In Vivo Gas Exchange Measurement of the Site and Dynamics of Nitrate Reduction in Soybean

Abstract

Abstract A gas analysis system was built to study the relationship between the reductant cost of NO3  − assimilation and the measured rate of CO2 and O2 exchange in roots, leaves, and stems+ petioles of soybean (Glycine max L. Merr. cv Maple glen) plants. The measurements were used to calculate the diverted reductant utilization rate (DRUR = 4*[measured rate of CO2 + measured rate of O2], in moles of high-energy electron [e  −] per gram per hour) in plants in the presence (N+) and absence (N−) of NO3  −. The differences in DRUR between the N+ and N− treatments provided a measure of the NO3  −-coupled DRUR of 25-d-old plants, whereas a 15NO3  −-enriched nutrient solution was used to obtain an independent measure of the rate of NO3  − assimilation. The measured reductant cost for the whole plant was 9.6 e  − per N assimilated, a value within the theoretical range of four to 10e  − per N assimilated. The results predicted that shoots accounted for about 55% of the whole-plant NO3  − assimilation over the entire day, with shoots dominating in the light, and roots in the dark. The gas analysis approach described here holds promise as a powerful, noninvasive tool to study the regulation of NO3  − assimilation in plant tissue.