Effects Of Combined Nitrogen And Potassium Application On Brassica Napus Yield And Quality And Its Photosynthetic Physiological Mechanism

Winter oilseed rape（Brassica napus L.）is an important crop in China for producing edible oils.The emphasis on N input while ignoring K fertilization in rapeseed production,coupled with high-intensity multiple cropping and rain heat synchronization in the main producing areas,consumed a large amount of soil K,resulting in stagnant rapeseed yield and limiting the improvement of N fertilizer utilization.Combining N and K applications is one of the important cultivation measures for high-yielding oilseed rape.Understanding the changes and mechanism of oilseed rape photosynthesis under conditions of N and K combination has important theoretical and practical significance for further tapping the yield potential of oilseed rape and improving N utilization efficiency.In this research,the field N-K interaction positioning experiment（containing four N rates:0,90,180,270 kg N/hm²and four K fertilizer dosages:0,60,120,180 kg K₂O/hm²in a complete design composed of sixteen treatments）was used to comprehensively analyze the effects of N-K combined application on the rapeseed yield,quality,growth phenotype,photosynthetic characteristics,N allocation and N turnover to elucidate the mechanisms by which N and K interactions increased rapeseed yield,improved rapeseed seed quality,promoted seedling growth,coordinated leaf N allocation and N turnover to improve leaf photosynthetic capacity and enhanced N fertilizer use efficiency.The main results are as follows:（1）Combining N and K fertilisation significantly increased the yield and N utilization efficiency of winter oilseed rape,and reduced the apparent surplus of N.When K supply was insufficient(K₀and K₆₀),seed yields increased 144.6%and117.2%by an average of Huayouza No.9（H9）and Zhongshuang No.11（Z11）after N application,while under sufficient K supply(K₁₂₀and K₁₈₀)conditions,the average yield increase rates of H9 and Z11 were as high as 212.4%and 199.4%after N fertilisation.In comparison with the treatment without K,the yield of H9 and Z11increased by 18.0%-132.9%and 14.7%-148.1%after K fertilisation,the N fertilizer recovery efficiency improved by 6.5-21.3 and 8.7-21.6 percentage points,and the apparent N surplus decreased by 18.2%-43.2%and 17.0%-35.1%.Compared to the treatment without N and K fertilizers,H9 and Z11 yields increased by153.2%-397.5%and 150.4%-322.9%after the synergistic application of N and K fertilizers,which was much higher than that of single N or K fertilizers.When reaching the same target yield（2500 kg/hm²）,K application reduces N input and apparent N surplus,and the economically optimal N and K fertilizer rates were lower for H9 than for Z11 at 165 kg N/hm²and 103 kg K₂O/hm².（2）Synergistic management of N and K fertilizers regulated the oil content,protein content and fatty acid composition of the seeds by adjusting the N and K nutrient concentration and ratio of the rapeseed.The K concentration of H9 and Z11seeds decreased by 13.6%and 18.5%on average,and resulted in a reduction of 7%and 9%in oil content after N application.The N concentration of H9 and Z11 seeds was decreased by an average of 7.9%and 7.4%,oil content increased by 8.6%and8.1%,and protein content was reduced by 12.3%and 12.0%after K application compared to the treatment without K application.Although the oil content decreased slightly after the combined application of N and K,the oil production of H9 and Z11improved by 1.8-4.1 and 1.9-3.7 times compared with the treatment without N and K.K application significantly increased the oleic acid content but decreased the linoleic and linolenic acid content;N fertilisation increased the monounsaturated fatty acid content and decreased the saturated and polyunsaturated fatty acid content,but there were no significant differences between N application rates.（3）N and K synergistically promoted the growth of oilseed rape at seedling stage,while the imbalance of N and K nutrition and symptoms of K deficiency in leaves were aggravated by heavy N and light K.The biomass,leaf area index and net photosynthetic rate of oilseed rape at seedling stage increased by 108.2%-328.1%,115.1%-243.2%and 68.2%-139.2%respectively after N and K combined application.Under K stress,increasing N fertilizer aggravated the symptoms of K deficiency in leaves.Compared with N₉₀K₀treatment,the proportion of chlorosis area caused by K deficiency in N₁₈₀K₀and N₂₇₀K₀treatments increased by 42.2%and 147.7%.N application promoted oilseed rape growth,amplified the growth dilution effect,and decreased the critical K concentration in leaves.Compared with N₉₀treatment,the biomass of N₁₈₀and N₂₇₀treatments increased by 39.2%and 41.4%,while the critical K concentration in leaves decreased by 38.5%and 45.3%.（4）In combination with N and K expanded the photosynthetic N pool of leaves,improved the N distribution ratio in the photosynthetic system,and greatly enhanced the photosynthetic capacity and photosynthetic N use efficiency（PNUE）of oilseed rape leaves.N application increased leaf N concentration and photosynthetic N pool capacity by 39.3%-155.0%and 14.2%-83.1%,while photosynthetic N share decreased by 4.0-11.0 percentage points,resulting in a 14.9%-31.9%decrease in leaf PNUE.On the contrary,K fertilisation promoted the expression of chlorophyll biosynthesis,photosynthetic electron transport and protein processing-related proteins,which increased the capacity and percentage of photosynthetic N pools by25.1%-43.4%and 3.4-11.4 percentage points and increased PNUE by 18.6%-112.2%.Although PNUE and photosynthetic N allocation ratio were slightly reduced after N-K combined application,photosynthetic area and photosynthetic N pool capacity increased by 134.1%and 77.0%compared to the treatment without N and K fertilizers,which were much higher than the enhancement of the leaf photosynthetic N pool by N fertilizer alone（26.0%）or K fertilizer alone（16.4%）,thus increasing leaf net photosynthetic rate by 63.5%-132.8%.（5）During leaf growth,the synergistic of N and K fertilizers increases stored N content and drives the turnover of stored N to photosynthetic N,which is essential for improving leaf photosynthetic rates and duration.The initial leaf expansion of N deficient rape leaves contained 19.1%less stored N than that of the N treatment,resulting in a 59.1%,19.8%and 18.8%reduction in leaf area,photosynthetic N pool capacity and photosynthetic rate,respectively,at full leaf expansion.Under K nutrient stress,although the stored N content at initial leaf expansion was 8.3%higher than in the K treatment,the turnover of stored N to photosynthetic N at full leaf expansion was 10.0%lower,resulting in an average reduction of 27.7%,19.6%and 15.1%in leaf area,photosynthetic N pool capacity and photosynthetic rate at full leaf expansion.On the contrary,the N and K combination greatly increased the photosynthetic capacity of the leaves by expanding the leaf photosynthetic N pool,promoting the turnover of stored N to photosynthetic N and improving the photosynthetic area while harmonizing the nutrient balance of N and K.In conclusion,the combination of N and K applications harmonized the nutrient balance of N and K,increased leaf photosynthetic area,expanded leaf photosynthetic N pool,optimized the N distribution ratio in the photosynthetic system,drove the turnover of stored N to photosynthetic N and greatly improved the photosynthetic capacity of rape leaves,thus promoting seedling rape growth,increasing rape yield,improving rapeseed quality and enhancing N fertilizer utilization efficiency.