Nitrogen Utilization in Lemna

Abstract

ABSTRACI'IN-labeled nitrate was used to trace short-term nitrate influx into Lemna gibba L. G3 in experiments where disappeaance of both radioactivity and total nitrate from the incubation medium was measured continuously and simultaneously. In plants performing net nitrate uptake from an initial nitrate concentration of 40 to 60 micromolar, there was no discrepancy between net uptake and influx, irrespective of the N status of the plants, indicating that concomitant nitrate efflux was low or nil. Plants treated with tungstate to inactivate nitrate reductase were able to take up nitrate following induction of the uptake system by exposure to a low amount of nitrate. Also, in this case, net uptake was equivalent to influx. In tungstate-treated plants preloaded with nitrate, both net uptake and influx were nil. In contrast to these observations, a clear discrepancy between net uptake and influx was observed when the plants were incubated at an initial nitrate concentration of approximately 5 micromolar, where net uptake is low and eventually ceases. It is concluded that plasmalemma nitrate transport is essentially unidirectional in plants performing net uptake at a concentration of 40 to 60 micromolar, and that transport is nil when internal nitrate sinks (vacuole, metabolism) are eliminated. The efflux component becomes increasingly important when the external concentration approaches the threshold value for net nitrate uptake (the nitrate compensation point) where considerable exchange between internal and external nitrate occurs.The kinetics of net NO3-uptake in plant tissue depends on the supply of NO3-to the plants during culturing, and can be drastically altered by changed NO3-availability during pretreatment or experiments (4, 18). Nitrogen starvation generally stimulates net 31). Influences of internal NO3-(2), accumulated Cl-or 8,11, 30), or N-metabolites other than NO3-or NH4+ (20) have been considered in relation to regulation of the rates of NO3-fluxes and net NO3-uptake.There are contrasting opinions regarding the effect of Navailability and plant N-status on the unidirectional components ofplasmalemma NO3-transport. Occurrence ofsignificant efflux of NO3-has been demonstrated in several species by the use of '5N03-(2, 18, 24), '3N03- (21,23,26, 27), as well as the N03-analog 36CI03- (6,7, 10,12). Models for efflux regulation of net NO3-uptake have been developed, involving N037/NO3 exchange and substrate cycling (5, 9). On the other hand, it has been concluded from experiments with '5N-and '3N-labeled N03-that regulation of N03-uptake mainly is related to modulation of N03-influx (18,22,26).General relations between net NO3 fluxes and NO3 assimi-'Supported by the Swedish Natural Science Research Council.lation in Lemna were considered in a preceding paper (16). In the present investigation, we have used '3N03 to study the unidirectional fluxes of NO3-in exponentially growing N-sufficient and N-limited Lemna gibba. Nitrogen-limited plants were also used for studies of fluxes of NO3-at the threshold ...

Nitrogen Utilization in Lemna

“…Assuming that the cytoplasmic pool of N03-does not become isotopically equilibrated with the external medium, within the time of physical equilibration of the experimental system after the addition of '3NO3 (14), the results presented here show that NH44 inhibits net NO3-uptake in Lemna by affecting NO3-influx. Since NRA was unaffected or to some extent even stabilized, we can conclude that NH44 acts primarily on the uptake mechanism, at least in short-term ex- (12).…”

“…Since NRA was unaffected or to some extent even stabilized, we can conclude that NH44 acts primarily on the uptake mechanism, at least in short-term ex- (12). The higher rate of N-assimilation in the presence of NH4+ might result in an increase of this compound, compared to the situation with NO3-alone as N source (13,14). However, contradictory results have been obtained with NR from Lemna minor, which was appreciably inactivated by NH4+ within 1 h (21).…”

“…In two accompanying papers we have presented data on the characteristics of NO3-uptake and NO3-reduction in Lemna gibba L. (12,14). The aim of the present study was to clarify the nature of the NH4' effect on membrane NO3-transport (using '3NO3-), and to investigate the effect of NH4' on accumulation and reduction of NO3-.…”

“…The flux experiments were performed as previously described, using '3N labeled NO3-prepared in the 225 cm cyclotron at the Research Institute of Physics, Stockholm (14). The experimental medium was the same N-free medium as used before (10) with the inclusion of KNO3 to an initial concentration of 100 ,M. The experiments were conducted 1 to 6 h after the daily NO3 addition, the intracellular NO3-level being between 1 and 6 ,imol g-' FW.…”