Impact of high temperature stress on grain-filling and the relief effect of foliage sprays during grain-filling stage of wheat

In order to relieve damages caused by frequent high temperatures at grain-filling stage to grain filling and yield decreasing of wheat in the North China Plain, a study was conducted in 20132014 and 20142015 growing seasons. A split block design was used with 4 different high temperature stress treatments (A1, A2, A3 and A4) produced by plastic sheet covering and no covering (A5) as control. Treatments A1, A2, A3, A4 and A5 were covered with plastic sheets at 12–25 d, 12–16 d, 15–20 d, 20–25 d after anthesis in 20132014 and 8–21 d, 8–12 d, 14–20 d, 16–21 d after anthesis in 20142015, respectively. Three foliar sprays 0.2% potassium dihydrogen phosphate (B1), 0.05% zinc sulfate (B2), water (B3) were applied at booting and early milking stages as the sub-treatments with no spray as the control (B4). The impacts of high temperature stress and the relieving effect of foliar spray on grain-filling during grain-filling stage were quantified via model simulation. The results showed that high temperature stress reduced grain weight, grain number per spike and grain yield of wheat. For yield losses under different temperature stress treatments, A1, A2, A3 and A4 were 15.34%, 13.11%, 14.93% and 12.64% in 20132014, and 9.41%, 3.89%, 4.93% and 2.04% in 20142015, respectively, compared with control. Yield of A1 was lowest among all the treatments and the difference between A1 and A5 (CK) was significant at P < 0.01. No significant differences existed among A2, A3, A4 and A5 in terms of yield. For 1000-grain weight, A1, A2, A3 and A4 respectively decreased by 1.96 g, 3.41 g, 1.71 g and 1.28 g in 20132014, and respectively by 4.27 g, 0.84 g, 1.23 g and 2.19 g in 20142015 compared with CK. Furthermore, grain numbers per spike decreased respectively by 5.45, 1.45, 0.87 and 0.71 in 20132014, and by 1.95, 2.30, 3.00 and 1.73 in 20142015. The established grain-filling process models showed that high temperature stress advanced the first inflection points by 0.48 d, 0.75 d, 0.46 d and 0.29 d and the second inflection points by 0.92 d, 1.42 d, 0.61 d and 0.22 d, respectively, compared with the control, which shortened the duration of wheat grain filling. The average grain-filling rate also decreased which resulted in lower 1000-grain weight. The application of sprays delayed the first and the second inflection points and prolonged grain-filling, which increased grain weight and yield. B1, B2 and B3 increased grains per spike by 2.30, 1.21 and 1.04 in 20132014, and by 2.01, 2.75 and 0.95 in 20142015, respectively, over B4. The 1000-grain weights of B1, B2 and B3 were respectively 1.10 g, 1.42 g and 0.89 g greater than B4. Based on the grain-filling process models, the times to maximum grain-filling rates delayed respectively by 0.73 d, 0.69 d and 0.61 d, and the average filling rate increased by 0.04 mg·grain^-1·d^-1, 0.03 mg·grain^-1·d^-1 and 0.01 mg·grain1·d1 over B4. Therefore, longer filling stage, higher grain weight and higher grain numbers per spike were main mechanisms of yield increase due to foliar spray treatments. Foliar spray mitigated the effects of high temperature stress on grain-filling. Yield promotion effects of B1 and B2 were better under high temperature than under normal temperature, and B1 had the best effect among all foliar spray treatments.