Relations between uptake and utilization of NO⁻₃ in <i>Pisum</i> growing exponentially under nitrogen limitation

Abstract

The utilization and translocation of nitrogen was investigated in exponentially growing, nitrogen‐limited Pisum sativum L. cv. Marma. The plants were given N daily at exponentially increasing, although suboptimal, relative nitrogen addition rates (R N ) calculated to yield a relative increment in N of 0.06 day −1 and 0.12 day −1 . After 10 days of NO − 3 additions (26 days after sowing), the relative growth rate more or less equaled R N . Uptake of NO − 3 was several‐fold higher than the N requirement for the growth rate set by R N . The daily addition of NO − 3 was taken up after 7 to 8 h, resulting in a cyclic behaviour in the NO − 3 utilization. During the phase of net NO − 3 influx, the filling phase (0 to 8 h), in vitro nitrate reductase activity (NR activity) and intracellular levels of soluble N in the root increased. In the phase of no net influx of NO − 3 the depletion phase (8 to 24 h), the plants were entirely dependent on stored N. During this phase both in vitro NR activity and intracellular levels of soluble N decreased. Also the calculated actual rate of NO − 3 reduction was high in the filling phase, while it was close to zero in the depletion phase. The pattern of these fluctuations indicates that the regulation of NO − 3 utilization involves an interplay between transmembrane fluxes of NO − 3 , the cytosolic NO − 3 concentration and NR activity. Cyclic fluctuations in N‐containing compounds were also found in the xylem. Nitrogen was mainly transported as amino acids. The pattern of NO − 3 transport in the xylem and the fluctuations in the shoot of in vitro NR activity indicate that a reasoning similar to that for the regulation of NO − 3 assimilation in the root also applies for the shoot. The results also indicate a substantial supply of amino acids to the xylem through recirculation from the shoot.