Foliar and root Cu supply affect differently Fe- and Zn-uptake and photosynthetic activity in soybean plants

A comparative study on the effect of excess copper (Cu) supplied through leaves and root treatments in soybean plants is presented. Although Cu was accumulated in leaves upon both treatments, important differences were observed between Cu treatment of leaves and Cu supplementation in hydroponic medium. The application of 50 μM CuSO4 on leaves increased chlorophyll content, oxygen evolution activity of thylakoids and fluorescence emission at 685 and 695 nm with respect to emission at 735 nm compared with control plants. In contrast, soybean plants grown in hydroponic medium supplemented with 50 μM CuSO4 exhibited lower chlorophyll content, decreased oxygen evolution activity of thylakoids and decreased fluorescence emission at 685 and 695 nm compared with control plants. On the other hand, the elemental analysis of leaves showed differences between the two treatments investigated. The results indicated that no antagonist interaction between Cu- and Fe-uptake took place in plants where excess Cu was applied on leaves but Cu compete with Fe-uptake in plants grown with excess Cu in the hydroponic medium. Furthermore, differences in Zn-uptake were also observed. Zn content decreased upon Cu treatment of leaves whereas the opposite was observed upon Cu treatment through roots. Interestingly, plants with Cu-treated leaves behaved similarly as cell suspensions grown in the presence of excess Cu. The results strongly indicate that different Cu-uptake and transport pathways must operate in leaf cells compared with root cells.