Effects of different Magnesium concentrations on chloroplast ultrastructure and photosynthetic response of Vitis vinifera cv.Red Globe

Chloroplast ultrastructure and photosynthetic response of Red Globe grape(Vitis vinifera L.cv.) leaves were studied under different concentrations of Mg2+(0 mmol·L-1,1 mmol·L-1,3 mmol·L-1,5 mmol·L-1)with sandy culture method.The results indicated that in the treatment of 3 mmol·L-1 Mg2+,the chloroplast had normal shape,containing lots of big starch grains,and the structures of grana and stroma were clear.No obvious change of the chloroplast ultrastructure in the 5 mmol·L-1 treatment was observed,while in the 0 mmol·L-1 and 1 mmol·L-1 treatment,a series of chloroplast configuration changed,and the structures of granum and stroma lamellae were destroyed.Additionally,the concentration of magnesium significantly affected chlorophyll content and photosynthetic parameters.The contents of chlorophyll a(Chla),chlorophyll b(Chlb) and total chlorophyll(Chl(a+b)),the light saturation point(LSP),the maximum net photosynthetic rate(Pnmax),apparent quantum yield(AQY) and carboxylation efficiency(CE) were significantly lower in the 0 mmol·L-1 Mg2+ treatment than in magnesium treatments,and they increased and then declined with the increase of magnesium concentration.The change tendency of the ratio of Chl a/Chl b,light compensation point(LCP),CO2 compensation points(CCP) and CO2 saturation point(CSP) was opposite.The turn point of increase or decrease was at the treatment of Mg2+ 3 mmol·L-1.Therefore,the ultrastructure of chloroplast was aberrant,the metabolism of chlorophyll and carbon was inhibited and in turn chlorophyll content and photosynthetic efficiency reduced under Mg2+ deficiency while suitable magnesium concentration could improve the light and CO2 utilization ability of grape.