Abstract: In order to clarify the effect of spraying (potassium dihydrogen phosphate, PDP, KH2PO4) on the grain setting characteristics of wheat under late spring coldness, Yannong 19 (YN19) with strong resistance to reversed late spring coldness and Xinmai 26 (XM26) with weak resistance to reversed late spring coldness were selected as materials. PDP was sprayed on the leaf surface after late spring coldness occurred on March 17−23, 2022 (booting stage). The distribution characteristics of spraying PDP on fertile grain number and grain weight with different spikelet and different grain positions under late spring coldness were compared and analyzed. The results showed that: (1) compared with CK, spraying PDP under late spring coldness increased the total fertile grain numbers of varieties XM26 and YN19. Among them, the total fertile grain numbers at the middle and lower spikelet of XM26 significantly increased by 8.33% and 33.33%, respectively, with no significant effect on the total fertile grain number at the upper spikelets. And the total fertile grain numbers of XM26 at G1 (the first grain position), G2 (the second grain position), G3 (the third grain position) and G4 (the fourth grain position) position increased by 8.11%, 3.13%, 4.35%, and 60.00%, respectively. The total fertile grain numbers at the lower spikelet of YN19 significantly increased by 23.08%, with no significant effect on the total fertile grain numbers at the upper and middle spikelets. The total fertile grain numbers of YN19 at G1, G2 and G3 position increased by 2.70%, 0 and 13.33%, respectively. (2) Compared with CK, spraying PDP under late spring coldness increased the grain weights of varieties XM26 and YN19. Among them, the total grain weights at the middle and lower spikelet of XM26 significantly increased by 18.46% and 46.16%, respectively, with no significant effect on the total grain weights at the upper spikelets. And the total grain weights of XM26 at G1, G2, G3 and G4 position increased by 2.70%, 4.44% and 13.33%, respectively. The total grain weights at the upper and lower spikelet of YN19 significantly increased by 21.70% and 33.63%, with no significant effect on the total grain weights at the middle spikelets. The total grain weights of YN19 at G1, G2 and G3 position increased by 15.97%, 13.12% and 17.55%, respectively. (3) Spraying PDP under late spring coldness mainly alleviates the yield loss by increasing the fertile grain numbers of XM26 with weak resistance to reversed late spring coldness, and increasing grain weight of YN19 with strong resistance to reversed late spring coldness. It was concluded that spraying PDP after the late spring coldness could increase the fertile grain number and grain weight of the lower spikelet and the weaker grain positions to reduce the yield loss.

Key words: Late spring coldness, Triticum aestivum L, Potassium dihydrogen phosphate, Spikelet and grain positions, Grain setting characteristics