Excess copper effect on growth, chloroplast ultrastructure, oxygen-evolution activity and chlorophyll fluorescence in Glycine max cell suspensions

The influence of excess copper on soybean photosynthetic cell suspensions was investigated. The cell suspensions grew well in the presence of 5–20 µ M CuSO₄ and developed tolerance to even higher levels of CuSO₄ (i.e. up to 50 µ M ), indicating that copper was not toxic to the cells at that high concentrations. Cu-adapted cell suspensions grew faster than the control in limiting light conditions and had higher content of chlorophyll per dry weight of cells. Copper was accumulated within the cells, and this event was accompanied by (1) increased oxygen evolution activity; (2) increased number of chloroplasts per cell, smaller chloroplasts, increased thylakoid stacking and grana size; (3) higher fluorescence emission of photosystem II antenna complexes and (4) stimulation of plastocyanin protein synthesis compared with untreated cells. Microanalysis of cross-sections revealed an increase of copper content in chloroplasts as well as vacuole, cytoplasm and cell wall in Cu-adapted cells. No antagonist interaction between copper and iron uptake took place in these cell suspensions. On the other hand, copper at subtoxic concentrations stimulated oxygen evolution activity in thylakoids from control cells, but this event did not take place in those from Cu-adapted ones. Furthermore, the loss of activity by copper inhibitory action at toxic concentrations was two-fold slower in thylakoids from Cu-adapted cells compared with the control ones. The data strongly indicate that copper plays a specific positive role on photosynthesis and stimulates the growth and the oxygen evolution activity in soybean cell suspensions.