J. Plant Physiol. Vol. 142. pp. 25 -30 (1993) Introduction Effect of Chromium on Photosynthesis, Respiration and Nitrogen Fixation in Pea (Pisum sativum L.) Seedlings N. R. BISHNOI, L. K. CHUGH, and S. K. SAWHNEY Department of Chemistry and Biochemistry, Haryana Agricultural University, Hisar-125004, India Received December 11, 1992 . Accepted March 5,1993 Summary Application of Cr 6 +, as K 2 Cr 2 07, 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 Cr 6 + as well as with the duration of exposure. Treat- ment 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 Cr 6 + had little effect on the activity of Photosystem II whereas that of Photosystem I was markedly inhibited. At low concentrations, Cr 6 + 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 treat- ment, efficiency of the developed nodules to fix nitrogen was also impaired. During initial stages of treat- ment with Cr 6 +, the order of relative sensitivity of these processes was: Photosynthesis > nitrogen fixa- tion > respiration, but upon extended exposure, nitrogen fixation was affected to the greatest extent fol- lowed by photosynthesis and then respiration. Key words: Photosynthesis, respiration, nitrogen fixation, chromium, Pisum sativum L. Abbreviations: Cr 6 + = Chromium; DAS = Days after sowing; PS I = Photosystem I; PS II = Photosys- tern II. Chromium is an important metal pollutant and is released into the environment primarily through industrial processes such as dyeing, tanning, paint manufacturing, electroplating and metallurgical operations (Hartford, 1979). It is estimated that world-wide about 1068 thousand metric tons of chro- mium are discharged annually into the biosphere (Nriagu, 1988). This metal gains entry into agricultural land through use of municipal waste-based composts as fertilizers and through irrigation with sewer water, which is quite often polluted with industrial effluents. Even at relatively low concentrations chromium depresses growth of plants (An- derson et aI., 1973; Davies et al., 1978; Barcelo et al., 1986) and their economic yield (Bishnoi et aI., 1993). Besides being mutagenic (Gebhart, 1984), this metal has been reported to depress protein and chlorophyll contents Gana et al., 1987), and to interfere with photosynthetic activity (Austenfeld, 1979) and mineral uptake by plants (Barcelo et al., 1985). It also diminishes the levels of several hydrolytic enzymes (Dua and Sawhney, 1991) and respiratory activity of germi- nating pea seeds (Rani et al., 1990). In the present studies, the influence of Cr 6 + on photosynthesis, respiration and sym- biotic nitrogen fixation has been ascertained to gain a better insight into the metabolic basis for its deleterious effect on plant growth. Materials and Methods Culture conditions Pea (Pisum sativum L. cv. Bonneville) plants were raised in earth- en pots filled with 5 kg of acid washed sand in a net house under two different culture conditions. One set of seedlings, referred to as N-fixing plants and used for determining the effect of Cr 6 + on nitro- gen fixation, was inoculated with Rhizobium leguminosarum (strain © 1993 by Gustav Fischer Verlag, Stuttgart