Effects of magnesium on chlorophyll fluorescence and active oxygen scavenging system of fruiting-node leaves of muskmelon

【Objectives】 Muskmelon leaves are often premature senescent at the fruit ripening stage, especially the leaves located on the fruiting-node, seriously decreasing the functions of photosynthetic system and fruit growth. The effects of magnesium on improving function of chlorophyll fluorescence and active oxygen scavenging system were studied. 【Methods】 Muskmelon cultivar ‘Yingchun(F1)’ was used as test material and sprayed with MgSO4 solution of 0, 2, 4 and 6 mmol/L on the day when female flowers on fruiting-nodes were opening and being hand-pollinated. The sprays were made once a week and 3 sprays in all. Every other week, the physiological indexes of leaves on fruiting-nodes were determined after the first spray. The chlorophyll fluorescence and photosynthesis characteristics were measured using imaging fluorescence spectrometer MINI-IMAGING-PAM and portable photosynthesis system Li-6400, the leaf Mg content by ICP-OES method, and the enzyme activities and the contents of malondialdehyde(MDA), soluble sugar and soluble protein in the fruits were measured referring to the LI He-sheng's experiment method, the soluble solids with a handheld sugar meter, and the Vc content by 2, 6-dichloro phenol indophenol titration method. 【Results】 From analyzing physiological indexes of leaves on fruiting-nodes and fruits, the following results can be obtained: the spray of 2 mmol/L and 4 mmol/L Mg2+ could significantly increase the Mg levels in fruiting-node leaves, and delay the degradation rate of chlorophyll during the process of leaf senescence at the same time. Some chlorophyll fluorescence and photosynthesis characteristic parameters, including the maximal photochemical efficiency(Fv/Fm), quantum yield of PSII electron transport(ФPSⅡ), apparent electron transport rate(ETR)and net photosynthetic rate are promoted at various degrees under the 2 mmol/L and 4 mmol/L Mg2+ treatments, while the minimal fluorescence(Fo) is decreased dramatically. The antioxidant enzymes activities are also significantly improved, while the accumulation of active oxygen and MDA content are reduced. Additionally, the treatments mentioned above increase the accumulation of soluble sugar and soluble solids, and the contents of Vc and soluble protein in varying degrees compared with the control, but there is no significant effect on single fruit weight. 【Conclusions】 The treatments with proper concentration of Mg2+ could increase the contents of Mg and chlorophyll of leaves on fruiting-nodes in muskmelon, improve chlorophyll fluorescence parameters and net photosynthesis rate, heighten the activities of POD, SOD and CAT and decrease the contents of H2O2, O-·2 and MDA, so that postpone the leaf senescence on fruiting-nodes and enhance the fruit quality indicators of the soluble sugar, soluble solids, Vc and soluble protein, but have no prominent effect on fruit yield.