Water Stress Impact on Winter Wheat Yield: Ear and Kernel Number Reduction Under Limited Irrigation

Winter wheat was subjected to varying water supply conditions under different treatments, as shown in Figure 3. The water stress index (WSI) of winter wheat during three critical growth stages, namely sowing-jointing, jointing-flowering, and flowering-maturity, was assessed. The WSI values for Con, Opt, and Min during sowing-jointing were 0.36, 0.42, and 0.35, respectively. During jointing-flowering, the WSI values for Con, Opt, and Min were 0.16, 0.19, and 0.53, respectively, with Min exhibiting significantly higher values than Con and Opt. The WSI values for Con, Opt, and Min during flowering-maturity were 0.31, 0.39, and 0.25, respectively. It is important to note that the jointing-flowering period is crucial for wheat's ear and kernel number formation. Thus, the water deficit environment during this phase in the Min treatment led to a substantial decrease in ear and kernel number.

Field trials conducted over a six-year period showed differences in fertility between treatments, as depicted in Figure 4. Fertility was synchronized in Con and Opt, while Min showed significantly earlier flowering and filling stages and an overall maturity that was five days earlier than Con and Opt. Weather conditions during winter wheat's growth season differed among irrigation treatments due to differences in fertility. The correlation between ear and kernel number and weather conditions at the formation stage was examined (Fig. 5). EN significantly correlated with Tmin, GDD, and VPD during jointing-heading (Fig. 5C), while KN correlated with Tmax, Tmin, and Solar during jointing-flowering (Fig. 5D). An analysis of weather conditions between treatments showed that Solar radiation was significantly lower in the Min treatment than in the Con (Opt) treatment between jointing-heading. GDD was also significantly lower in the Min treatment than in the Con (Opt) treatment between jointing-flowering, which was another reason for the significant reduction in the EN and KN under the Min treatment.

To quantify the importance of the factors affecting the yield, further analysis was conducted. Results showed that EN was the critical factor responsible for the yield reduction in the Min treatment compared to the Con treatment (Fig. 6). The reduction in EN accounted for 90% of the yield reduction. Biomass analysis also indicated significantly lower values in the Min treatment until the heading stage (Fig. S1). The difference in water stress and solar radiation during the jointing to 5 days after heading resulted in reduced EN in the Min treatment as compared to Con, as demonstrated by Fig. 3, Fig. 4, and Fig. 5. Further analysis indicated that water stress during the nodulation stage to 5 days after heading was the primary contributor (58%) to the EN reduction.