Nitrogen Absorption, Utilization And Related Enzymatic Activity In Soybean

The experiment was conducted with frame tests(pot without bottom),sand culture and tissue culture during the period from 2006 to 2007 year.Labeled ¹⁵N was used in the frame test and the experiment design was as following:genotype and nitrogen were chosen as the main factors,and five genotypes were chosen(high protein,high oil,high yield,forage and root noduleless soybean), and three nitrogen levels,low,mid and high,were chosen.One genotype was used to study the nitrogen source and accumulation in soybean,determine the activity of nitrate reductase and glutamine synthetase;Labeled ¹⁵N fertilizer was used in sand culture,the nitrogen source and composition,the activity of glutamine synthetase and nitrate reductase,and the mRNA content induced by nitrate nitrogen were mensurated;In the tissue culture experiment,Labeled ¹⁵N ammonium sulfate or potassium nitrate and mixture of them were used as nitrogen source to mensurate the ratio of NH₄⁺-N:NO₃^--N which were absorbed by plants.The results were shown as following:Nitrogen uptake of soybean could be divided into there periods,there was evidence difference between cultivated and forage soybean:(1) Inorganic nitrogen period,when the plants absorbed mainly soil-N and fertilizer-N.The period of cultivated soybean began from seedling to beginning bloom(R₁).The proportion of soil-N and fertilizer-N was between 83.8%～85.2%and 12.7%～15.0%respectively in R₁,and the nodulation-N was below 3.0%at this stage;The period of MSD began from seedling to V₄ period,the proportion of soil-N,fertilizer-N and nodulation-N was 75.4%,15.7%and 8.9%in V₄.(2) Nodulation-N fast increasing period,when the plants absorbs mainly soil-N and nodulation-N,nodulation-N increased fast.During the period from R₁ to R₅, nodulation-N accumulated fast and the proportion increased in cultivars,both soil-N and fertilizer-N accumulations still increased,but their proportions in plant decreased;The proportions of soil-N and nodulation-N in cultivation soybean were between 60.0%～67.9%and 28.3%～36.3%respectively in R₅,while that of fertilizer-N was less than 4.0%;The period of MSD was from V₄ to R₅,the proportion of nodulation-N already achieved 69.1%in R₅,soil-N was 29.7%, fertilizer-N was only 1.2%.(3) soil-N and nodulation-N synchronization absorption period,the plants absorbed both soil-N and nodulation-N,cultivation soybean began from beginning size stage to full maturity stage(R₅—R₈),when the nodulation-N and soil-N still had the massive accumulations,and the accumulation of fertilizer-N stagnated;The proportions of soil-N and nodulation-N were between 39.3%～60.5%and 37.5%～59.2%respectively in R₈,and fertilizer-N was less than 2.0%;In this period,MSD absorbed mainly nodulation-N,the accumulation of soil-N and fertilizer-N were few.The proportion ofnodulation-N already reached as high as 71.4%in R₈.N-level used had a significant effect on N-absorption and N-accumulation of soybean. (1)Under frame conditions,the order of nitrogen accumulative amount of DN47 is low-N＞mid-N＞high-N,the difference reached significant level;the order of nodulation-N accumulative amount is low-N＞mid-N＞high-N,and the nodulation-N accumulative amount of low-N was significantly higher than that of mid-N and high-N,but there was no significant difference between mid-N and high-N.(2)Under sand culture,the proportion of fertilizer-N increased as nitrogen level heightened, but that of nodulation-N was on the contrary;Applying higher N level during these periods:from seedling to V₃,seedling to R₁,and seedling to R₂,then applied lower nitrogen level during the periods from V₃ to R₈,R₁ to R₈,and R₂ to R₈,accumulative nodulation-N amount decreased significantly as the higher nitrogen level period prolonged,but that of fertilizer-N had little difference;Applying lower nitrogen level during the periods:from seedling to R₄,and seedling to R₅,then applied higher nitrogen level during the periods from R₄—R₈,and R₅—R₈,accumulative nitrogen amount were higher than that of lower-N treatment in whole period,the accumulative nitrogen amount of the treatment that applying higher nitrogen level during the period from R₅—R₈ was significantly higher than that of the period from R₄—R₈,but there was no difference between accumulative nodulation-N amount of the two treatments.Yield of soybean was affected by N levels(2) Under frame condition,the order of yield of DN47 is low-N＞mid-N＞high-N,and the yield of low-N and mid-N was significantly higher than that of high-N,but there was no significant difference between low-N and mid-N.(2) Under sand culture,the yield of low-N was lower because nitrogen requirement could not be fulfilled,higher-N could not reach high yield became of the inhibition of higher nitrogen on the growth of nodules. Applying higher N level during these periods:from seedling to V₃,seedling to R₁,and seedling to R₂,then applied lower nitrogen level during the periods from V₃ to R₈,R₁ to R₈,and R₂ to R₈,the yield decreased as the higher nitrogen level period prolonged;applying lower nitrogen level during the periods:from seedling to R₄,and seedling to R₅,then applied higher nitrogen level during the periods from R₄—R₈,and R₅—R₈,the yield was higher than that of lower-N in whole period,but there was no significant difference,which indicated that heightened exogenous nitrogen level between R₄ and R₅ and lower nitrogen level in prophase could increase yield of soybean.The regression of yield and proportion of fertilizer-N in seed,yield and proportion of nodulation-N in seed were analyzed,and regression equations were y=-0.017x²+1.329x+25.294 and y=-0.017x²+2.091x-12.828 respectively.The two regression equation provided a better fit,and the result of derivation analysis shown that highest yield could be obtained when the proportion of soil-N and nodulation-N was 39.1%and 61.5%respectively.We could deduce:when the contribution rate of exogenous nitrogen and nodulation-N were about 40%and 60%in seeds respectively,these could satisfy the requirement of soybean and benefit to soybean yield formation. There were a difference between ammonium and nitrate nutrition,and more ammonium was absorbed by soybean than nitrate.Tissue culture in sterile condition was conducted and the result shown that when the ratio of NH₄⁺-N:NO₃^--N in culture medium was 1:1,the rates of NH₄⁺-N: NO₃^--N absorbed by soybean were approximately 1.5:1,and the ratios of NH₄⁺-N and NO₃^--N absorbed by leaf,stem and root were 1.6:1,1.4:1 and 1.6:1.The activity of nitrate reductase(NR) exhibited similarly among different varieties,higher in tender leaf and lower in old leaf,but there were some difference of the activity dynamic of nitrate reductase.Gene of iNR could be induced by NO₃^-,accordingly mRNA of iNR1and iNR2 isoenzymes could be enhanced.The activity of glutamine synthetase(GS) exhibited similarly among different varieties,higher in tender and middle leaves and lower in low leaves.The change trends of activity of glutarnine synthetase in different parts were similar,but there was a great difference among different years.