EFFECT OF CHROMIUM ON PHOTOSYNTHESIS, RESPIRATION AND NITROGEN-FIXATION IN PEA (PISUM-SATIVUM L) SEEDLINGS

Application of Cr6+, as K2Cr2O7, to pea seedlings severely depressed the overall capacity of plants to carry out photosynthesis, respiration and symbiotic nitrogen fixation. The deleterious effect became more pronounced with increasing concentrations of Cr6+ as well as with the duration of exposure. Treatment with the metal caused a reduction in chlorophyll content, photosynthetic activity per unit leaf area and in the activities of Photosystems I and II. In isolated chloroplasts, exogenously added Cr6+ had little effect on the activity of Photosystem II whereas that of Photosystem I was markedly inhibited. At low concentrations, Cr6+ diminished the capacity of plants to fix atmospheric nitrogen largely by interfering with the formation of nodules, but at higher concentrations and with a prolonged period of the treatment, efficiency of the developed nodules to fix nitrogen was also impaired. During initial stages of treatment with Cr6+, the order of relative sensitivity of these processes was: Photosynthesis > nitrogen fixation > respiration, but upon extended exposure, nitrogen fixation was affected to the greatest extent followed by photosynthesis and then respiration.