。适量增施氮肥，可提高Pn和Gs，进而提高WUE。
It is well known that water shortage is a main limited factor to crop production, therefore it is important to improve the crop water use efficiency (WUE), especially under water limited condition. Gaodan grass, a hybrid of sorghum and Sudan grass, is a widely planted forage crop in Inner Mongolia, where water deficiency is a common problem. The primary aim of this study was to investigate the relationships between WUE and leaf physiological characteristics in Gaodan grass, and furthermore to understand the potential for increasing Gaodan grass water use efficiency. From booting to flowering stages of Gaodan grass under different water supply conditions, water use efficiency (WUE), net photosynthetic rate (Pn), transpiration rate (Tr), stomata conductance (Gs), intercellular CO2 concentration (Ci), leaf temperature (Tl), and relative water content of leaves (RWC) were measured. The results showed that the water use efficiency presented a quadratic curve with photosynthetic rate. When Pn was 27 µmol CO2 m-2 s-1, and Tr was 3.5–4.0 mmol H2O m-2 s-1, WUE reached maximal point (8.4 µmol CO2 mmol-1 H2O). The relationship between the photosynthetic rate and the transpiration rate presented a parabolic curve. The transpiration rate at the maximal photosynthesis rate was a critical value, beyond which the water utilization would be luxurious. When the stomatal conductance was 0.4 mol H2O m-2 s-1，WUE reached 8.39 µmol CO2 mmol-1 H2O, if Gs was higher than 0.4 mol H2O m-2 s-1 WUE would be decreased. It is necessary to take some measures for increasing stomatic resistance and inhibiting transpiration, so as to save water and improve photosynthesis and yield as well. The increase of temperature promoted the photosynthesis and transpiration, when temperature was in the range of 35℃ to 36℃, the photosynthetic rate reached the maximum. With the increase of RWC, WUE was increased, when RWC was 84%–86%, WUE was the maximum. Proper N fertilization was also proved to increase Pn and Gs, and WUE further.
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