In vivo gas exchange measurement of the site and dynamics of nitrate reduction in soybean

A gas analysis system was built to study the relationship between the reductant cost of NO3- assimilation and the measured rate of CO2 and O-2 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 (DRUB = 4*[measured rate of CO2 + measured rate of O-2], 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 DRUB of 25-d-old plants, whereas a (NO3-)-N-15-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 10 e(-) 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.