Nitrate Reduction in Response to CO(2)-Limited Photosynthesis : Relationship to Carbohydrate Supply and Nitrate Reductase Activity in Maize Seedlings

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.