Effects of Cd~(2+) Stress on Photosynthesis and Chlorophyll Fluorescence Characteristics of Cucumber Seedlings

The effects of different concentrations of cadmium(0,25,50,100,200 μmol/L) on the photosynthesis and chlorophyll fluorescence characteristics of cucumber seedlings were studied with a solution culture.The results showed that net photosynthetic rate(Pn),stomatal conductance(Gs) and transpiration rate(Tr) greatly decreased but intercellular CO2 concentration(Ci) showed no change at the 4th day of cadmium stress,relative chlorophyll content only decreased at 200 μmol/L cadmium treatment,while major PSII florescence parameters(the maximal photochemical efficiency of PSII Fv/Fm,the actual photochemical efficiency of PSII in the light Yield and electron transport rate ETR) did not change,indicating a stronger stomatal limitation in the decline of photosynthesis for those treatments with cadmium lower than 200 μmol/L.On the 8th day of cadmium,net photosynthetic rate,stomatal conductance and transpiration decreased more sharply with the increase of intercellular CO2 concentration(Ci),relative chlorophyll content and major fluorescence parameters except NPQ also decreased,implying a stronger nonstomatal limitation such as decreased chlorophyll content and destroyed PSII primary reaction.The lower value of main fluorescence parameters except NPQ at 50 μmol/L cadmium treatment on the 8th day might be related with serious Fe deficiency caused by cadmium stress.