| The reason why soybean seed yield per unit is low and total yield is not stable in Heilongjiang province is that there is short of suitable high-yield cultivation technology and extension speed is low.Narrow row soybean cultivation technology with high density can promote the increase of soybean yield by decreasing row spacing,enlarging seed spacing,rationally increasing planting density and using soybean varieties resistant to lodging.Two soybeans varieties,Hefeng42 and Kennong30 were used to comprehensively study the correlation between root trait and photosynthesis,leaf N-metabolism,leaf senescence physiology,to discuss contributions that soybean root made to leaf C-N metabolism coordination,leaf senescence delay and the increase of grain yield,to confirm the correlation between leaf,root and grain yield,to clarify physiological mechanism of grain yield difference of different soybeans varieties under interaction of fertilizer-N and density.This study was conducted to confirm the optimum treatment for two soybean varieties,the optimum nitrogen fertilizer rate,optimum planting density and grain yield in order to provide theoretical reference for research and extension of narrow-row seeding technology with high density.The results were as follows:Our study showed that there was significant regression relationship between nitrogen fertilizer rate and grain yield at medium density(D2),and the equation is y =-0.2583x2 + 25.125x+ 3023.2.The results showed that the yield for Hefeng42 was 3634.18 kg/hm~2 with the optimum nitrogen rate of 48.64 kg/hm~2.There was also significant regression relationship between nitrogen fertilizer rate and grain yield at medium density(D1),and the equation is y =-0.2473x2 + 23.296x+ 2853.1.The results showed that the yield for Kenong30 was 3401.67 kg/hm~2 with the optimum nitrogen rate of 47.09 kg/hm~2.Our results indicated that there was significant regression relationship for Hefeng42 between planting density and grain yield at medium nitrogen level(N2)and the equation is y =-0.389x2 + 36.258x-477.35.The yield for Hefeng42 was 3675.36 kg/hm~2 with the optimum planting density of 46.60 plants/m~2.Our results also indicated that there was significant regression relationship for Kennong30 between planting density and grain yield at medium nitrogen level(N2),and the equation is y =-0.1725x2 + 13.52 x + 73.827 The yield for Kennong30 was 3387.41 kg/hm~2 with the optimum planting density of 39.19 plants/m~2.Fertilizer-N significantly increased soybean grain yield.Grain yield first increased,and thendecreased with an increase in nitrogen fertilizer rate at the same density level.Medium nitrogen level gave highest grain yield for two soybean varieties at medium and high density level.While low nitrogen level produced highest yield for Hefeng42 at low density level.However,medium nitrogen level had highest yield for Kennong30 and the yield difference between low nitrogen level and medium nitrogen level for Kennong30 was not significant at low density level.The response of Hefeng42 yield to planting density was D2>D3>D1,while the response of Kennong30 to planting density was D1>D2>D3 under the same nitrogen treatment.Moreover,the yield at D2 for Hefeng42 was higher than that at D1 for Kennong30 at the same nitrogen level.Planting density posed significant impact on grain yield of different soybean varieties,and there was significantly interactive effect between nitrogen and density.N2D1 treatment for Kennong30 produced highest yield with 3356.75 kg/hm~2,while N2D2 for Hefeng42 gave the highest yield with 3665.33 kg/hm~2,was 8.42% higher than that of N2D1 treatment for Kennong30.Two soybean varieties responded differently to fertilizer-N and density.The lodging grade for Hefeng42 is grade one(no lodging problem)for all treatments.The lodging grade for Kennong30 is grade one at low density level,grade two(mild lodging problem)at medium density level,and grade three at high level density except for N0D3 treatment.Grain protein content for Kennong30 was higher compared to Hefeng42,while fat content was lower compared with Hefeng42 at the same treatment.Grain protein content and fat-protein content first increased,and then declined with the increase of nitrogen fertilizer rate,while fat content gradually decreased with an increase of fertilizer-N rate at the same density level.The grain protein content,fat content and fat-protein content decreased with the increase in planting density under the same nitrogen level.N2D1 treatment for Hefeng42 produced highest protein content and total fat-protein content,with 43.83%,and 64.28%,respectively,and N0D1 for Hefeng42 gave highest fat content with 21.01%.N2D1 treatment for Kennong30 had highest protein content and total fat-protein content,with 46.45%,and 64.82%,respectively,and N0D1 for Kennong30 produced highest fat content with 18.48%.Two soybean varieties responded significantly to fertilizer-N and density,and there was significant interact effects of fertilizer-N and density on photosynthetic rate at V4 and R2 stages.The photosynthetic rate for Kennong30 at low density was higher compared to Hefeng42 at the developmental,while the photosynthetic rate for Hefeng42 at medium and high density was higher compared to Kennong30.Our results showed that low and medium nitrogen level was beneficial to leaf photosynthesis for Hefeng42 and Kennong30 at the same density level.The optimum planting density for Hefeng42 and Kennong30 leaf photosynthesis was medium density level and low density level at the same nitrogen level,respectively.Leaf photosynthetic indicators such as chlorophyll content,photosynthetic rate,leaf specific weight and leaf area index at R2 and R4 stages was higher than that at V4 stage.There was significantly positive correlation between grain yield and leaf photosynthetic indicators at R2 and R4 stages,showing that an increase in photosynthetic indicators was beneficial to yield increase at R2 and R4 stages.Leaf NRA,GSA,and nitrogen content of different varieties responded significantly to density at the developmental stage,and there was significant interaction between fertilizer and density.Fertilizer-N had significant effects on leaf NRA of different varieties at V4 and R2 stages,while leaf NRA showed no significant response to fertilizer-N at R4 stage.There was significantly positive relationship between leaf nitrogen content,NRA,and yield,while significant positive quadratic parabola in one unknown between GSA and yield w as observed at V4 and R2 stages,Nitrogen fertilizer had significant effects on leaf CAT activity,POD activity,SOD activity,and MDA content for two soybean varieties.The activity of CAT,POD,and SOD at medium density level(D2)for Hefeng42 was highest at the same nitrogen level.While the activity of CAT,POD,and SOD at low density level(D1)for Kennong30 was highest at the same nitrogen level.The response of CAT activity,POD activity,and SOD activity to planting density for Hefeng42 and Kennong30 was medium density>low density>high density and low density>medium density>high density,respectively.However,leaf MDA content showed opposite response to planting density compared to CAT,POD,and SOD.Except at R4 stage for Hefeng42 leaf POD activity,there was significantly positive relationship between CAT activity,POD activity,SOD activity and grain yield in the study.The correlation between MDA content and yield was significantly negative.Nitrogen fertilizer had significant impact on root activity and dry root weight per unit area at the same density level at different stages.The root activity and dry root weight for Kennong30 at low planting density was higher compared to Hefeng42 at V4,R2,and R4 stages.However,the root activity and dry root weight for Kennong30 at medium and high planting density was lower than that for Hefeng42 at V4,R2,and R4 stages.Excessive rate of nitrogen fertilizer inhibited the increase of root nodule number at V4 and R2 stages,while excessive rate of fertilizer-N inhibited the dry root nodule weight at the developmental stage.Fertilizer-N exerted no significant influence on root nodule number and dry root nodule weight for Hefeng42 at R4 stage,while it was opposite for Kennong30.There was significant interaction between nitrogen and density for root activity.The effects of planting density on root nodule per plant and dry root nodule weight per plant was greater compared to nitrogen fertilizer.Our results also showed that there was significantly positive correlation between dry root nodule weight per unit area and grain yield. |
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