Deterioration of Fruit Quality of Tomato by Excess Cobalt and Its Amelioration

Abstract

Tomato (Lycopersicon esculentum L.) cv. Alivikas was grown in refined sand at two levels of phosphorus (P): 1.5 mM (adequate) and 3.0 mM (high); 0.5 mM cobalt (Co) was supplied at each P level after 35 days of growth. Visible symptoms of Co toxicity appeared as interveinal chlorosis of young leaves after 8 days of metal supply. Biomass was depressed at both P levels with excess Co, although less at the high P level. Discontinuation of high P or Co or additional iron (Fe) supply also increased the biomass. The fresh weight of fruits was decreased, but dry weight was increased in high P supply. Quality of fruits was poor with Co‐treated plants as Co depressed the fruit fresh weight and dry weight, volume and size of fruits, concentrations of chlorophyll a, b and carotene, ascorbic acid, lycopene, reducing sugars, and starch, and increased acidity and phenols. These effects were greater with adequate vs. high P. High P (without Co) improved fruit quality by increasing fruit dry weight, volume, and size of fruits, ascorbic acid content, and reducing sugars and by decreasing acidity. Accumulation of cobalt in leaves and fruits was greater, and these plant parts developed visible Co toxicity symptoms. Fruits developed black patches with prolonged Co supply. New fruits developed after the discontinuation of the Co supply appeared green and normal, but were few in number compared with increased vegetative growth of plants (cobalt supplied with the normal level of phosphorus). Earlier flowering, fruiting, and ripening were observed in plants with high phosphorus rather than adequate P. Brown spots in fruits developed after 18–20 days of Co supply, and in the most severe case, young emerging fruits were totally brown. Amelioration of cobalt toxicity was better with discontinuation of Co than withdrawal of high P with additional Fe.