Distribution of NO3− reduction between roots and shoots of peachtree seedlings as affected by NO3− uptake rate

The distribution of NO₃^? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings (Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO₃^?, together with transport through xylem and phloem of the newly reduced N were estimated, using ¹⁵N labellings, in intact plants supplied for 90 h with 0.5 mM NH₄⁺ and 0.5, 1.5 or 10 mM NO₃^?. Xylem transport of NO₃^? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO₃^? reduction at all 3 levels of NO₃^? nutrition. However, the contribution of the shoots to the whole plant NO₃^? reduction increased with increasing external NO₃^? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 mM NO₃^?, respectively. Both ¹⁵N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO₃^? translocation from roots to shoots. It is proposed that the lack of NO₃^? export to the shoots at low NO₃^? uptake rate resulted from a competition between NO₃^? reduction in the root epidermis/cortex and NO₃^? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO₃^? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO₃^? assimilation, principally occurring in the roots, was mainly incorporated in the shoots.     

The influence of enriched rhizosphere CO2 on N uptake and metabolism in wild-type and NR-deficient barley plants

Positive influences of high concentrations of dissolved inorganic carbon (DIC) in the growth medium of salinity-stressed plants are associated with carbon assimilation through phosphoenolpyruvate carboxylase (PEPc) activity in roots; and also in salinity-stressed tomato plants, enriched CO₂ in the rhizosphere increases NO^?₃uptake. In the present study, wild-type and nitrate reductase-deficient plants of barley (Hordeum vulgare L. cv. Steptoe) were used to determine whether the influence of enriched CO₂ on NO^?₃ uptake and metabolism is dependent on the activity of nitrate reductase (NR) in the plant. Plants grown in NH₄⁺and aerated with ambient air, were transferred to either NO₃^? or NH₄⁺ solutions and aerated with air containing between 0 and 6 500 μmol mol^?1 CO₂. Nitrogen uptake and tissue concentrations of NO₃^? and NH₄⁺ were measured as well as activities of NR and PEPc. The uptake of NO^?₃ by the wild-type was increased by increasing CO₂. This was associated with increased in vitro NR activity, but increased uptake of NO₃^? was found also in the NR-deficient genotype when exposed to high CO₂ concentrations; so that the influence of CO₂ on NO₃^? uptake was independent of the reduction of NO₃^? and assimilation into amino acids. The increase in uptake of NO₃^? in wild-type plants with enriched CO₂ was the same at pH 7 as at pH 5, indicating that the relative abundance of HCO₃^? or CO₂ in the medium did not influence NO₃^? uptake. Uptake of NH₄⁺ was decreased by enriched CO₂ in a pH (5 or 7) independent fashion. Thus NO₃^? and NH⁺₄ uptakes are influenced by the CO₂ component of DIC independently of anaplerotic carbon provision for amino acid synthesis, and CO₂ may directly affect the uptake of NO₃^? and NH₄⁺ in ways unrelated to the NR activity in the tissue.     

Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations

We examined the hypothesis that elevated CO₂ concentration would increase NO₃^– absorption and assimilation using intact wheat canopies (Triticum aestivum cv. Veery 10). Nitrate consumption, the sum of plant absorption and nitrogen loss, was continuously monitored for 23 d following germination under two CO₂ concentrations (360 and 1000 μmol mol^–1 CO₂) and two root zone NO₃^– concentrations (100 and 1000 mmol m³ NO₃^–). The plants were grown at high density (1780 m^–2) in a 28 m³ controlled environment chamber using solution culture techniques. Wheat responded to 1000 μmol mol^–1 CO₂ by increasing carbon allocation to root biomass production. Elevated CO₂ also increased root zone NO₃^– consumption, but most of this increase did not result in higher biomass nitrogen. Rather, nitrogen loss accounted for the greatest part of the difference in NO₃^– consumption between the elevated and ambient [CO₂] treatments. The total amount of NO₃^–-N absorbed by roots or the amount of NO₃^–-N assimilated per unit area did not significantly differ between elevated and ambient [CO₂] treatments. Instead, specific leaf organic nitrogen content declined, and NO₃^– accumulated in canopies growing under 1000 μmol mol^–1 CO₂. Our results indicated that 1000 μmol mol^–1 CO₂ diminished NO₃^– assimilation. If NO₃^– assimilation were impaired by high [CO₂], then this offers an explanation for why organic nitrogen contents are often observed to decline in elevated [CO₂] environments.     

Growth and Major Nutrient Concentrations in Brassica campestris Supplied with Different NH4^＋/NO3 Ratios

In the present study, we investigated whether growth and main nutrient ion concentrations of cabbage （Brassica campestris L.） could be increased when plants were subjected to different NH4^＋/NO3- ratios. Cabbage seedlings were grown in a greenhouse in nutrient solutions with five NH4^＋/NO3- ratios （1：0; 0.75：0.25; 0.5：0.5; 0.25：0.75; and 0：1）. The results showed that cabbage growth was reduced by 87% when the proportion of NH4^＋-N in the nutrient solution was more than 75% compared with a ratio NH4^＋/NO3- of 0.5：0.5 35 d after transplanting, suggesting a possible toxicity due to the accumulation of a large amount of free ammonia in the leaves. When the NH4＋/NO3- ratio was 0.5：0.5, fresh seedling weight, root length, and H2PO4- （P）, K^＋, Ca^2＋, and Mg^2＋ concentrations were all higher than those in plants grown under other NH4^＋/NO3- ratios. The nitrate concentration in the leaves was the lowest in plants grown at 0.5： 0.5 NH4^＋/NO3-. The present results indicate that an appropriate NH4^＋/NO3- ratio improves the absorption of other nutrients and maintains a suitable proportion of N assimilation and storage that should benefit plant growth and the quality of cabbage as a vegetable.     

