Cadmium at high dose perturbs growth, photosynthesis and nitrogen metabolism while at low dose it up regulates sulfur assimilation and antioxidant machinery in garden cress (Lepidium sativum L.)

Metal contamination of soils has become a worldwide problem and great environmental threat, as these metals accumulate in soils and plants in excess, and enter the food chain. Increased cadmium (Cd) uptake from contaminated soils leads to altered plant metabolism and limits the crop productivity. The experimental crop, Lepidium sativum L. (Garden Cress, Family: Brassicaceae) is a medicinally and economically important plant. An experiment was conducted to examine the effect of different concentrations of Cd (0, 25, 50 or 100 mg kg−1 soil) on the performance of L. sativum. Cd accumulation in roots and leaves (roots > leaves) increased with the increaseing Cd concentration in soil. High Cd concentration (100 mg Cd kg−1 soil) inhibited the leaf area and plant dry mass and significant decline in net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO2 (Ci), chlorophyll (Chl a, Chl b, total Chl) content, carbonic anhydrase (CA; E.C. 4.2.1.1) activity, nitrate reductase (NR; E.C. 1.6.6.1) activity and nitrogen (N) content was also observed. However, ATP-sulfurylase (ATP-S; EC. 2.7.7.4) activity, sulfur (S) content and activities of antioxidant enzymes such as superoxide dismutase (SOD; E.C. 1.15.1.1); catalase (CAT; E.C. 1.11.1.6); ascorbate peroxidase (APX; E.C. 1.11.1.11) and glutathione reductase (GR; E.C. 1.6.4.2) and glutathione (GSH) content were increased. Specifically, the decrease in NR activity and N content showed that Cd affects N metabolism negatively; whereas, the increase in ATP-S activity and S content suggests the up-regulation of S assimilation pathway for possible Cd tolerance in coordination with enhanced activities of antioxidant enzymes and GSH. High Cd concentration (100 mg Cd kg−1 soil) perturbs the L. sativum growth by interfering with the photosynthetic machinery and disrupting the coordination between carbon, N and S metabolism. On the other hand, at low Cd concentration (25 mg Cd kg−1 soil) co-ordination of S and N metabolism complemented to the antioxidant machinery to protect the growth and photosynthesis of L. sativum plants.