Effect of Carbon Dioxide on Nitrate Accumulation and Nitrate Reductase Induction in Corn Seedlings

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Abstract
Exposure of the leaf canopy of corn seedlings (Zea mays L.) to atmospheric CO2 levels ranging from 100 to 800 μl/l decreased nitrate accumulation and nitrate reductase activity. Plants pretreated with CO2 in the dark and maintained in an atmosphere containing 100 μl/l CO2 accumulated 7-fold more nitrate and had 2-fold more nitrate reductase activity than plants exposed to 600 μl/l CO2, after 5 hours of illumination. Induction of nitrate reductase activity in leaves of intact corn seedlings was related to nitrate content. Changes in soluble protein were related to in vitro nitrate reductase activity suggesting that in vitro nitrate reductase activity was a measure of in situ nitrate reduction. In longer experiments, levels of nitrate reductase and accumulation of reduced N supported the concept that less nitrate was being absorbed, translocated, and assimilated when CO2 was high. Plants exposed to increasing CO2 levels for 3 to 4 hours in the light had increased concentrations of malate and decreased concentrations of nitrate in the leaf tissue. Malate and nitrate concentrations in the leaf tissue of seven of eight corn genotypes grown under comparable and normal (300 μl/l CO2) environments, were negatively correlated. Exposure of roots to increasing concentrations of potassium carbonate with or without potassium sulfate caused a progressive increase in malate concentrations in the roots. When these roots were subsequently transferred to a nitrate medium, the accumulation of nitrate was inversely related to the initial malate concentrations. These data suggest that the concentration of malate in the tissue seem to be related to the accumulation of nitrate.