Physiological And Biochemical Mechanisms Of Manganese Toxicity And Detoxification In Cucumis Sativus L.

Mn toxicity easily occurs in the soils of succesive cropping greenhouses and acidity. Cucumber (Cucumis sativus L.) is widely planted in open field and greenhouse and easily affected by excess Mn toxicity. Therefore, it is very important to study the physiological mechanisms of Mn toxicity in cucumber and take measures to decrease the toxicity. In this study, hydroponic culture experiments were conducted to investigate the physiological mechanisms of Mn toxicity to cucumber and the effects of pH, light intensity, silicon and salicylic acid. The main results obtained were presented as follows:1. Effects of excess Mn on plant growth and nutrient element uptake in cucumber at different pHExcess Mn in nutrient solution inhibited the growth of shoots and roots, especially at low pH. The Mn content increased with the increasing Mn concentration in nutrient solution, and low pH increased the Mn distribution in the shoots. Excess Mn inhibited the absorption of Ca, Mg and Fe, and low pH decreased Mg content. Because Mg and Fe deficiency influence chlorophyll synthesis, the decrease in chlorophyll content induced by excess Mn may be due to the decreased absorption of Mg and Fe by excess Mn. As a result, the net photosynthesis rate was decreased.2. Combined effects of excess Mn and low pH on oxidative stress in cucumberThe MDA content, as an index of lipid peroxidation, dramatically increased with increasing Mn concentration, especially at low pH. Excess Mn led to an elevation in activities of Mn-SOD and Fe-SOD, which play an important role in removing O2 . Cu,Zn-SOD and CAT were sensitive to excess Mn, and their activity significantly decreased under excess Mn and low pH, while activities of GPX, APX, DHAR and GR increased under low pH and higher concentrations of Mn, which indicated their important roles in scavenging reactive oxygen species in cucumber roots tolerance to low pH and excess Mn.3. Effects of excess Mn on photosynthesis characteristics and nutrition uptake in cucumber under different light intensityExcess Mn inhibited plant growth and the plants under high light intensity showed more serious symptoms of Mn toxicity compared to those under low light intensity. Low light intensity decreased the Mn content in cucumber plant and decreased the inhibitory effects forabsorption of Ca, Mg, Fe, and Zn under excess Mn. Under high light intensity the primary maximum photochemical efficiency of PSII (Fv/Fm), the quantum efficiency of non-cyclic electron transport of PSII ( φ PSII) and photochemical quenching (qP) significantly decreased in excess Mn treatment, however, under low light intensity no significant effects on Fv/Fm and qP were observed. Excess Mn significantly decreased the chlorophyll content, and low light intensity increased chlorophyll content. Excess Mn decreased net photosynthetic rate (Pn) and stomatal conductance (Gs), particularly under high light intensity. Excess Mn increased intracellular CO2 (Ci) under high light intensity and decreased Ci under low light intensity. Therefore, we could conclude that stomatal limitation was a dominant factor on Pn decrease in excess Mn treatment under high light intensity and non-stomatal limitation was a dominant factor on Pn decrease in excess Mn treatment under low light intensity.4. Oxidative stress and antioxidant system in cucumber influenced by excess Mn under different light intensitiesLow light intensity decreased the O2- producing rate and H2O2 content in cucumber leaves, and reduced the lipid peroxidation induced by excess Mn. Under the same Mn concentration, low light intensity decreased the activity of antioxidant enzymes, which adapted to the low O2- producing rate and H2O2 content. Compared to normal Mn treatment, excess Mn significantly decreased ascorbate and glutathione content, particularly under the high light intensity. Antioxidant enzymes showed different changes in different organelles. Activities of SOD, GPX, APX, DHAR and GR in cytosol and chloroplast were stimulated by excess Mn treatment. APX, DHAR and GR activities in chloroplast were...