Studies On The Photosynthetic Physiological Characteristics Of Citrus Sinensis Cv. Newhall Leaves Under Low Light Stress

Citrus is an important commercial fruit crop in the southern China,as well as in the world.With the development of modern protected horticulture,low light is one of the most important factors limiting the photosynthesis,growth,fruit yield and quality of plant in protected cultivation.Plant photosynthesis is one of the most important physiological processes,and photosynthetic rate decreased,photosynthate reduced under low light stress.Therefore the mechanism of citrus under low light conditions need to be studied.In this paper,Citrus(Citrus sinensis cv.Newhall) plants were grown under different light intensities to investigate the changes of growth,mineral content, photosynthesis mechanism,absorbed light allocation and metabolism of active oxygen. The main results were as follows: @1) Decreased light irradiance of treatment was accompanied with an increase of leaf area,but with a significant decrease of specific leaf weight,leaf thickness,spongy tissue, palisade tissue,upper epidermis and lower epidermis thickness.With a decrease of light irradiance,leaf water potential increased,and water use efficiency reduced.The mineral element content was also changed under different light intensities.The content of Potassium,Phosphor,and Magnesium increased,but Calcium and Zinc decreased.At 12%of full sunlight in greenhouse,both leaf nitrogen content per square meter and gram increased,but it reduced nitrogen use efficiency and the partitioning of leaf nitrogen between the different photosynthetic functions.It was shown that the Newhall navel orange plants can adapt the environment of low light on some extent. @2) The results showed that the curves of diurnal change of net photosynthetic rate(Pn) were bimodal,showing an obvious "midday depression" phenomenon at 100%and 34% of full sunlight in greenhouse.The curves of diurnal change of Potential quantum yield of photosystemâ…¡(Fv/Fm) and relative quantum efficiency of PSâ…¡photochemistry(φPSâ…¡) were "V" figure,indicating a phenomenon of photoinhibition,but it was recoveried at 17:00 PM almostly.Non-photochemical quenching(NPQ) increased while Fv/Fm andφPSâ…¡decreased.The curve of Diumal change of Pn was unimodal,diurnal change of Fv/Fm,φPSâ…¡or NPQ did not fluctuate at 12%of full sunlight in greenhouse.With a decrease of light intensity,Diurnal integral of Pn and Transpiration rate(E) decreased significantly,showing that photosynthesis functions of the Newhall Navel orange was restricted.3) low light significantly decreased light saturation point,light compensation point, and maximal photosynthetic rate.Except sucrose,reducing sugar,starch and total non-structural carbohydrates all decreased.Net photosynthesis(Pn) decreased,but Stomatal conductance(Gs) showed no significant change,intercellular CO₂ concentration(Ci) increased.After the treatment for 30 days,the maximum carboxylation rate of Rubisco(V_cmax) and the capacity for ribulose-1,5-bisphosphate regeneration(J_max) also significantly decreased at 12%of full sunlight in greenhouse. With a decrease of light intensity,Fv/Fm,φPSâ…¡,Fm and qP increased.Pn was higher in 2%oxygen than that of 21%oxygen.It is likely that 12%of full sunlight in greenhouse restrained ribulose-1,5-bisphosphate activity and content.The maximum carboxylation rate of Rubisco(V_cmax) and the capacity for ribulose-1,5-bisphosphate regeneration(J_max) decreased.4) Chlorophyll a,chlorphyll b and carotenoid increased at 34%and 12%of full sunlight in greenhouse.Ratio of chlorophyll a and chlorphyll b decreased,it was shown that capture light capability of Newhall navel orange increased,especially the utilizing capability of shortwave light.It was shown that imbalance existed about distribution of excited energy at 12%of full sunlight in greenhouse,the excited energy distributed to PSâ… increased.The decrease in light intensity was consistent with an increase of the allocation of light absorbed by PSâ…¡antennae to the photochemical reaction,and a decrease of the allocation of absorbed light to thermal dissipation and fluorescence dissipation.Compared with the control,the photochemistry electron flow of low light treatment plants decreased significantly.The excitation pressure(1-qP) and non-photochemical quenching(NPQ) reduced at 34%and 12%of full sunlight in greenhouse.5) Compared with the control plants,activities of SOD,CAT and GR reduced at 34% and 12%of full sunlight in greenhouse significantly,the content of GSH,AsA and AsA/DHA also decreased.APX showed no significant change,but the difference of GSH. AsA content and AsA/DHA between 34%and 12%of full sunlight in greenhouse was not remarkable.Activities of POD and ratio of GSH/GSSH significantly increased at 12%of full sunlight in greenhouse.As light intensity decreased,O₂^? content increased at 34%of full sunlight in greenhouse,reduced at 12%of full sunlight in greenhouse. Hydrogen peroxide(H₂O₂) content decreased,but MDA content significantly increased. The results indicated that plants of the Newhall navel orange growing at 34%and 12%of full sunlight in greenhouse had been damaged.