The influence of NH4', in the external medium, on fluxes of NO3-and K were investigated using barley (Hordeum vulgare cv Betzes) plants.NWV was without effect on NO3-(3'10C3-) influx whereas inhibition of net uptake appeared to be a function of previous N03-provision. Plants grown at 10 micromolar NO3-were sensitive to external NHW when uptake was measured in 100 micromolar NO3-. By contrast, NO3-uptake (from 100 micromolar NO3-) by plants previously grown at this concentration was not reduced by NI-14 treatment. Plants pretreated for 2 days with 5 millimolar NO3-showed net efflux of NO3-when roots were transferred to 100 micromolar NO3-. This efflux was stimulated in the presence of NH4'. NH4' also stimulated NO3-efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO3-occur via effects upon efflux.By contrast to the situation for NO3-, net K' uptake and influx of 'Rb+-labeled K' was inhibited by NH4' regardless of the nutrient history of the plants. Inhibition of net K' uptake reached its maximum value within 2 minutes of NH4' addition. It is concluded that the latter ion exerts a direct effect upon K' influx.In barley, Lewis et al. (15), for example, showed that NH4' inhibited NO3-accumulation in the root and shoot and NO3-translocation to shoots although NO3-uptake was not measured directly. Recently, MacKown et al. (17,18) have shown that NH14' inhibits NO3-uptake and NO3-reduction in corn plants.Their results suggest that almost all the effect of NH4' on uptake of N03-is via its effect on the reduction step. There have also been reports of stimulated NO3-uptake arising as a consequence of NH4' pretreatment as in the case of seedlings of wheat (21) and corn (18).The almost universal choice of relatively long experimental periods (usually 2 to 24 h) for the investigation ofeffects of NH4+ upon nitrate uptake has made it extremely difficult to distinguish between effects on nitrate uptake per se and subsequent effects upon the properties (selectivity, turnover, etc.) of the nitrate transporter or effects on nitrate reduction and translocation (e.g. 17, 18). Both inhibition (9, 23, 24, 26) and stimulation (21) of in vitro nitrate reductase activity, following NH4' pretreatment, have been documented. Other workers using corn have shown that NH4+ inhibits NO3-reduction in vivo (18). However, the possible direct or indirect effects of NH4+ on the turnover of the nitrate transport proteins (presuming they are in fact proteins) will remain an open question until such proteins can be biochemically characterized.The potential importance of NH4, in the regulation of N03-metabolism has been widely recognized and explored in numerous studies of nitrate assimilation in micro-organisms and higher plants (e.g. 2-4, 12, 14). Considering the intermediary role of NH4+ in nitrate utilization, the former ion must represent a likely candidate for feedback effects on uptake and/or reduction of nitrate. Both short term (direct) effects of NH4' on...

The effect of various anions on the short-term influx on NO3− into barley roots was examined by the use of 36ClO3− as a tracer for nitrate. NO3− (36ClO3−) influx was found to be extremely sensitive to inhibition by external Cl− (Ki = 0.035 mM). By contrast 36Cl− influx was not significantly affected by external NO3−. Other anions, SO42− and H2PO4−, had no effect on NO3− (36ClO3−) influx, while HPO42−exerted a slight inhibitory effect. In the range from pH 4 to pH 8 no significant effect of OH− concentration on NO3− (36CIO3−) influx was detected. When barley roots were pretreated for periods up to 4 h with 10 mM NO3−, there was a significant reduction in NO3− (36ClO3−) and 36Cl− influx. KCl at the same concentration (10 mM) caused similar reductions of NO3− (36ClO3−) and Cl− influx. When plants were pretreated with lower NO3− or Cl− concentrations (0.05–5.0 mM) for 2 days, Cl− influx was found to be more sensitive than NO3− (36ClO3−) influx to nitrate or chloride pretreatment.

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...

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This paper argues that a genuine engagement of Christianity with evolution needs to include a discussion of Christology. Further, it develops a particular approach to Christology through a theo-dramatic account of incarnation. The somewhat static post-Chalcedon theological categories of divine and human natures are hard to square with contemporary evolutionary accounts of human origins. Once the divine Logos is portrayed in the active categories of Wisdom it becomes easier to envisage divine and creaturely wisdom coexisting in the person of Christ. I argue, in particular, that a focus on God's agency through a modified version of Hans Urs von Balthasar's account of theo-drama invites participation and affirms human agency in a way that grand narratives do not. More particularly, drawing on examples from hominid evolution, contemporary discussion of paleontology and cooperative evolutionary theories, I suggest that the most convincing accounts of evolutionary biology fit into this theodramatic account more readily than alternatives. As such, in the spirit of Robert Boyle, this paper deliberately blurs the categories of revealed and natural theology by arguing that we can make sense of the former through concentration on the latter.