Temperature Effects on Growth and Manganese Tolerance in Tobacco1

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Abstract
Symptoms of Mn toxicity in tobacco (Nicotiana tabacum L.) are often observed in the mountain region of North Carolina at soil acidity and soil test levels of Mn which do not produce toxicity in the piedmont. Although genotypic differences between tobacco types grown in the regions may contribute to the differential response, variations in temperature between the regions may also be involved. To investigate the effects of temperature on plant growth and Mn tolerance, tobacco plants of a prevalent cultivar (‘Coker 347’) for the piedmont region were grown in sand culture at day/night temperatures of 22/18, 26/22, and 30/26 C. Manganese was applied daily at rates of 10, 20, 40, 80, and 120 mg/liter in a complete nutrient solution. Plants were sampled weekly during a 6‐week growth period and evaluated for visual toxicity symptoms, dry weight. Mn concentration, relative growth rate (RGR), and relative accumulation rate (RAR) of Mn.Plant tolerance to high rates of applied Mn increased with increasing temperature despite significantly greater tissue concentrations of Mn during early growth. Tissue levels of Mn in leaves associated with the appearance of visual toxicity symptoms were 700 to 1,200 mg/kg dry wt at 22/18 C, 2,000 to 3,500 mg/kg at 26/22 C, and 5,000 to 8,000 mg/kg at 30/26 C. Mn toxicity symptoms were more severe in immature leaves and often decreased as the leaves matured even though the concentration of Mn in the leaves increased. The RGR of leaves during the exponential growth phase increased with increasing temperatures at each Mn rate, but the RAR of Mn was unaffected by either temperature or applied Mn rate. It is proposed that the increased tolerance to high levels of tissue Mn by plants grown in warm temperatures is associated with more rapid rates of leaf expansion accompanied by increased vaculor capacity for disposal of accumulated Mn.