Study On The Reasons For Photosynthesis Of ’Guifei’ Mango Leaves Promoted By Enhanced UV-B Radiation

Ultraviolet radiation B area is also known as UV-B radiation,the wavelength is 280～320 nm.Most UV-B radiation is absorbed by the ozone layer and a few will reach the earth surface.In recent years,due to the influence of human activities,a large number of chemical substances such as chlorofluorocarbons have been produced,resulting in the thinning of the atmospheric ozone layer.Therefore,the UV-B radiation reaching the surface increases At this time,the UV-B radiation is called“enhanced UV-B radiation”.Although UV-B radiation is a secondary component of sunlight,it has disproportionate effects on higher plants（such as morphogenesis,growth and development,photosynthesis,secondary metabolism,etc.）.Therefore,in view of the potential harm of enhanced UV-B radiation in the future,the effects of enhanced UV-B radiation on adult trees of‘Guifei’mango（Mangifera indica Linn cv.Gu Ifei）were artificially simulated in the field.The natural light was used as the control,and the enhanced UV-B radiation of 96 kJ·m^-2·d^-1was used as the treatment.The changes in fruit maturity,fruit yield and quality,leaf microstructure,photosynthetic rate,leaf photochemical reaction,leaf photosynthetic key enzyme activity and gene expression level were observed.To explore the response characteristics of‘Guifei’mango to enhanced UV-B radiation,and provide a theoretical basis for the future development of cultivation techniques for mango and other crops to adapt to enhanced UV-B radiation.The findings are as follows:1.96 kJ·m^-2·d^-1enhanced UV-B radiation treatment increased the fruit ripening rate of early harvest batches,reduced the single fruit weight,and there was no significant difference in the yield per crown volume,indicating that the radiation treatment would lead to fruit smaller,but did not cause yield reduction in orchards.The fruit quality indexes of the first batch of harvested fruits were significantly higher than those of the control except titratable acid content,and the titratable acid content was significantly lower than that in the control.There was no significant difference in the fruit quality between the treatment and the control in the second batch.The Vc content of the third batch of harvested fruits in the treatment was also significantly higher than that in the control,and there was no significant difference in the quality index between the treatment and the control.The results showed that enhanced UV-B radiation treatment improved the nutritional flavor quality of‘Guifei’mango fruit.2.The net photosynthetic rate（Pn）and intercellular CO₂concentration（Ci）of96kJ·m^-2·d^-1enhanced UV-B radiation treatment were significantly higher than the control on 22 April,29 April and 7 May,the transpiration rate（Tr）was significantly higher than the control on 29 April and 22 May,and the stomatal conductance（Gs）was significantly higher than the control on 7 May;there was no significant difference in photosynthetic physiological indexes between treatment and control.In conclusion,enhanced UV-B radiation treatment may promote leaf photosynthesis and transpiration by increasing Gs.3.There was no significant difference in stomatal length,stomatal width,stomatal area and stomatal density between 96 kJ·m^-2·d^-1enhanced UV-B radiation treatment and control,indicating that enhanced UV-B radiation treatment did not affect stomatal morphology and distribution density.4.The thickness of the upper cuticle,spongy tissue,lower epidermis and lower cuticle of the leaves treated with 96 kJ·m^-2·d^-1enhanced UV-B radiation had no significant change,and the upper epidermis was thickened.The long columnar cells in the palisade tissue were longer,the arrangement compactness increased by 8.49%,and the palisade tissue thickened by 30.5μm;porosity of sponge tissue decreased by 6.43%;leaf blade thicker than 36μm,increased by 18%.It is suggested that enhanced UV-B radiation may promote photosynthesis by improving the morphology and structure of leaf photosynthetic tissue.5.The values of chlorophyll a,chlorophyll b,chlorophyll a+b,chlorophyll a/b,carotenoid content,Hill reaction activity and photochemical quenching coefficient of 96kJ·m^-2·d^-1enhanced UV-B radiation treatment were significantly higher than those of the control.It indicated that enhanced UV-B radiation treatment may promote the leaf photoreaction.6.96 kJ·m^-2·d^-1enhanced UV-B radiation significantly enhanced the activities of Rubisco activation enzyme（RCA）and ribulose-1,5-diphosphate carboxylase/oxidase（Rubisco）in leaves,and up-regulated the gene expression of large subunit gene（rbc L）and small subunit gene（rbc S）of Rubisco.Therefore,enhanced UV-B radiation treatment promoted leaf dark response by up-regulating the expression levels of key enzyme genes of photosynthetic carbon assimilation in‘Guifei’mango,thus promoting leaf photosynthesis.In conclusion,96 kJ·m^-2·d^-1enhanced UV-B radiation treatment can improve the photosynthetic pigment content and microstructure of adult leaves of‘Guifei’mango,enhance the ability of light energy capture,hydrolysis,electron transfer and light energy conversion,promote light reaction and form stronger assimilation;the activity of photosynthetic key enzyme was increased by increasing the expression of photosynthetic key enzyme genes such as rbc L and rbc S,thereby enhancing the CO₂fixation ability of leaves.Based on these two aspects,enhanced UV-B radiation treatment enhanced the photosynthesis of‘Guifei’mango leaves,promoted the rapid accumulation of photosynthetic products,and finally made’Guifei’mango mature earlier,and improved the fruit quality of early harvest batches.