Effect Of Leaf Chlorophyll Content On Photosynthetic Physiology And Grain Yield Of Rice

Population increase and economic development have brought great pressure to the increase in grain production under the situation of decrease in the existing arable land,and the crop yield per unit area in China must be increased to meet the security of food supply.Rice is one of the most important food crops in China,and the increase of rice yield is especially urgent.Photosynthesis is the most important way to produce dry matter,which is an important factor to determine rice yield potential.Therefore,how to increase crop photosynthesis has become a research hotpoint.In this study,a rice mutant with low chlorophyll content(YL)and its wild type(WT,Zhefu 802)were used as materials,and they were grown in the field and pots.Three treatments,nitrogen fertilizer rate,planting density and light intensity,were conducted.Grain yield and its components,some agronomic characters,photosynthetic characteristics,carbon dioxide conductance,chloroplast development,photoinhibition and antioxidant mechanism of mutant rice and its control were investigated systematically.Main results are as follows:1.Leaf area and grain yield Compared with those of WT,the number of tillers and leaf area index(LAI)of YL were lower,but the percentage of productive tillers and spikelets per panicle of YL were significantly greater or more.With,the increase of planting density,the leaf area index and grain yield were increased,with more increases in YL than in WT.2.Leaf photosynthetic traits(1)Compared with that of WT,the chlorophyll content of YL was only 50%,but the light harvesting capacity accounted for more than 80%.(2)Fluorescence data indicated that decreased chlorophyll content in YL resulted in the increases in exposure level of reaction center and actual efficiency in PSII,leading to a decreased thermal dissipation in YL.(3)The ultrastructure of chloroplast and thylakoid membrane protein gene expression showed that lower chlorophyll content did not affect the electron transfer chain.(4)Compared with that of WT,increase in the photosynthetic rate in YL was much larger than the increase in leaf N content.(5)The stomatal conductance,mesophyll conductance and rubisco enzyme content in YL were significantly higher than those in WT.The decrease of chlorophyll content in the leaves for YL resulted in the redistribution of nitrogen in the photosynthetic system,which increased the content of Rubisco.In addition,the developed vascular bundle structure and the high expression level of aquaporins genes in the mutant contribute to a higher CO2 conductance,and ultimately improve the photosynthetic rate.3.Photosynthetic characteristics of population compared with WT,the mutant had a higher canopy photosynthetic capacity.(1)The decrease in chlorophyll content in the leaves of YL improved the light transmittance,which could be beneficial to the distribution of light in the canopy and increase in the photosynthesis of population.(2)The higher transpiration rate and less heat dissipationin in YL could have a larger buffering capacity to canopy temperature,which could automatically adjust the temperature of the flag leaf,maintain a higher leaf photostynthesis,reduce photosynthetic "midday depression" at summer noon due to the high temperature,increase the stomatal opening time,and maintain a high-efficiency of photosynthetic operation of canopy for a long time.4.Photoinhibition WT leaves absorbed excess light energy,and the phenomenon of light suppression was more serious.(1)Compared with WT,YL produced less superoxide anion and malodialdehyde(MDA)under high irradiance,which could prevent photosystem I from some damage.(2)Under high light intensity,the activities of superoxide dismutase(SOD)and catalase(CAT)in the leaves of YL were significantly higher than those of the WT.The results suggest that a high chlorophyll content is not necessary for a high photosynthetic rate of leaves.Appropriate reduction in chlorophyll content in rice leaves could benefit to reduce the consumption of nitrogen used in the chlorophyll synthesis process,optimize nitrogen distribution in the blade,improve the efficiency of photosystem II and alleviate the photoinhibition,and consequently,increase the leaf photosynthetic rate.The leaf photosynthesis would not be affected if the chlorophyll content is decreased though reducing the input of nitrogen fertilizer and adjusting the nitrogen application proportion,which could increase grain yield and nitrogen use efficiency in rice.