Study On The Effect And Potential Mechanism Of Nitrogen And Potassium Combined Application On Canopy Growth Of Brassica Napus

Oilseed rape（Brassica napus L.）is an important oil crop in China.The phenomenon of emphasize the application of nitrogen（N）fertilizer and neglect the application of potassium（K）fertilizer was found in rape production,however the high-intensity crop rotation and the same period of rain and heat in the main oilseed rape producing areas,leading to a large amount of soil K being consumed away which resulted in the unbalanced application of N and K limiting the exploitation of oilseed rape yield potential.N and K combined application is one of the important agronomic measures to improve the yield of oilseed rape and photosynthesis is completed in canopy,it is of great theoretical and practical significance to analysis the effects of N and K combined application on canopy growth and photosynthetic performance of oilseed rape for further exploring the yield potential of oilseed rape and improving N fertilizer use efficiency.The field experiment of N and K interaction was conducted in this study（including four N rates:0,90,180,270 kg N/hm² and tow K rates:0,120 kg K₂O/hm²,in a complete design composed of eight treatments）,and the effects of N and K combined application on yield and fertilizer use efficiency of oilseed were clarified through two years of field experiments and the physiological mechanism of N and K combined application to improve the yield was preliminarily analyzed from the perspectives of canopy growth,photosynthetic capacity,light and N distribution and N allocation in the canopy.The main results are as follows:（1）N and K combined application significantly increased the yield,shoot N and K accumulation and fertilizer utilization efficiency of oilseed rape.Compared with no N and K fertilizer,the oilseed rape yield,shoot N and K accumulation of N and K combined application increased by 1.2-4.7,2.3-6.8 and 4.0-5.3 times,which was significantly higher than that of N or K fertilizer alone.Under the condition of insufficient K supply（K₀）,the average yield of oilseed rape was increased 89.5%after N application,while under the condition of sufficient K supply(K₁₂₀),the average yield increase was 214.0%after N application.Compared with the treatment no K application,the N fertilizer recovery efficiency increased by 9.4-32.5 percentage points after K application.Compared with no N application,ensuring sufficient N supply(N₁₈₀ and N₂₇₀)the K fertilizer recovery increased by 42.1-51.3 percentage points.When ensuring the application of K fertilizer,the rate of N fertilizer used to achieve the regional average yield has been reduced by 25.4%-30.5%.The optimal N and K application rates under the conditions of this experiment are 180 kg N/hm² and 120 kg K₂O/hm²,respectively.（2）The combined application of N and K improved the growth and photosynthetic capacity of oilseed rape canopy,maintaining stable canopy temperature.The photosynthetic area,net photosynthetic rate,and daily range of canopy temperature during each growth period of oilseed rape is significantly correlated with its biomass and yield.For every 0.1 increase in leaf or silique area index during over-wintering,bolting,flowering and podding stage,the yield of oilseed rape can be increased by 67.7,45.8,33.5 and 30.7 kg/hm²,respectively.For every 1μmol m^-2 s^-1 increase in net photosynthetic rate during over-wintering,bolting,and podding stages,the yield of oilseed rape can be increased by 88.3,56.1 and 348.9 kg/hm²,respectively.For every1℃increase in the daily range of canopy temperature during over-wintering,bolting,flowering,and podding stage,the grain yield reduced by 599.1,719.4,509.9 and 369.5kg/hm²,respectively.Compared with the N₀K₀ treatment,the N₁₈₀K₁₂₀ treatment increased the photosynthetic organ area and net photosynthetic rate by an average of80.4%and 66.6%,respectively,and the daily range of canopy temperature decreased by an average of 7.0%during the entire growth period.（3）Under the optimal N fertilizer rate(N₁₈₀),ensuring K supply(K₁₂₀)can improve the leaf structure of the oilseed rape canopy,increase canopy light interception,and coordinate N utilization during the bolting stage.Compared with N₀K₀ treatment,N₁₈₀K₁₂₀ treatment reduced the light transmittance of the middle and lower canopy of oilseed rape by 74.6%and 96.4%,respectively,significantly improving the light interception of the canopy.As N supply increases,the canopy light extinction coefficient（K_L）and leaf N content per unit area（SLN）gradually increase,while the N extinction coefficient（K_N）decreases,resulting in a relative deterioration of light and N coordination（K_N/K_L）.However,comparing applying N fertilizer but no K fertilizer,the SLN,K_N and K_N/K_L of oilseed rape increased by 66.0%,43.8%and 15.5%after N and K combined application,respectively and the increase in K_N/K_L is due to the supply of K promoting the transfer of N,especially water-soluble protein N,from the lower leaves to the upper and middle leaves of the canopy,and improving the distribution of SDS-soluble protein N,thereby fully utilizing the N nutritional function and promoting the coordinated utilization of light and N.While the fitting relationship indicates that under the conditions of ensuring N supply(N₁₈₀ and N₂₇₀),K_N/K_L is significantly positively correlated with the net photosynthetic rate and biomass of oilseed rape.In summary,the combined application of N and K improved the photosynthetic capacity of the canopy,optimized the light and N distribution and N allocation in the canopy,thereby promoting the growth of the oilseed rape canopy,improving the yield and fertilizer utilization efficiency of the oilseed rape.