The Combined Effects Of Maize Straw Mulch And Phosphorus Fertilization On Photosynthetic Characteristics Of Flag Leaves And Grain Yield Of Winter Wheat

Phosphorus（P）affects the anatomical structure and carboxylation efficiency of leaves,and is an important factor in regulating net photosynthetic rate.However,the southwest hilly dryland climate is dry in winter and dry in spring,and the soil available phosphorus is deficient,which severely limits the photosynthetic efficiency and yield of wheat flag leaves.This study conducted a two-factor split-plot field positioning experiment with corn straw mulch（0,8000 kg/hm²）in the main area and phosphorus application rate（0,75,120 kg/hm²）in the split-plot at Renshou Experimental Station in a typical hilly and dry land in Southwest China in 2020–2022.Study the effects of straw mulching and phosphorus application on phosphorus absorption and utilization efficiency,flag leaf photosynthetic characteristics,sucrose synthesis,and yield of wheat.Clarify the impact of straw mulching combined with phosphorus fertilizer on phosphorus absorption and utilization efficiency of wheat under purple soil in hilly and arid areas of southwest China and its relationship with flag leaf photosynthetic characteristics.Provide theoretical basis and technical support for the high yield,high efficiency,and green sustainable development of southwest winter wheat.The main results are as follows:（1）Straw mulch increased the proton absorption potential of wheat root tips from0-20 cm,thereby increasing the accumulation of phosphorus and dry matter in the aboveground parts.Compared with the result for no-mulch treatments,the proton absorption potential of wheat root tips in 0-20 cm after anthesis under straw mulching increased by 29.4%.In the two experimental years,the phosphorus accumulation,mature dry matter accumulation,and post-anthesis dry matter accumulation of wheat covered with straw increased by 32.7,29.9,and 27.8%,respectively,compared to wheat without straw mulch.The accumulation of dry matter increases with the increase of phosphorus application.Straw mulch combined with phosphorus application reduced the pre-flowering dry matter transport rate and contribution rate to grains.（2）Straw mulch increases the phosphorus content in wheat flag leaves,improving chlorophyll content and net photosynthetic rate.In the two experimental years,the leaf mass per area and phosphorus content per unit area of wheat flag leaves increased by3.9-4.1%and 16.9-24.5%,respectively,with straw mulching compared to no mulch.During the grain-filling period of wheat,the chlorophyll content and net photosynthetic rate of flag leaves under straw mulching treatment were significantly higher than those without straw mulch.At 20 days post-anthesis,straw mulch increased chlorophyll a,chlorophyll b,chlorophyll,and net photosynthetic rate by 12.0,8.9,11.1,and 20.8%,respectively,compared to without straw mulch.Compared to no phosphorus application,the net photosynthetic rate increased by 4.9–21.5%under different phosphorus application rates.Correlation analysis shows that the phosphorus content of flag leaves is significantly positively correlated with specific leaf weight and net photosynthetic rate.（3）Straw mulch combined with phosphorus application increases the stomatal size and density of flag leaves,thereby improving stomatal and mesophyll conductance.Compared with wheat flag leaves without straw mulch,straw mulch increases the maximum net photosynthetic rate,apparent quantum efficiency,light compensation point,and light saturation point by 10.0,27.0,18.8,and 18.2%,respectively.Compared with the result without straw mulch,the stomatal size and density of wheat flag left under straw coverage increased by 5.5 and 8.0%,respectively,and the thickness of palisade and sponge tissue increased by 8.0 and 5.1%,respectively.The maximum carboxylation efficiency,initial carboxylation efficiency,intercellular CO₂concentration,chloroplast CO₂ concentration,maximum photochemical efficiency,and actual photochemical efficiency of flag leaves with phosphorus application increased by 24.1–61.1%,6.5–10.4%,1.4–6.6%,9.0–42.7%,6.9–18.4%,and 5.8–16.8%compared to those without phosphorus application.Straw mulch improves the carboxylation efficiency and stomatal conductance of flag leaves but does not affect electron transfer efficiency.Biochemical limitations are the main limiting factors for the net photosynthetic rate of flag leaves under straw mulching and phosphorus application treatments,accounting for 0.41–0.55 of the photosynthetic limiting factors.（4）Straw mulch combined with phosphorus application increased the activity of sucrose synthase in wheat flag leaves,thereby increasing the sucrose content in grains.In the two experimental years,the non-structural carbohydrate content in flag leaves and grains increased by 13.8 and 19.8%,respectively,compared to those without straw mulching 20 days after anthesis.In different phosphorus fertilizer treatments,the decrease in sucrose content in flag leaves with ent.Under straw coverage,there was no significant difference in sucrose synthase activity between 75 kg/hm² and 120 kg/hm²of phosphorus application.（5）Straw mulch improves the efficiency of phosphorus absorption and utilization in wheat,thereby increasing yield:compared to results without straw mulch,straw mulch increases phosphorus fertilizer productivity and phosphorus absorption efficiency by 25.0 and 35.2%,respectively,while phosphorus grain production efficiency decreases by 8.9%.Applying phosphorus significantly increased phosphorus fertilizer productivity and phosphorus absorption efficiency compared to not applying phosphorus,and reduced phosphorus grain production efficiency.However,applying120 kg/hm² of phosphorus under straw cover significantly reduced phosphorus absorption and utilization efficiency compared to 75 kg/hm².At the same time,straw mulching increased the effective panicles by 15.2 and 8.6%,the number of grains per panicle by 8.7 and 10.7%,and the yield increased by 25.0 and 1%compared to no mulch.The yield increase effect of straw mulching was greater than that of phosphorus application.There is no difference in yield between 75 kg/hm² and 120 kg/hm² of phosphorus application under straw mulch（two-year average）.Correlation analysis shows that there is a significant positive correlation between net photosynthetic rate and effective panicle,grain number per panicle,and yield.The above results indicate that straw mulch combined with phosphorus fertilizer improve the proton absorption capacity of wheat root tips after anthesis,and increase the accumulation of dry matter and phosphorus in wheat aboveground parts.Straw mulch improves wheat phosphorus fertilizer production efficiency and phosphorus absorption efficiency while reducing phosphorus grain production efficiency and harvest index.Under straw mulching combined with phosphorus fertilizer,the photosynthetic efficiency of flag leaf increased,and stomatal and mesophyll restrictions decreased significantly.Straw mulch combined with phosphorus fertilizer are beneficial for improving the activity of sucrose synthase in wheat flag leaves,thereby promoting the synthesis and output of sucrose in flag leaves.Under the conditions of this experiment,straw mulching combined with 75 kg/hm² phosphorus fertilizer can improve wheat phosphorus utilization efficiency and flag leaf light energy utilization,while promoting yield improvement.The results of this study clarify the role of leaf CO₂ conductivity in regulating flag leaf photosynthetic capacity under straw mulching and phosphorus fertilizer application conditions,providing a theoretical basis for improving wheat high yield and phosphorus efficiency.