Effects Of Cu(~2+) Stress On Growth And Photosynthesis Of Beta Vulgaris

Copper is essential micronutrient elements to plants growth and development,which is components and activator of a variety of enzymes and involvesin photosynthesis, electron transport, protein synthesis and other metabolic activities.However, excessive copper is toxic to plants. In this study, in order tounderstand the tolerance and response of Beta vulgaris to Cu stress and to lookinsight into the mechanism of copper toxicity in higher plants, Beta vulgaris cv KWS3418was used to examine the effect of Cu（²+）concentration on growth,photosynthetic characteristics, cell membrane permeability, superoxide dismutase（SOD）, organic acids, inorganic ions.The main results are showed as follows:1. Plants grown well and no significant difference was detected between control (0.3μmol·L^-1) and copper-treated plants when the concentration of Cu（²+）≤250μmol·L^-1,However, with the increase of Cu（²+）concentration (Cu（²+）＞250μmol·L^-1), the growthof plants such as leaf area, leaf fresh and dry weight, fresh and dry weight per plant alldecreased significantly. These findings suggseted that red beet can endure certaincopper stress, but under higher copper stress the seedings were seriously damaged.2.Under condition of concentration of250μmol·L^-1Cu（²+）,net photosynthetic rate andphotosynthetic oxygen evolution rate of red beet seedlings significantly declined,and the higher of copper concentration, the lower of the value. Stomatalconductance and chlorophyll content under250μmol·L^-1copper treatment wassignificantly lower than that of the control, and decreased withincreasing copper concentration, but intercellular CO2concentration did not changesignificantly. Copper stress reduced the photosynthesis capacity of red beet seedlingsand the reasons for this decline is stomatal closure and photosyntheticpigments degradation.3. φPSⅡ significantly decreased when the plants were treated with the concentrationof250μmol·L^-1Cu（²+）, with the increase of copper concentration, this reductionbecome more obviously. These results suggests that the photochemical activity of PSⅡ is restrained, which affected carbon fixation and assimilation of the dark reaction, eventually leading to the lower photosynthesis of red beet.4. Fv/Fm decreased and Fo increased with the increase of the copper concentration,when copper concentration increases to1000μmol·L^-1, changes in Fv/Fm andFo were significantly different between control and copper-treated plants. However,copper stress had little effect on the number of the activitive PS II reaction centerper unit area of leave（RC/CSo）,and under concentration of1000μmol·L^-1Cu（²+）,thedegree of closure of the reactive center （VJ） increased significantly.This illustratesthe photosystem Ⅱ damaged under the high concentrations of copper(1000μmol·L^-1Cu（²+）). Wk significantly increased under condition of copper concentration of1000μmol·L^-1suggesting that oxygen-evolving complex OEC was seriously hurt.5. When the concentration of Cu（²+）was250μmol·L^-1, Sm dropped but had nosignificant difference with control. Sm aws significantly lower than control when theconcentration of Cu（²+）increased to500μmol·L^-1and1000μmol·L^-1. This resultssuggest that electron transfer from QAto QBinhibited and electron transport activityof PS Ⅱ diminished when the concentration of copper higher than250μmol·L^-1.6. Changes in ABS/CSm, TRo/CSm and ETo/CSm were similar after copper treatment.Under condition of250and500μmol·L^-1copper treatment, each parameterslightly increased compared with control, but has no significant difference.When Cu（²+）reached to1000μmol·L^-1, a significant reduction was observed in thevarious parameters. This illustrates that copper treatment reduced light energy whichreaction center of PS II can capture, and then the excitation energy used to deoxgenizeQAand the energy for electron transfer decreased per unit area. DIo/CSm increasedgradually with the increase of concentration of copper, and when the concentration ofcopper reached to1000μmol·L^-1, DIo/CSm showed a signifiavnt increase comparedwith control. These results suggest that plants start to initiate the heat dissipationmechanism under copper stress to prevent damage to plant photosynthetic apparatus.7. Copper treatment enhanced total activity of SOD mainly due to higher expressionlevels of Cu/Zn-SOD1. The MDA content increased with increase of copper, reacheda significant level under high concentrations of (500,1000μmol·L^-1) copper. Theseresults suggest that the high concentration of copper treatment induced a certaindegree of oxidative stress, causing damage to cell membrane system a increasesof membrane permeability of red beet seedlings although copper stress enhances SODactivity.8. Organic acid metabolism of sugar beet was affected by copper stress. When the Cu（²+）concentration exceeds250μmol·L^-1, oxalic acid content of the leaves and rootswas significantly lower than that of the control. However acetic acid content of redbeet under500,1000μmol·L^-1copper treatment is significantly higher than that ofcontrol. Inorganic ions were less affected by copper stress, only under the highconcentrations (1000μmol·L^-1Cu（²+）)significant changes occured in the content ofinorganic ions, the original balance system was interferenced resultingin the disorder of metabolic processes.