Effect Of Phosphorus Application On Floret Development And Grain Setting Of Winter Wheat Under Different Water Treatments

To clarify the regulatory effect of phosphorus application on spike floret development and grain setting under different water treatments,three water treatments: severe drought W0（relative water content 35%-45%）,moderate drought W1（relative water content 55%-65%）,and normal water W2（relative water content75%-85%）,and two phosphorus levels（no phosphorus fertilizer P0,phosphorus fertilizer P1）were set up in 2020-2022 with the massive-spike variety Zhoumai 16（ZM 16）and the multi-spike variety Yumai 49-198（YM 49-198）as test materials.The effects of phosphorus application under different water treatments on the developmental dynamics of fertile florets,nutrient uptake and transport,leaf photosynthetic characteristics,osmoregulatory substances,enzyme activities of carbon,nitrogen and phosphorus metabolism,spike endogenous hormones and yield of winter wheat were investigated by observing the florets development process using a dissecting microscope and measuring the dry matter,soluble sugar content,nitrogen content,phosphorus content,chlorophyll content,net photosynthetic rate（Pn）,maximum photochemical efficiency（Fv/Fm）,actual photosynthetic efficiency [Y（II）],sucrose content,proline content,pyrroline-5-carboxylic acid synthase,carbon,nitrogen and phosphorus metabolizing enzymes,spike endogenous hormones and yield and its components.The main results are as follows:（1）The results showed that drought affected the floret development and grain set,and significantly reduced yield.And the application of phosphorus fertilizer optimized the development of spike and floret and effectively reduced the degeneration and abortion of florets.At anthesis,compared with no phosphorus fertilizer,the number of fertile florets increased by 8.93%,17.29% and 10.55% and the number of grains by7.21%,20.97% and 11.99% in the W0,W1 and W2 conditions,respectively,for the massive-spike variety;and the number of fertile florets treated with phosphorus increased by 9.31%,17.41% and 10.88%,while yield increased by 7.56%,21.84%and 11.66% for the multi-spike variety.With non-significant differences in spike number between treatments of phosphorus application under severe drought and significant differences between treatments under moderate drought and normal water.The difference in 1000-grain weight between phosphorus application treatments under the three water treatments was not significant.The increased application of phosphorus fertilizer significantly increased wheat yield by increasing the spike number,grain number per spike and thousand grain weight under the three water treatments of two varieties,and yield increased by 13.41%,29.32% and 16.85%（V1）and 12.66%,29.76% and 17.63%（V2）for phosphorus application compared to no phosphorus application,respectively.（2）Drought severely affects above-ground dry matter accumulation and the uptake and distribution of carbon,nitrogen and phosphorus in wheat.Phosphorus application promotes the production of photosynthetic products in the plant and improves their proportional distribution in the spike.Phosphorus application promoted the dry matter weight,nitrogen and phosphorus content of spikes and shoot,soluble content of spike,soluble sugar content of shoot under normal water,as well as the spike/shoot ratio of soluble sugar and phosphorus content,while soluble sugar content of shoot under drought stress,and the spike/shoot ratio of nitrogen content showed the opposite trend from 18 days before flowering to the anthesis in both wheat varieties.Phosphorus application enhances nitrogen uptake and conversion in wheat plants,promotes sucrose synthesis and enhances plant carbon,nitrogen and phosphorus material accumulation and nutrient supply.The application of phosphorus fertiliser increased carbon and nitrogen metabolising enzyme activity and the effect on acid phosphatase activity varied according to water treatment,with phosphorus application under W1 and W2 conditions decreasing acid phosphatase activity and phosphorus application under W0 conditions increasing acid phosphatase activity.（3）Drought stress resulted in impaired photosynthetic organ function,reducing leaf chlorophyll content,net photosynthetic rate（Pn）,maximum photochemical efficiency（Fv/Fm）and actual photochemical efficiency [Y（II）].The application of phosphorus fertilizer can alleviate the damage caused by drought stress to the photosynthetic function of the plant,enhance the photosynthetic capacity of the plant and promote photosynthesis.At anthesis,chlorophy II content increased by2.76%-5.70%（V1）and 2.78%-5.77%（V2）,Pn increased by 5.66%-12.14%（V1）and5.49%-10.93%（V2）,Fv/Fm increased by 0.72%-1.65%（V1）and 1.02%-1.64%（V2）,and Y（II）increased by 2.02%-5.43%（V1）and 2.86%-4.62%（V2）.（4）Drought stress significantly reduced plant water content and increased leaf proline content and P5 CS activity.Phosphoric fertilizer increased the water content of the plants.The water content of the stems of the two varieties increased,with increases of 2.98%,5.16% and 3.48%（V1）and 2.19%,4.44% and 3.59%（V2）for stems,2.76%,3.34% and 2.89%（V1）and 2.70%,4.92% and 3.02%（V2）for leaves,and 1.59%,3.56% and 1.91%（V1）and 2.30%,5.35% and 3.32%（V2）for spikes under W0,W1 and W2 conditions,respectively.The additional application of phosphorus fertiliser at different water levels enhanced P5 CS activity and promoted proline synthesis.The proline content of leaves of two varieties increased by 12.94%,40.17% and 25.53%（V1）and 17.70%,37.95% and 20.72%（V2）under W0,W1 and W2 treatment.（5）Drought can cause plan hormone metabolism disorders,with increased ABA content,decreased IAA,GA₃ and CTK content,and decreased inter-hormone ratios IAA/ABA,GA₃/ABA and CTK/ABA.Phosphorus fertilisation improved hormone metabolism under different water treatments,with varying increases in the contents of endogenous IAA,GA₃ and CTK and in the IAA/ABA,GA₃/ABA and CTK/ABA ratios compared to no phosphorus fertilisation.The correlation analysis showed that the number of fertile florets were significantly positively correlated with IAA,GA₃ and CTK content and IAA/ABA,GA₃/ABA and CTK/ABA ratios,and negatively correlated with ABA content.In summary,phosphorus application under drought stress promotes aboveground nutrient production and uptake,optimizes the distribution of soluble sugars,nitrogen and phosphorus in the spike,improved photosynthetic system damage caused by drought stress,enhanced photosynthetic production capacity and promoted photosynthesis,while increasing the plant’s water content and P5 CS activity and promoting proline production.In addition,phosphorus fertilisation increases the IAA,CTK and GA₃ content and decreases the ABA content,and increases the IAA/ABA,CTK/ABA and GA₃/ABA ratios,which provide information and material supply for promoting the development and growth of spikes and florets.Phosphorus regulates the photosynthesis of leaves,the accumulation and distribution of assimilates,osmoregulatory substances and the content of enzymes and hormones to alleviate the damage caused by drought stress,thus reducing the degeneration and abortion of fertile florets,promoting the spike floret development and grain setting,ultimately leading to increased yields.