LIMITING STEP ON PHOTOSYNTHESIS OF RICE PLANTS TREATED WITH VARYING COPPER LEVELS

The photosynthetic process of rice (Oryza sativa L.) plants grown over a 30-day period in a nutrient solution containing concentrations of copper ranging from 0.002 to 6.25 mg/L was investigated. The PSII-dependent H2O to DCIP electron transport showed its maximum in the 0.05 mg/L Cu treatment and decreased thereafter with increasing copper levels in the solution; however, when DPC was used, the electron transfer rate to DCIP was less inactivated. The DCIP to MV, as well as the over-all H2O to MV electron transfer, showed a continuous rate decrease from the 0.01 to the 6.25 mg/L Cu treatments. The experimental data indicate that copper inhibits the photosynthetic electron transport mainly before the DPC donation site in the PSII. The activities of Rubisco were also determined and showed a progressive decrease with increasing copper levels in the solution medium. We conclude that although excess copper affects the photosynthetic process in multiple ways, the prevailing effect is that on the activity of Rubisco, which in turn limits the over-all photosynthetic activity.