Different Deficit Irrigation Lower Limits and Irrigation Quotas Affect the Yield and Water Use Efficiency of Winter Wheat by Regulating Photosynthetic Characteristics

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Abstract
To determine suitable thresholds for deficit irrigation of winter wheat in the well-irrigated area of the Huang-Huai-Hai Plain, we investigated the effects of different deficit irrigation lower limits and quotas on the photosynthetic characteristics and grain yield of winter wheat. Four irrigation lower limits were set for initiating irrigation (i.e., light drought (LD, 50%, 55%, 60% and 50% of field holding capacity (FC) at the seedling-regreening, jointing, heading and filling-ripening stages, respectively), medium drought (MD, 40%, 50%, 55% and 45% of FC at the same stages, respectively), adequate moisture (CK1, 60%, 65%, 70% and 60% of FC at the same stages, respectively), heavy drought (CK2, 35%, 40%, 45% and 40% of FC at the same stages, respectively)) and five irrigation quota per event (30, 60, 90, 120 and 180 mm) were set for each lower limit. We found that the increase of drought stress is conducive to normal photosynthesis of winter wheat leaves which is supported by the following findings. First, photosynthetic rate (Pn) of LD60 treatment was higher than that of LD30, LD90, LD120, LD180, MD30, MD60, MD90, MD120 and MD180. Then, Under the 90 mm irrigation quota treatment, the yield of winter wheat basically increased with the increase of irrigation’s lower limit. Moreover, With the increase in irrigation quota, the yield of winter wheat increased, and the water use efficiency (WUE) of winter wheat increased at first and then decreased. In addition, compared with the LD30, MD30, MD60, MD90, MD120, and MD180, the yield of winter wheat in LD60 treatment increased by about 3.23% (3-year average), 32.3%, 19.9%, 11.7%, 10.1%, and 14.6%. At the same time, the WUE with LD60 treatment of winter wheat was significantly higher than LD90, LD120, LD180, MD30, MD60, MD90, MD120, MD180 treatments. There was a positive correlation between soil volumetric water content and Pn and between yield and Pn. The key period for yield formation in winter wheat is 180 days after sowing. In conclusion, to achieve the dual goals of stable winter wheat yield and efficient utilization of water resources in this region, the suitable threshold for initiating deficit irrigation of winter wheat is the LD60 treatment. This conclusion provides data support for water-saving and stable yield of winter wheat in this area.

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