Effect of limited irrigation on diurnal variation in flag-leaf photosynthesis and yield of dryland wheat

A field experiment was conducted to study the effects of five irrigation treatments on diurnal variations in flag-leaf photosynthesis and yield of “Qingmai 6” dryland wheat variety. The results showed that diurnal variations in net photosynthetic rate (Pn) in each treatment followed a double-peak curve. However, the degree of midday depression in photosynthesis increased with increasing amount of irrigation. Increasing irrigation amount enhanced flag-leaf Pn and net stomatal conductance (Gs) under ≤3 irrigation times. The maximum Pn and Gs appeared in W3 treatment (i.e., 60 mm irrigation each at jointing, booting and grain-filling stages). With increasing supplementary irrigation, intracellular CO₂ concentration and stomatal limitation (Ls) decreased. Flag-leaf Pn under W4 treatment (i.e., 60 mm irrigation each at sprouting, jointing, booting and grain-filling stages) was significantly lower than that under W3 treatment. This suggested that superfluous irrigation had negative effects on flag-leaf photosynthesis in dryland winter wheat at grain-filling stage. In all the treatments, yield under W2 treatment (i.e., 60 mm irrigation each at jointing and booting stages) was the highest. The difference in yield between W2 and W1 (i.e., 60 mm irrigation at jointing stage) treatments was insignificant. Under ≤2 times of irrigation, yield increased with increasing irrigation amount. Yields under W3 and W4 treatments were lower than that under the dryland control treatment. This suggested that superfluous irrigation was unsuitable for high yield dryland winter wheat cultivation. In all the treatments, water production efficiency (WPE) declined with increasing irrigation amount. The experiment suggested that irrigation at jointing, booting and grain-filling stages was crucial for high flag-leaf Pn in dryland winter wheat. However, the effect of irrigation at sprouting stage on flag-leaf photosynthesis of dryland winter wheat was insignificant. Jointing was the most pivotal stage for high yield of wheat. Jointing and booting were the most essential stages for irrigating of winter wheat. Dryland wheat irrigation at these stages was very important for high yield and WPE. In terms of yield and WPE, the optimal supplementary irrigation mode for high yield in dryland winter wheat was 60 mm at jointing stage.