Effects of copper on growth and on photosynthesis of mature and expanding leaves in cucumber plants

The aim of this study was to study the relationship between growth inhibition in Cu-treated Cucumis sativus L. seedlings and photosynthesis. Twenty-two days after sowing, copper was added to the nutrient solution for 5 days, leading to final Cu supplementary concentration in sand of, respectively 0 (control) and 10 mug g(-1) (Cu stress). The responses of cucumber leaves to copper addition depend on their growth stage. Young expanding leaves showed a reduction in leaf area, while mature leaves exhibited a significant decline in photosynthesis. Sucrose and starch content rose in both types of leaves. For mature leaf, net CO2 assimilation declined at a nearly constant intercellular CO2 mole fraction, indicating that stomatal closure did not account for the inhibition of photosynthesis. Maximal photochemical yield of PSII in dark-acclimated leaves was unaffected, indicating that the dark phase of photosynthesis rather than the light phase was affected. Relative growth rate and leaf area ratio of stressed plants were significantly below that of control plants while no significant decrease of net assimilation rate was observed for Cu-stressed plants. Thus despite a reduction in mature leaf photosynthesis, growth reduction is more likely due to a reduction in whole plant leaf area. This decline in photosynthesis is probably a consequence of an altered source-sink relationship, rather than due to a toxic effect of copper on photosynthesis. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.