Abstract: In order to explore the alleviating mechanism of phosphorus fertilizer postpone on the growth of wheat flag leaves under late spring coldness at connectivum stage, the low temperature simulation experiment of pot was carried out by using the artificial climate chamber with the wheat variety "Yannong 19" (YN19, strong resistance to reversed late spring coldness) and the wheat variety "Xinmai 26" (XM26 weak resistance to reversed late spring coldness) as experimental materials. In the experiment, two temperature treatments were set at connectivum stage of wheat: control(average daily temperature 15℃, minimum night temperature 11℃, CK) and low temperature(−4℃, LT), and two phosphorus fertilizer application modes: conventional phosphorus fertilizer application which all of phosphorus fertilizer was applied as base fertilizer and phosphorus fertilizer postpone which half of phosphorus fertilizer was applied as jointing fertilizer. The low temperature treatment lasted for 4h·d−1. Photosynthetic and antioxidant physiological indices of flag leaves at flowering and grain filling stage and yield were analyzed. The results showed as follows: (1) compared with CK, the relative chlorophyll content (SPAD value), net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of flag leaves in LT treatment significantly decreased(P<0.05), while the intercellular CO2 concentration (Ci) significantly increased. (2) Compared with CK, LT treatment significantly decreased the activities of SOD and CAT, and significantly increased the content of malondialdehyde (MDA). (3) Under the experimental conditions, LT treatment significantly reduced the number of grains per spike, 1000 grains weight and single stem yield of the two varieties compared with CK treatment. (4) Compared with LTR1, the SPAD value of XM26 flag leaves at flowering stage and YN19 flag leaves at grain filling stage under LTR2 treatment were significantly increased by 7.3% and 10.1%, respectively. Pn of XM26 and YN19 flag leaves was significantly increased by 8.5% and 19.0% at flowering stage, 8.0% and 8.7% at grain filling stage, and there was no significant difference in Gs, Tr and Ci. (5) Compared with LTR1, SOD activity, CAT activity and MDA content of XM26 flag leaves at flowering and grain filling stages of LTR2 in were significantly increased by 9.0% and 5.7%, 12.4% and 30.9%, and 7.9% and 7.3%, respectively; SOD activity, CAT activity and MDA content of YN19 flag leaves at flowering and grain filling stages of LTR2 were significantly increased by 7.7% and 8.2%, 20.8% and 25.5%, and 6.8% and 7.0%, respectively. (6) Compared with R1, R2 significantly increased the grain number per spike of XM26 by 8.4% after LT treatment, but had no significant effect on the 1000 grains weight of the two cultivars. The yield recovery rates of XM26 and YN19 per stem could reach 8.9% and 9.9%. In conclusion, delayed phosphorus transfer can effectively enhance the antioxidant capacity of wheat flag leaf cells, reduce the degree of membrane lipid peroxidation, and improve the photosynthetic capacity of wheat flag leaf cells. Through the synergistic alleviation of antioxidant and photosynthetic capacity, the growth of flag leaves in the middle and late stage of wheat growth under reversed late spring coldness at connectivum stage, the number of grains per spike and 1000 grains weight of main stem were increased, the "source" was maintained and the "reservoir" was increased, and the effects of disaster free, stable and increased yield, disaster reduction and loss stopping were achieved.

Key words: Late spring coldness, Phosphate, Wheat, Photosynthetic, Antioxidant system