Nitrate Reduction in Response to CO2-Limited Photosynthesis

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Abstract
The effects of CO2-limited photosynthesis on 15NO3- uptake and reduction by maize (Zea mays, DeKalb XL-45) seedlings were examined in relation to concurrent effects of CO2 stress on carbohydrate levels and in vitro nitrate reductase activities. During a 10-hour period in CO2-depleted air (30 microliters of CO2 per liter), cumulative 15NO3- uptake and reduction were restricted 22 and 82%, respectively, relative to control seedlings exposed to ambient air containing 450 microliters of CO2 per liter. The comparable values for roots of decapitated maize seedlings, the shoots of which had previously been subjected to CO2 stress, were 30 and 42%. The results demonstrate that reduction of entering nitrate by roots as well as shoots was regulated by concurrent photosynthesis. Although in vitro nitrate reductase activity of both tissues declined by 60% during a 10-hour period of CO2 stress, the remaining activity was greatly in excess of that required to catalyze the measured rate of 15NO3- reduction. Root respiration and soluble carbohydrate levels in root tissue were also decreased by CO2 stress. Collectively, the results indicate that nitrate uptake and reduction were regulated by the supply of energy and carbon skeletons required to support these processes, rather than by the potential enzymatic capacity to catalyze nitrate reduction, as measured by in vitro nitrate reductase activity.