Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ³⁶ClO₃⁻

Abstract

A computer-controlled multichannel data acquisition system was employed to obtain continuous measurements of net nitrate or chlorate uptake by roots of intact barley plants (Hordeum vulgare cv Betzes) using nitrate-specific electrodes. Plants, previously grown in solutions maintained at 10 or 200 micromolar N03-(low N or Rates of uptake of the major inorganic nutrients, by roots of higher plants, are extremely sensitive to the availability of these nutrients in the ambient solution. Generally, the capacity for uptake increases with low availability so that internal concentrations tend to be maintained at prescribed levels (2, 13). Such responses have been interpreted to reflect the operation of regulatory mechanisms which may function via negative feedback effects upon the transport systems involved (3,4,11,19,23 (14) demonstrated that NO3-'influx' was reduced and NO3-efflux enhanced by prior growth of wheat seedlings at increasing levels of NO3-(0.25 to 15 mM). However, the term influx in these studies referred to rates of '5NO3 uptake based upon exposures ranging from 0.5 to 6 h. Unfortunately, if the half-life for cytoplasmic exchange is short by comparison to the uptake period, then the measured flux would approximate net uptake rather than unidirectional influx. In these experiments, NO3 efflux increased as the nitrate concentration of the growth medium was increased from 1.0 to 15 mm. Here also, the long term basis ofthe flux measurements (20.5 h) would inevitably lead to an underestimate ofefflux if t1/2for cytoplasmic exchange is short by comparison to flux measurements. Our recently reported estimate of cytoplasmic half-life for NO3 exchange in barley roots (7) suggest that this is indeed the case; t/2 was estimated to be 17 min. Nevertheless, the former study as well as earlier and more recent work from the same laboratory (16-18) attest to the potential importance of efflux in determining net uptake rates.Recently, two techniques which facilitate the measurement of nitrate fluxes over relatively short time periods (610 min) have been reported from this laboratory. First, 36C103-has been demonstrated to be a tracer for nitrate (7). Second, a microcomputer-based system has been described (12) which permits rapid and automatic measurement of ion uptake, using ion-specific electrodes. The frequency with which measurements can be obtained by the latter method leads to extremely sensitive estimates of uptake even in experiments of short duration (5-10 min). The present work was undertaken, using these methods, to explore the relationships between nitrogen status and NO3 flux into barley roots. MATERIALS AND METHODSPlant Culture. Barley plants (Hordeum vulgare L. cv Betzes or Bonanza) were grown hydroponically, in 36-L tanks containing 0.01x Johnson's modified inorganic medium in which NO3 was maintained at 10 gM (low N) or 200 gM (high N) as described in a previous communication (6). Tanks were contained in a Conviron growth cabinet at 26°C and 70% RH and illumination was provided by fluorescent l...

Studies of the Uptake of Nitrate in Barley

“…Our current observations and those of Lee and Drew (18) regarding the changes of '3N03-influx associated with increased N status are in conflict with earlier reports suggesting that NO3-influx is insensitive to negative feedback control and that net NO3-uptake is regulated by alterations of efflux (3,8 (26). Investigating the influence of N03-status on NO3-influx by completely withholding NO3-may further complicate the interpretation of results because of the decay of influx ('deinduction') associated with complete withdrawal of NO3- (29).…”

“…Net uptake studies should be interpreted with caution because they are the resultant of two separate fluxes, namely influx and efflux, and these processes may be mediated by two distinct transport systems. It is only recently that radioactive tracers (13N03-and 36C103-) have been used to measure unidirectional N03-fluxes with a view to probing the phenomena of induction and regulation of NO3-uptake (3,8,11,18,20,23,29, and references therein).…”

Nitrogen Utilization in Lemna

“…Differences in net NO3-uptake in relation to N-status of the Lemna cultures are directly associated with alterations in NO3-influx, whereas no assoiation to efflux is seen. These observations corroborate other studies with I"N03- (22,26), but contradict the contention of efflux being a major regulator of net NO3-uptake, as proposed previously (5,9,12).…”

“…The N-sufficient or NO3-loaded plants (22). Interpretations based on other experiments with 13NO3- (15) or 'T003- (7,9,12), however, rather suggested that the main control of net N03-uptake was related to modulation of NO3-efflux. The reasons for these discrepancies are obscuree; however, some of the contradictions might be anributed to different pretreatment ofthe plant material (22).…”

“…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).…”