Studies On The Effects Of Different Leaf Nitrogen Content On Photosynthesis Under Steady-state And Fluctuating Light Environments In Brassica Napus L.

As the world's third-largest oil crop,canola is widely cultivated all over the world.Due to the continuous increase of population and the decrease of arable land,human consumption and industrial demand for canola have increased.It is essential to increase the yield of oilseeds by cultivating new varieties and optimizing cultivation measures.The yield of canola is mainly determined by the biomass and harvest index.Although the harvest index of canola has room for improvement compared with wheat and rice,it is considered that the energy contained in canola oil is much greater than that of cereal crops.The energy of starch and the energy harvest index of canola is already close to cereal crops,so to further increase the yield,we need to increase the dry matter accumulation of canola.Most of the biomass accumulation of crops comes from the photosynthesis of crop leaves,so increasing the photosynthetic rate of crop leaves is considered an effective way to enhance crop yields in the future.This dissertation first explored the variation of leaf biochemical characteristics and photosynthetic rate among canola genotypes and their relationships,providing a theoretical basis for canola breeding with high photosynthetic efficiency.Secondly,the light environment under natural conditions is always in dynamic fluctuations,and the response rate of leaves to fluctuating light will affect its total carbon assimilation efficiency and water use efficiency in a day.Therefore,another purpose of this study is to explore the response mechanism of canola leaves to the change of light intensity.According to the above research purposes,four pot experiments were conducted:(1)The leaf net photosynthetic rate,CO₂ diffusion efficiency,biochemical characteristics,nitrogen content,and stomatal morphology was investigated in 22 canola varieties;(2)The relationship between leaf nitrogen content and the response rate of photosynthesis to changes in light intensity was investigated using eight canola lines showing a large variation in leaf nitrogen content;(3)The effects of nitrogen fertilizer application rates on steady-state photosynthesis,dynamic photosynthesis,and nocturnal transpiration rate were investigated using Huayouza 9 widely cultivated in the Yangtze River Basin;(4)The effects of nitrogen fertilizer application rate on leaf net photosynthetic rate,CO₂ diffusion efficiency,and functional nitrogen allocation.The main results are as follows:1.The genotypic variation of photosynthetic rate is mainly determined by stomatal conductance and mesophyll conductance.Under the same fertilizer supply conditions,the leaf net photosynthetic rate varied widely among 22 canola genotypes.The lowest value of Zhenyou 5 was 25.2?mol m^-2 s^-1,and the largest that of SWU102 was as high as 35.7?mol m^-2 s^-1.The total leaf nitrogen content,Rubisco enzyme content,Rubisco enzyme maximum carboxylation rate,chloroplast maximum electron transfer rate,stomatal conductance,and mesophyll conductance all showed significant varietal differences.Further analysis indicated that the correlations between net photosynthetic rate and diffusion efficiency of CO₂ were observed;however,there was no relationship between net photosynthetic rate and leaf nitrogen content based on leaf area.These results suggested that the variation of net photosynthetic rate among canola genotypes is mainly caused by the difference in diffusion efficiency of CO₂ rather than leaf nitrogen content.Besides,the leaf photosynthetic nitrogen use efficiency is significantly positively correlated with the distribution ratio of nitrogen to the carboxylation system and electron transport system,but not closely related to the distribution ratio of nitrogen to the light interception system.2.The photosynthetic response rate to light intensity change was positively correlated with leaf nitrogen content.There was a considerable genotypic variation in the response rate of photosynthesis to a light intensity change.After the light intensity changed from 0?mol m^-2 s^-1to 1000?mol m^-2 s^-1,the time required for the photosynthetic rate to reach 50% of the maximum photosynthetic rate(T_A50)varied from 2.6 min for Quanzi to 5.9 min for Zheyou 21.Besides,the leaf main mineral element content,the initial stomatal conductance under darkness,the response rate of the stomata,the lag time of the initial stage of the stomatal response,and the stomatal morphology showed widely varietal difference.The strong correlations of T_A50 with leaf nitrogen content,Rubisco enzyme content,and stomatal conductance under darkness were observed;however,there was no relationship between T_A50 and the lag time of the initial stage of stomatal response.3.Nitrogen application rate mainly affected the photosynthetic rate of mature leaves but have little effect on the photosynthetic rate of young leaves.Nitrogen application rate had little effect on the nitrogen content of the third leaf from the top(T3),but high nitrogen treatment can significantly increase the nitrogen content of the sixth leaf from the top(T6).Under low light conditions,nitrogen fertilizer treatments had no significant effect on the net photosynthetic rate of both two leaf positions.In contrast,under saturated light intensity,although the net photosynthetic rate of T3 was not significantly different under the two nitrogen fertilizer treatments,the net photosynthetic rate of T6 under high nitrogen treatment was markedly higher than that of low nitrogen treatment, indicating that nitrogen application rates mainly affect the photosynthetic rate of mature leaves.Besides,there was no difference in photosynthetic response rate to light intensity changes under two nitrogen fertilizer treatments,which may be due to the higher stomatal conductance of low nitrogen treatment plants under weak light,which made up for the deficiency of insufficient leaf nitrogen content.There was an apparent internal rhythm in the nocturnal transpiration,which gradually increased from 0.2 mmol m^-2 s^-1 at the early night to 2.0 mmol m^-2 s^-1 at the dawn of the next day.Compared with the high-nitrogen treatment,the low nitrogen treatment had a faster rate of increase in nocturnal transpiration.Besides,there was a significant positive correlation between leaf stomatal conductance at dawn and steady-state stomatal conductance under low light conditions.And varieties with higher stomatal conductance at dawn can increase their photosynthetic rate faster in the daytime.4.When the leaf nitrogen content of canola was higher than 2.12 g m^-2,the photosynthetic rate remained constant.The net photosynthetic rate,photosynthetic nitrogen content,and mesophyll conductance of the two genotypes of canola increased with the increase of nitrogen supply.Under low nitrogen treatment,the genotype Quanzi with higher leaf nitrogen content had higher photosynthetic nitrogen allocation ratio,mesophyll conductance,and the net photosynthetic rate than genotype Zheyou 21, showing lower leaf nitrogen content;under high nitrogen treatment,the leaf nitrogen content of Quanzi was 51% higher than that of Zheyou 21,but there was no significant difference in the net photosynthetic rate between these two genotypes,that is,the photosynthetic nitrogen use efficiency of Quanzi under high nitrogen treatment was significantly lower than that of Zheyou 21.The reason,we found that when the leaf nitrogen content was less than 2.12 g m^-2,net photosynthetic rate and leaf nitrogen content were significantly positively correlated,ant then with the increase of leaf nitrogen content,the net photosynthetic rate no longer increased but maintained a maximum value of 22.6?mol m^-2 s^-1;meanwhile,the leaves tended to increase the ratio of storage nitrogen.Based on the above research results,we believe that exceeding a certain range of leaf nitrogen content will prompt plants to increase the storage nitrogen distribution ratio and reduce the photosynthetic nitrogen distribution,which is not conducive to the effective utilization of nitrogen,but also leads to a decline in plant biomass and yield.Therefore,within a certain range,choosing canola varieties with relatively low leaf nitrogen content and high mesophyll conductance will increase the possibility of cultivating high photosynthetic canola varieties.For the same reason,planting varieties with relatively low leaf nitrogen content is expected to appropriately reduce the application of nitrogen fertilizers and improve nitrogen use efficiency while ensuring high yields.