Changes in rubisco, cysteine-rich proteins and antioxidant system of spinach (Spinacia oleracea L.) due to sulphur deficiency, cadmium stress and their combination

Rita Bagheri, Javed Ahmad, Humayra Bashir, Muhammad Iqbal, M. Irfan Qureshi

Sulphur (S) deficiency, cadmium (Cd) toxicity and their combinations are of wide occurrence throughout agricultural lands. We assessed the impact of short-term (2 days) and long-term (4 days) applications of cadmium (40 μg/g soil) on spinach plants grown on sulphur-sufficient (300 μM SO4 (2-)) and sulphur-deficient (30 μM SO4 (2-)) soils. Compared with the control (+S and -Cd), oxidative stress was increased by S deficiency (-S and -Cd), cadmium (+S and +Cd) and their combination stress (-S and +Cd) in the order of (S deficiency) < (Cd stress) < (S deficiency and +Cd stress). SDS-PAGE profile of leaf proteins showed a high vulnerability of rubisco large subunit (RbcL) to S deficiency. Rubisco small subunit (RbcS) was particularly sensitive to Cd as well as dual stress (+Cd and -S) but increased with Cd in the presence of S. Cysteine content in low molecular weight proteins/peptide was also affected, showing a significant increase under cadmium treatment. Components of ascorbate-glutathione antioxidant system altered their levels, showing the maximum decline in ascorbate (ASA), dehydroascorbate (DHA), total ascorbate (ASA + DHA, hereafter TA), glutathione (GSH) and total glutathione (GSH + GSSG, hereafter TG) under S deficiency. However, total ascorbate and total glutathione increased, besides a marginal increase in their reduced and oxidized forms, when Cd was applied in the presence of sufficient S. Sulphur supply also helped in increasing the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT) under Cd stress. However, their activity suffered by S deficiency and by Cd stress during S deficiency. Each stress declined the contents of soluble protein and photosynthetic pigments; the highest decline in contents of protein and pigments occurred under S deficiency and dual stress respectively. The fresh and dry weights, although affected adversely by every stress, declined most under dual stress. It may be concluded that an optimal level of S is required during Cd stress for better response of SOD, APX, GR and CAT activity, as well as synthesis of cysteine. RbcS is as highly sensitive to S deficiency as RbcL is to Cd stress.