Effects Of UV-B Radiation On Mango Fruit Quality And Leave Photosynthesis And Its Mechanism

Ultraviolet radiation region B（UV-B radiation,wavelength 280³20 nm）is usually absorbed by the ozone layer and only a small amount reaches the ground.In recent years,ozone decreases due to the pollution of chlorofluorocarbons in the stratosphere,which leads to an increase in UV-B radiation,which is called"UV-B radiation enhancement".Because UV-B radiation is easily absorbed by many important macromolecules（such as nucleic acids,proteins,lipids and plant hormones）,even if the dose of UV-B radiation increases slightly,it may have significant biological effects.In view of this,the treatment of artificial simulation of enhanced UV-B radiation(24and 96 kJ·m^-2·d^-1)were conducted with the adult trees of‘Tainong 1’in the field,and the plant yield,fruit quality,physiological damage,antioxidant protection mechanismphotochemical reactions,activities of key enzymes in carbon assimilation and the expression of gene were observed.The characteristics of cultivation performance and photosynthesis changes of mango under enhanced UV-B radiation were studied,which enriched the physiological theories of photosynthesis and light stress resistance of fruit trees,and provided a scientific basis for the formulation of UV-B radiation enhanced mango cultivation techniques in the future.The findings are as follows:1.compared to the control（natural lighting）,there was a decrease of tree yield and soluble sugar and sugar-acid ratio of fruits under the 96 kJ·m^-2·d^-1treatment and no significant changes with 24 kJ·m^-2·d^-1tratment except that vitamin C was significantly higher than the control.2.MDA content and relative electrical conductivity in leaves treated with 96kJ·m^-2·d^-11 UV-B were significantly higher than the control and no significant changes with 24 kJ·m^-2·d^-1tratment.MDA content in pericarp of UV-B radiation enhanced was not significantly different from that of the control,but was significantly higher than the control in flesh.3.The activities of SOD,POD and CAT treated with 96 kJ·m^-2·d^-11 tratment were significantly higher than the control in leaves,but lower than the control in flesh.The activities of POD in pericarp were significantly lower than the control,while SOD and CAT activities no significant changes with the control.The activities of SOD,POD and CAT in leaves and pericarp of 24kJ·m^-2·d^-1treatment were no significant with the control,but the activities of SOD and CAT in flesh were significantly lower than the control,but the activity of POD was not significantly different from he control.4.The contents of polyphenols,flavonoids,reduced GSH and other reduced protective components were significantly higher than control treated with 96kJ·m^-2·d^-1UV-B.The contents of flavonoids and reduced GSH in flesh were significantly lower than the control,while the polyphenol contents was not.The contents of flavonoids and reduced GSH in leaves treated with 24kJ·m^-2·d^-11 were significantly higher than the control.Flavonoids and reduced GSH contents in flesh were significantly lower than the control only at the later ripening stage,and polyphenols contents was almost not affected.The two UV-B radiation enhanced treatments had no significant effect on the content of reducing protective components in mango pericarp.5.The mangiferin content in leaves treated with 96 kJ·m^-2·d^-1was significantly higher than the control,while the Vc content was significantly lower than the control.Unknown compounds（May belong to xanthones）contents in pericarp and Vc contents in pericarp and flesh were significantly higher than the control.There was no significant difference of Vc and mangiferin contents in leaves,Vc contents in pericarp and flesh and unknown compound contents between 24 kJ·m^-2·d^-1treatment and the control.6.Net photosynthetic rate,stomatal conductance,transpiration rate and intercellular CO2 concentration in leaves treated by 96 kJ·m^-2·d^-1were significantly lower than the control,while no significant changes with 24kJ·m^-2·d^-1tratment.7.Chlorophyll a/b value in leaves treated by 96 kJ·m^-2·d^-11 were significantly lower than the control,while photosynthetic pigment contents,the vitality of Hill reaction,the quenching coefficient of photochemistry the activities and of Rubisco were significantly higher than the control.The Hill activity and Rubisco activity treated by 24 kJ·m^-2·d^-11 were significantly higher than the control,while photosynthetic pigment contents showed no significant difference compared to the control.The expression of coding gene of Rubisco big subunit（RbcL）could be inhibited by the 96 kJ·m^-2·d^-1tratment.It is clear that stomatal limitation and non-stomatal limitationcould be induced by 96 kJ·m-2·d-1 treatment directly,and then photosynthesis would be inhibited.96 kJ·m-2·d-1 treatment significantly caused active oxygen damage to the leaves.The antioxidant protection mechanism was activated in the early stage,especially the accumulation of rich mangiferin to reduce active oxygen damage as much as possible.However,in the later stage,damage to the active oxygen in the leaf is caused by the destruction of the antioxidant protection system,which may be damaged by the photosynthetic membrane system in the later stage.As a result,photosynthesis is inhibited,which in turn leads to reduced tree yield and worsened fruit flavor quality.Pericarp is protected from damage by accumulating a xanthone compound which may be the same as mangiferin.In spite of the increased Vc content,the flesh caused damage due to its antioxidant enzyme activities such as SOD,POD,and CAT,and decreased levels of flavonoids and GSH.