Cobalt toxicity effects on growth and metabolism of tomato

Tomato (Lycopersicon esculentum) cv. Pusa ruby was grown in refined sand with complete nutrition (control) and at 0.05, 0.1, 0.2, 0.4, and 0.5 mM of cobalt sulfate. Visible effects of excess cobalt (Co) manifested on tomato at 0.5 mM Co, after 3 days of metal supply to 40 day old plants. The symptoms of excess cobalt diffused chlorosis of young leaves from base and later necrotic spots appeared on chlorotic areas. The necrotic spots enlarged in size, coalesced and in due coarse most of the entire leaf turned necrotic and withered. With excess Co, there was loss of lamina and marginal scorching of affected leaves. The affected leaves were distorted and appeared hook like with rudimentary leaflets at the top. These effects were most severe at 0.5 mM and the intensity of symptoms gradually diminished with a decrease in Co supply from 0.5 to 0.05 mm. At the latter level of Co, no visible effects could be seen except for growth depression. Excess Co restricted the biomass, concentration of phosphorus (P), sulfur (S), and iron (Fe), chlorophyll a and b, DNA and RNA, reducing and non-reducing sugars, starch, total soluble proteins, protein and non-protein nitrogen, and increased phenol and Co concentrations. In excess Co treated tomato leaves, the activity of catalase decreased and peroxidase, ribonuclease, and acid phosphatase increased. The accumulation of Co was greatest in roots and old leaves and lowest in stem of tomato.