Effects of sowing date and rate on grain filling characteristics of winter wheat under chilling injury

In Huaibei Area of Anhui Province, chilling injury occurred twice in April 2013. The winter wheat cultivars 'Jimai22' and 'Wanmai52' planted in 2012 to 2013 were used to analyze the effects of sowing date and rate on grain filling characteristics of winter wheat under chilling injury. Comparative experiments were conducted for different sowing dates early sowing (on October 3) and timely sowing (on October 15) and sowing rates N1 (6×10⁴ plant·666.7m^-2), N2 (10×10⁴ plant·666.7m ^-2), N3 (14×10⁴ plant·666.7m^-2) and N4 (18×10⁴ plant·666.7m ^-2) in Mengcheng and Taihe Counties. During the grain filling stage of winter wheat, field investigations of chilling injury showed that 'Jimai22' and 'Wanmai52' early sowed severely suffered injuries. The cold resistance of 'Jimai22' was higher than that of 'Wanmai52'. Under the same sowing dates, the single-grain weight of the two cultivars had the same variation trend, which decreased with increasing sowing rate. In addition, the single-grain weight of 'Jimai22' was higher than that of 'Wanmai52'. Richards equations combined with Matlab program was used to estimate the secondary parameters of wheat grain filling. The results showed that various parameters at different sowing dates had certain regularities. Specifically, initial grain-filling potential (R₀), average grain-filling rate (V_mean), duration of middle grain-filling stage (T₂), early stage grain-filling rate (Vp1) and middle stage grain-filling rate (V_p2) under timely sowing treatment were much higher. Contrarily, early grain-filling stage duration (T₁), late grain-filling stage duration (T₃) and late stage grain-filling rate (V_p3) under early sowing treatment were much higher. This suggested that early sowed winter wheat was more likely to suffer more serious chilling injury. Under the same sowing date, the two wheat cultivars had similar variation trends with changing sowing rate. Specifically, T₁ and V_p3 increased gradually with increasing sowing rate. Furthermore, R₀, V_mean, T₂, T₃, V_p1, V_p2, and W_max (final dry weight) followed a decreasing trend. Also compared with 'Wanmai52', 'Jimai22' showed higher V_mean, T₂, T₃ and V_p1, and lower R₀, T₁, V_p2 and V_p3. Correlation analysis showed that T₂ (0.871 1), T₃ (0.809 6), V_mean (0.777 5) and V_p2 (0.761 6) of early sowing treatment had higher correlations with single-grain weight, while T₂ (0.906 1), R₀ (0.873 8), V_mean (0.837 2) and V_p2 (0.805 6) of timely sowing treatment had higher correlations with single-grain weight. Also path analysis suggested that in terms of chilling injury stress, initial grain-filling potential had positive effects on single-grain weight under both timely and early sowing treatments. Prolonged middle and late grain-filling stages were beneficial for increasing single-grain weight. However, prolonged early grain-filling stage had negative effects on single-grain weight. Also enhanced grain-filling rates at early and middle stages were conducive for increasing single-grain weight, but grain-filling rate at the late stage exerted negative effects on single-grain weight. Therefore supplementation with quick-acting fertilizers in combination with leaf fertilizer application at late growth stage under chilling injury stress were critical for improving initial grain-filling potential, prolonging middle and late grain-filling stages and increasing grain dry weight.