Effects of enhanced UV-B radiation on photosynthetic physiology and ultrastructure of leaves in mango (Mangifera indica L.)

Abstract:【Objective】With the deterioration of global climate, the attenuation of stratospheric ozone re- sults in the enhancement of ultraviolet (UV-B) radiation on the surface of the earth, which has a wide impact on the physiology and growth of surface plants. In recent years, there were extensive studies about the adverse effects of enhanced UV-B radiation on the leaf structure and photosynthesis in man- go, but few research have been carried out to compare the impacts of UV-B on mango varieties with different leaf texture. Thus, the objective of this study was to explore the effects of enhanced UV-B ra- diation on the growth and development, photosynthetic physiology, and leaf ultrastructure of two man- go varieties with different leaf texture, so as to provide reference for the selection of mango varieties with UV-B radiation tolerance under dry and hot valley climate.【Methods】The field experiments were conducted in an experimental orchard of Institute of Tropical Eco-agriculture, Yunnan Academy of Ag-ricultural Sciences from November 2018 to March 2020. Two-years-old mango seedlings cultivar San- nianmang and Maqiesu were used as experimental materials to investigate their leaf texture and re- sponse to UV- B treatments. Three UV- B radiation intensities were sett, including natural radiation (CK), and two enhanced UV- B radiation on the basis of natural light, which were 2.5 kJ · m- 2 · d- 1 (T1) and 5.0 kJ · m- 2 · d- 1 (T2), respectively. The outdoor rainproof lamp holder and 40 W ultraviolet lamp were used to simulate and supplement the UV-B radiation. The distance between the lamp and mango canopy was kept at 0.6 m. The mango canopy was irradiated for 6 h every day from 9:00 to 12:00 and 14:00 to 17:00 (irradiation was suspended in rainy days), and the irradiation lasted for 16 months. The measurement indices included the growth and development, photosynthetic pigment content, photosyn- thetic characteristics, light response curve, fluorescence induction kinetic parameters, and leaf ultra- structure.【Results】There were significant differences in growth and development, photosynthetic pig- ment content, photosynthetic characteristics and leaf ultrastructure between the two mango varieties un- der different UV-B radiation intensity. Compared with CK, the increment of the plant height, stem di- ameter and leaf number of Sannianmang significantly increased under T1 and T2 treatments, while the growth of Maqiesu significantly decreased with the enhancement of UV-B radiation intensity. This re- sult indicated that enhanced UV-B radiation could promote the growth of Sannianmang, but could in- hibit the growth of Maqiesu. The contents of chlorophyll a, chlorophyll b, carotenoid and chlorophyll a+b in Sannianmang were not significantly different under T1 treatment from those under the CK treat- ment, but significantly increased under T2 treatment. When the UV-B radiation intensity was enhanced, leaf photosynthetic pigment content of Maqiesu decreased significantly. There was no significant differ- ence in the chlorophyll a/b of Sannianmang and Maqiesu under different UV-B radiation intensities. Compared with CK, T1 treatment increased the net photosynthetic rate (Pn), stomatal conductance (Gs) and water use efficiency (WUE) of Sannianmang by 16.1%, 25.0% and 7.9%, respectively, but there was no significant difference in photosynthetic characteristics between T2 treatment and CK. The Pn,Gs, Ci and Tr of Maqiesu decreased with the enhancement of UV-B radiation intensity, but WUE firstly decreased and then increased with the enhancement of UV-B radiation intensity. There was no signifi- cant difference in the light response curve of Sannianmang under different UV-B radiation intensities, but the difference was significant in that of Maqiesu when photosynthetically active radiation (PAR) was higher than 200 μmol · m- 2 · s- 1. The enhancement of UV- B radiation intensity decreased the Pn of Maqiesu under higher PAR. The initial fluorescence value (F0), maximal fluorescence value (Fm) and variable fluorescence value (Fv) of Sannianmang and Maqiesu significantly decreased with the enhance- ment of UV-B radiation intensity, and T2 treatment had the highest decline range. Under different UV- B radiation intensities, there were no significant differences in the PSII maximum photochemical quan- tum yield (Fv/Fm) and PSII potential photochemical activity (Fv/F0) of Sannianmang, but the Fv/Fm andFv/F0 of Maqiesu significantly decreased with the enhancement of UV-B radiation intensity. The ultra- structure of leaf cells was damaged in varying degrees under the enhanced UV-B radiation. Under T1 treatment, the epicuticular wax increased, stomata atrophied, chloroplasts tended to be oval, and more osmiophilic particles were accumulated in the leaves of two mango varieties, among them, the leaves of Maqiesu were partly broken, and a large number of black lipid droplets were formed in the cells. The arrangement of cell structure was disordered, the number of black lipid droplets significantly in- creased and the photosynthetic capacity decreased in the leaves of Maqiesu under T2 treatment.【Conclusion】Sannianmang with thicker leaf texture had higher tolerance to the enhanced UV-B radiation, while Maqiesu with thinner leaf texture was sensitive to the enhanced UV-B radiation. Therefore San- nianmang was more suitable for cultivation in the environment with stronger UV-B radiation than Maqiesu.