Study On Regulatory Of Magnesium On Photosynthesis Of Cucumber Under Low Temperature And Low Light Intensity

Low magnesium stress, foliar spray and applying in soil of magnesium on the growth, physiological characteristics and photosynthesis of cucumber(Cucumis sativa L) seedlings under low temperature were investigated with’Jinyou3’plants as experimental materials. The results are as follows:1. Low temperature (day/night12℃/8℃) decreased the growth of cucumber seedlings significantly. The Mg2+content in leaves, stems and roots of cucumber seedlings increased at the initial stage of low temperature stress, but3d later, it decreased rapidly. Chlorophyll content declined with low temperature stress days. Low temperature caused by cold current also decreased the Mg2+content in leaves, but increased in stems and roots. This indicated that magnesium deficiency caused by the hindered transportation under low temperature is the main reason for the chlorosis of cucumber leaves. The30%Mg2+treatment showed a significant less growth, and lower chlorophyll and Mg2+contents in leaves compared with the control. However, foliar spray0.1%Mg2+(+Mg) alleviated the inhibition of growth by low temperature, increased the Mg2+and chlorophyll contents. Applied MgSO4in soil showed the same effect as foliar spray. The optimum amount of MgSO4for the application was1-2kg/667m2.2. Low temperature significantly decreased net photosynthetic rate (Pn), stomatal conductance (Gs), carboxylation efficiency (CE) and RuBP carboxylase activity(RuBPCase), while increased intercelluar CO2concentration (Ci). Low temperature also led to dectrease in actual photochemical efficiency in lumination (φPS II), maximum photochemical efficiency in darkness (Fv/Fm), photochemical quenching (qP) and electron transport rate (ETR), while increase in initial fluorescence (Fo) and non-photochemical quenching (NPQ). These data suggested that the decrease in Pn is mainly caused by the non-stomatal limitation. The damage of PS II reactive center is one of the important machenisms of the reduction in photosythetic function.The30%Mg2+treatment showed a significant lower Pn, Gs CE, and RuBPCase activity in comparison with the control. No markedly change was found in Ci between the30%Mg2+treatment and the control. Foliarspray0.1%Mg2+(+Mg treatment) and applied MgSO4for1-2kg/667m2in soil(M1、M2)decreased the reduction in Pn, Gs, Tr, CE and RuBPCase activity, and showed a significantly increase in φPSII, Fv/Fm, qP and ETR, compared with the control. These data indicated that low magnesium stress resulted in a more seriously damage in photosynthetic apparatus during low temperature, consequently increased the reduction in Pn. It is the important machenism of the effection in photosynthesis.3. Low temperature led to a significant damage in chloroplast membrane, and showed a decrease in chloroplasts, grains, and lamellas, while an increase in starch grains in the cucumber seedlings. The chloroplasts swelled, and the starch grains were larger in size under low temperature. Low magnesium stress resulted in a more seriously damage in chloroplast membrane during low temperature, and showed lower chloroplants, grains, and lamellas, while more and larger starch grains in cucumber seedlings.4. Malondialdehyde (MDA) content, and the activities of superoxide dismutase (SOD), peroxidase (POD) enhanced gradually, whereas, the catalase (CAT) activity tend to decrease with stress days of low temperature. The activities of ascorbate perosidase (APX) and glutathione reductase (GR) increased in the first days, then decreased. Low magnesium stress also led to an increase in MDA content, while a decrease in antioxidant enzyme activities.+Mg treatments plants showed a lower MDA content, whereas, higher activities of SOD, POD, CAT, APX and GR than the control plants. This data suggested that MgSO4could reduce the lipid peroxidation of membrane, and alleviate the injury in cellmembrane of cucumber by low temperature.5. In solar-greenhouse condition, the M1cucumber plants revealed the most friuts, however the M2plants showed the largest friut weight amoung the four treatments. No significant differences were found in friutsor friut weight between the M3and Mo plants.The total yield of M1and M2increased by13.8%and16.1%respectively than that the Mo. Therefore, we concluded that applied MgSO4for1-2kg/667m2in soil could significantly alleviate the low temperature obstacle, and inhance the yield of cucumber. So we suggest extension in cucumber production in solar-greenhouse.