Effects of spring limited irrigation on grain yield and root characteristics of winter wheat in groundwater-overexploitation areas in the North China Plain

Understanding the effects of limited irrigation on wheat yield and root characteristics is crucial for maintaining sustainable agricultural development in the groundwater-overexploitation zone of the North China Plain. We evaluated these effects in two winter wheat cultivars: Shimai22 (SM22, deep root system) and Shinong086 (SN086, shallow root system). Irrigation treatments were traditional irrigation twice (W2), no irrigation (W0), and single irrigation (W1) based on the number of leaves that unfolded in spring (3 L, 4 L, 5 L, and 6 L). Wheat grain yield, root characteristics, and root-shoot relationships were investigated according to growth chamber and field experiments. The results showed that the wheat yield of W0 and W1 were 50.9% and 23.0% less than that of W2, respectively. However, SM22 and SN086 exhibited the smallest yield reduction effect under 4 L and 3 L, with significant reductions of 10.4% and 16.2% compared to W2, respectively. The results of the growth chamber experiment showed that compared to W2, limited irrigation reduced the total root quantity, whereas no significant reductions were observed with the 3 L and 4 L. Meanwhile, SN086 increased root distribution in the deep layer at early single irrigation. Moreover, limited irrigation mainly affected the roots in the soil depth of 0–40 cm. Therefore, we analyzed the effects of limited irrigation on the wheat root system of the 0–40 cm soil depth under certain field conditions. Similarly, we found that limited irrigation reduced wheat root quantity. However, root weight density (RMD), root length density (RLD), and root surface area density (RSD) at 3 L and 4 L were significantly higher than those at 5 L and 6 L at the booting; those at 5 L and 6 L were significantly higher than those at 3 L and 4 L at milk ripening. The root quantity of SN086 was significantly higher than that of SM22. Root–shoot relationships showed that irrigation increased RMD, RSD, and RLD at the booting stage, while augmenting aboveground biomass, spike number, kernel number per spike, and grain yield. The earlier irrigation time in W1 was beneficial for increasing the drought resistance of roots and shoots; however, the later irrigation time resulted in higher RLD, RSD, and RMD at the milk-ripening stage, increasing kernel weight. Furthermore, SM22 had a lower root–shoot ratio, and SN086 had a higher root–shoot ratio under high-yield conditions. Therefore, 4 L single irrigation in spring for deep-rooted cultivars or 3 L single irrigation in spring for shallow-rooted cultivars is a suitable water-saving irrigation method to promote the establishment of the wheat root system, improve the drought resistance of the root system, increase aboveground biomass, and prolong spike development time, increasing spike and kernel numbers to improve the grain yield of winter wheat.