Abstract:

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 (P_n), transpiration rate (T_r), stomata conductance (G_s), intercellular CO₂ concentration (C_i), leaf temperature (T_l), 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 P_n and G_s, and WUE further.

Key words: Gaodan grass, Leaf, Physiological characteristics, Water use efficiency