| Crop yield is derived from photosynthesis,improving crop photosynthesis efficiency is an effective way to increase yield,in order to alleviate food crisis.The photosynthetic rate of crops is affected by crop varieties,ecological factors,water and nutrient availability and other factors.The nitrogen is the most critical mineral nutrition for photosynthesis,and nitrogen application plays important roles in yield increase.The response mechanism of photosynthesis to nitrogen is the basis for improving nitrogen utilization efficiency and increasing yield.Tobacco is an important leafy cash crop and its yield and quality are greatly affected by nitrogen application.In this study,the effects of nitrogen level on biomass or yield by affecting photosynthesis process were investigated in Nicotiana Tabacum L.The light harvesting,electron transfer and carboxylation reaction process of photosynthesis in tobacco leaves were investigated in different nitrogen levels,and the physiological mechanism of nitrogen levels affecting photosynthesis performance of tobacco leaves was clarified.The following results are obtained:(1)The total chlorophyll content and net photosynthetic rate in the newly developed leaves of tobacco seedlings showed differences between nitrogen levels at 8h,and the stable difference appeared at 9d after the treatments.The chlorophyll content in the mature functional leaves appeared from 4h to 3d,the net photosynthetic rate was significantly different at 9d;stable difference in aboveground biomass appeared at 9d,and the stable difference in root biomass appeared at 6d after treatments.Thus,with the comprehensive consideration,it took at least 9d to appear the stable differences of biomass and photosynthetic and physiological characteristics of tobacco seedlings after treatment.The response of tobacco seedlings to a series of nitrogen levels showed that the optimal nitrogen levels for biomass accumulation in roots and leaves ranged from 4-6 mmol mmol/L.The optimal nitrogen levels of stem biomass accumulation were 4 mmol/L,followed by 8 to 14 mmol/L.There were significant differences between the total chlorophyll and chlorophyll a contents in the nitrogen level ranges of 0.8-1.6 mmol/L,2.4-12 mmol/L and 14-20 mmol/L.According to the sensitivity of each charateristics to the nitrogen levels,the optimal ranges of nitrogen levels for the biomass accumulation of tobacco seedlings was obtained through analysis of classification:2.4-8 mmol/L;both the nitrogen leves below 1.6 mmol/L and above 12 mmol/L could inhibite grownth.(2)The biomass of roots,stems and leaves was significantly lower in higher nitrogen(HN)and lower nitrogen(LN)than that of control group(MN),in the leaves of tobacco seedlings,and that of LN was significantly lower than that of HN.The net photosynthetic rate of leaves was significantly different at different nitrogen levels:HN>MN>LN in low light and MN>HN>LN under higher light.The higher apparent quantum efficiency and lower light compensation point of the HN indicated that leaves of HN could effectively use lower light.The stomatal limitation value Ls of HN was significantly higher,and the carboxylation efficiency of LN treatment was significantly lower,indicating that HN had obvious stomatal limitation,while LN had obvious mesophyll limitation.The light harvesting capacity,antenna conversion efficiency,photochemical efficiency of PSII and carboxylation rate in LN were significantly lower than MN,which might be the reasons for the lower net photosynthetic rates in LN.The lower conversion efficiency of antenna and PSII photochemical efficiency of HN might be the reasons for the inhibited photosynthetic rate.The distribution ratio of nitrogen in the light-harvesting system(PNp)in the leaves of HN is about 1.2 times higher than that of MN,while that in the electron transport system(PNe)is about 50%of MN,and that in the carboxylation system(PNc)is also significantly lower in LN than that in MN.Compared with MN,PNp increased by 36.7%,while the PNe and PNc decreased by 44.3%and 20.9%,respectively.(3)The leaf chlorophyll content in HN was significantly higher than MN,the synthesis of chlorophyll b was accelerated in HN,while the content of chlorophyll a was not significantly different from that of MN.The total chlorophyll conten of LN decreased significantly,the content of chlorophyll a decreased significantly,but there was a little rise of the chlorophyll b content.The expression of Hem A1 gene was strongly related to the total chlorophyll content and increased with the nitrogen levels.The expression levels of CAOl in HN and LN were significantly increased,resulting in the increased proportion of chlorophyll b.(4)The maximum photosynthetic rate of leaves increased with the nitrogen levels,resulted in the increasing harvested biomass in the field conditions.PNc in leaves was positively correlated with nitrogen application level,while PNe showed no significant difference between the nitrogen levels,indicating that nitrogen application level in fields did not inhibit tobacco growth in our study.This study provided a basis for the physiological mechanism of tobacco photosynthetic response to nitrogen levels,and provided references for tobacco nitrogen application and field management. |
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