Changes of photosynthetic parameters in cucumber leaves under Cu, Cd, and Pb stress

The effects of Cu, Cd, and Pb toxicity on photosynthesis in cucumber leaves (Cucumis sativus L.) were studied by the measurements of gas exchange characteristics, chlorophyll (Chl) fluorescence parameters, and Chl content. Concentrations of metals in sequence of 20 M Cu, 20 and 50 M Cd, and 1 000 M Pb decreased the plant dry mass to 50–60 % after 10 d of treatment whereas 50 M of Cu decreased it to 30 %. The content of Cd in leaves of plants treated with 50 M Cd was three times higher than the contents of Cu and Pb after plant treatment with 50 M Cu or 1 000 M Pb. Hence Cd was transported to leaves much better than Cu and Pb. Nevertheless, the net photosynthetic rate and stomatal conductance in leaves treated with 50 M Cu or Cd were similarly reduced. Thus, Cu was more toxic than Cd and Pb for photosynthesis in cucumber leaves. None of the investigated metals decreased internal CO₂ concentrations. Also the effect of metals on potential efficiency of photosystem 2, PS2 (F_v/F_m) was negligible. The metal dependent reduction of PS2 quantum efficiency (_PS2) after plant adaptation in actinic irradiation was more noticeable. This could imply that reduced demand for ATP and NADPH in a dark phase of photosynthesis caused a down-regulation of PS2 photochemistry. Furthermore, in leaves of metal-treated plants the decrease in water percentage as well as lower contents of Chl and Fe were observed. Thus photosynthesis is not the main limiting factor for cucumber growth under Cu, Cd, or Pb stress.This revised version was published online in March 2005 with corrections to the page numbers.