Mechanisms Underlying The Effects Of Nitrogen Supply On Leaf Nitrogen-forms Partitioning And Photosynthetic Nitrogen Use Efficiency Of Brassica Napus

About 95% of the plant's dry matter is derived from photosynthesis,and improving photosynthetic capacity is the key to increase crops yield and nitrogen(N)use efficiency.Photosynthetic capacity of leaves is usually positively correlated with N content.Although the photosynthetic area is increased gradually with the increase of N supply,the photosynthetic N use efficiency(PNUE,photosynthetic capacity per unit N)is gradually decreased.Optimization N partitioning in leaves plays an important role in the further improvement of photosynthetic capacity and PNUE.Winter oilseed rape(Brassica napus L.)is an important oil crop in China and it is also a crop with a high N requirement.Maintaining high output levels while reducing N input is crucial to ensure the competitiveness of agriculture,environment,and economy in winter oilseed rape.Based on field experiments,this study investigated the effect of N supply on N partitioning within photosynthetic apparatus and PNUE in winter oilseed rape leaves by using N allocation model.Combined with the chemical grouping method,the dynamic of photosynthetic N and storage N and the turnover mechanisms between the two during the leaf growth were analyzed.The vertical distribution of different forms of N within canopy of two different N partitioning genotypes was further studied,and the key factors affecting the coordination of light and N in canopy were analyzed.The effect of environmental changes on N utilization and growth of winter oilseed rape was initially explored.Overall,the above studies will be helpful to understand in depth the mechanisms underlying the effects of N supply on N-forms partitioning and PNUE.The main results are listed as follows:(1)The decrease in PNUE of winter oilseed rape leaves with the increase of N supply is due to the decline of the fractions of N allocated to photosynthetic apparatus(e.g.,carboxylation,electron transport components and light capture system).The photosynthetic capacity was raised with the increase of N supply,however,PNUE was significantly decreased.Compared with N0 treatment,the fractions of N allocated to photosynthetic apparatus for N180 and N360 treatments were decreased 29.3% and 34.5%,respectively;while the fractions of N allocated to the carboxylation,electron transport components and light capture system were reduced 24.1%,23.3%,34.6% and 31%,26.7%,38.5%,respectively.The correlation analysis indicates that the PNUE had a dramatically positive correlation with the fractions of N allocated to carboxylation and electron transport components,photosynthetic apparatus,however,it was negatively correlated with the non-photosynthetic components.(2)The fractions of N allocated to carboxylation and electron transport components are the major limiting factor of PNUE at high N level.More N was allocated to the photosynthetic apparatus and the photosynthetic N can be degraded earlier in the lower leaves under N deficiency.Suitable N level could maintain the distribution of photosynthetic proteins within the types of their proteins.The influence degrees of the fractions of N allocated to carboxylation and electron transport components on PNUE were 26.8% and 42.6%,which was higher than that of light capture system.The influence of N nutrition on PNUE was dominated by the fractions of N allocated to carboxylation and electron transport components,which the average proportion reached 77.8%.The fraction of N allocated to carboxylation was the principal limiting factor of PNUE in upper leaves,which the influence degree reached 83.3%,while dominated by the fractions of N allocated to electron transport components and light capture system in the lower leaves.(3)Storage N in leaf is critical to coordinate leaf expansion and photosynthetic capacity.Turnover between storage N and photosynthetic N could maintain high photosynthetic rate and duration.Storage N content(Nstore)was increased with the increase of N supply and decreased gradually along with leaf expansion.The relative growth rate based on leaf area was positively correlated with Nstore during leaf expansion.When Nstore was decreased to 1.32 g m-2(1.20 g m-2 for N deficiency and 1.89 g m-2 for N sufficiency),the leaves stopped expanding.Water-soluble protein form of storage N was the main N sources for leaf expansion.After the leaves fully expanded,when Nstore was above 1.83 g m-2,net photosynthetic rate(An)was stabilized at the highest value of 23.9 ?mol m-2 s-1.Once Nstore was below the threshold,An began to decline with the degradation of SDS(Sodium dodecyl sulfate)-soluble protein form of storage N.N supply can improve Nstore,thereby promoting leaf growth and biomass.(4)The partitioning of N in SDS-soluble protein in leaves and its attenuation within the canopy are the main factors affecting the coordination of light and N.With the increase of N supply,the N content of SDS-soluble protein(Ns)in the light leaf color genotype(R5)and the deep leaf color genotype(H9)gradually increased.The average distribution ratio of R5 in Ns for all N treatment was only 14.6%,which was 40.4% lower than that of H9.With the increase of N supply,the total light interception rate and light extinction coefficient(KL)of the canopy gradually increased.The KL of R5 was less than H9 in both years,especially in 2015/2016,which was 9.8% lower than H9.N supply significantly increased the N content in the top leaves of canopy,but decreased the N extinction coefficient(KN).The average KN of R5 for all N treatments was 23.8% higher than that of H9.The coordination of light and N(KN/KL)gradually decreased with the increase of N supply,and the KN/KL of R5 for each treatment was higher than that of H9.There was a significant positive correlation between KN/KL and N utilization efficiency.The average KN for SDS-soluble protein of R5 was 15.4% higher than that of H9.Through local sensitivity analysis,H9 could increase KN by reducing the partitioning of N in SDS-soluble protein.As SDS-soluble protein is mainly related to light capture system on thylakoid membrane,it is helpful to delay the light extinction of canopy and accelerate N decay by reducing the partitioning of SDSsoluble protein in leaves,and thus improving the coordination of light and N and N utilization efficiency.(5)The leaf area and photosynthetic capacity in winter haze was significantly decreased,due to the large amount of storage N in the leaves transferred to light capture,N application could increase the N content of storage and photosynthesis.Winter haze reduced irradiation intensity and stability.The proportion of N allocated to light capture was increased by 14.2% in winter haze than that in the normal year.However,N proportion allocated to storage(decreasing by 9.0%)and carboxylation(decreasing by 3.5%).The proportion of N allocated to storage under N sufficiency was enhanced by 5.8%.Although the proportion of N allocated to carboxylation system was declined,the N content in carboxylation was much higher than that under N deficiency,thereby improving the photosynthetic performance.Increase top dressing of N application could be benefit for plant growth after winter haze.