Evaluation of Cr stress tolerance in Portulaca oleracea L. based on growth, photosynthesis indexs and the activity of antioxidant enzymes

In order to investigate effects of Cr stress on growth, photosynthetic indexs and the activity of antioxidant enzymes in Portulaca oleracea L.(Purslane), an experiment in the greenhouse of Ferdowsi University of Mashhad, was done in a completely randomized design with three replications of five levels (0, 25, 50, 75 and 100 ppm). The amount of Cr+6 in the soil, root and shoot at different levels of stress were measured. By increasing the chromium content in shoot and root were significantly increased. Chromium accumulation at root was more than shoot. Stomatal conductance, chlorophyll a, carotenoids, transpiration and efficiency of photosystem II decreased with increasing the amount of Cr. However, the Co2 assimilation, water use efficiency, growth, coefficient stability index (CSI) and chlorophyll b and total chlorophyll concentration in 100 ppm was significantly higher than 75 ppm. Also, the activity of catalase and peroxidase increased by increasing the amount of Cr+6. Based on the results, chromium in low concentrations possibly effected on biosynthesis of photosynthetic pigments and caused disorder of photosynthetic electron transport, decreased photosynthetic efficiency, thereby reduced the production of reactive oxygen species and growth. But at higher levels of chromium (100 ppm), purslane by activating defense mechanisms such as increasing the activity of antioxidant enzymes and prevent a sharp decline in total proteins could reduce somewhat the damage and maintained the water use efficiency. The purslane is likely to be as tolerant plants of chromium and it was effective in refining the contaminated soils with this heavy metal.