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

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.     

Reduction in phosphoribulokinase activity by antisense RNA in transgenic tobacco: effect on CO2 assimilation and growth in low irradiance

To quantify the importance of the Calvin cycle enzyme phosphoribulokinase (PRK) in photosynthesis and to perturb photosynthesis without large direct reductions in leaf protein content, tobacco plants (Nicotiana tabacum L.) were transformed with an inverted cDNA encoding tobacco PRK. A population of plants expressing antisense RNA and a range of PRK activities from wild-type to less than 5% of wild-type were obtained. CO₂ assimilation under the growing conditions (330 µmol photons m^?2 sec^?1, 350 µbar CO₂, 25°C) was not inhibited until more than 85% of PRK activity had been removed. With reduction in PRK activity of between 85 and 95%, assimilation rates and amounts of chlorophyll compared with wild-type were reduced by up to half. Decreased absorption of light by leaves with less chlorophyll accounte0d for only a small part of the reduction in assimilation rate. When PRK activity was below 15% of wild-type, amounts of ribulose-5-phosphate, ribose-5-phosphate, ATP and fructose-6-phosphate were 1.5- to fivefold higher and levels of ribulose-1,5-bisphosphate, 3-phosphoglyceric acid and ADP 1.5- to fourfold lower than in wild-type. It is estimated that these changes maintained flux through PRK to realise the assimilation rates observed. A possible shift of control within the Calvin cycle towards fructose-1,6-bisphosphatase in plants with low PRK is discussed. Amounts of hexoses and starch in particular were reduced in plants expressing the lowest PRK activities; amounts of sucrose were little affected. Lower CO₂ assimilation in plants with low PRK activity correlated with reduced relative growth rate of shoots and delayed flowering, but there was no effect on specific leaf area. It is concluded that (i) in wild-type plants grown in constant low light, PRK has a flux-control coefficient for CO₂ assimilation of zero, and that even when amounts of PRK are reduced 20-fold relative to wild-type, altered amounts of metabolites compensate for much of the reduction in PRK protein; (ii) in plants where there is a 95% reduction in amounts of PRK, photosynthesis was reduced twofold without large changes in leaf protein content or leaf geometry.     

Differential responses of root uptake kinetics of NH4+ and NO3− to enriched atmospheric CO2 concentration in field-grown loblolly pine

The nitrogen requirement of plants is predominantly supplied by NH₄⁺ and/or NO₃^? from the soil solution, but the energetic cost of uptake and assimilation is generally higher for NO₃^? than for NH₄⁺. We found that CO₂ enrichment of the atmosphere enhanced the root uptake capacity for NO₃^?, but not for NH₄⁺, in field-grown loblolly pine saplings. Increased preference for NO₃^? at the elevated CO₂ concentration was accompanied by increased carbohydrate levels in roots. The results have important implications for the potential consequences of global climate change on plant-and ecosystem-level processes in many temperate forest ecosystems.     

Photosynthetic energy supply for NO3− assimilation in Scenedesmus

NO₃^?-dependent O₂ in synchronous Scenedesmus obtusiusculus Chod. in the absence of CO₂ is stoichiometric with NH₄⁺ excretion, indicating a close coupling of NO₃^? reduction to non-cyclic electron flow. Also in the presence of CO₂, NO₃^? stimulates O₂ evolution as manifested by an increase in the O₂/CO₂ ratio from 0.96 to 1.11. This quotient was increased to 1.36 by addition of NO₂^?, without competitive interaction with CO₂ fixation, indicating that the capacity for non-cyclic electron transport at saturating light is non-limiting for simultaneous reduction of NO₃^? and CO₂ at high rates. During incubation with NO₃^?+ CO₂, no NH₄⁺ is released to the outer medium, whereas during incubation with NO₂^?+ CO₂, excess NH₄⁺ is formed and excreted. NO₃^? uptake is stimulated by CO₂, and this stimulation is also significant when the cellular energy metabolism is restricted by moderate concentrations of carbonyl cyanide-p-trifluoromethoxyphenylhydrazone, whereas NO₃^? uptake in the absence of CO₂ is severely inhibited by the uncoupler. Also under energy-restricted conditions NO₃^? uptake is not competitive with CO₂ fixation. Antimycin A is inhibitory for NO₃^? uptake in the absence of CO₂, and there is no enhancement of NO₃^? uptake by CO₂ in the presence of antimycin A. It is assumed that the energy demand for NO₃^? uptake is met by energy fixed as triosephosphates in the Calvin cycle. Antimycin A possibly affects the transfer of reduced triose phosphates from the chloroplast to the cytoplasm. Active carbon metabolism also seems to exert a control effect on NO₃^? assimilation, inducing complete incorporation of all NO₃^? taken up into amino acids. This control effect is not functional when NO₂^? is the nitrogen source. Active carbon metabolism thus seems to be essential both for provision of energy for NO₃^? uptake and for regulation of the process.     

Effect of NO3- supply on N metabolism of potato plants (Solanum tuberosum L.) with special focus on the tubers

The response of the tubers to NO₃^– was studied in comparison to the other organs of Solanum tuberosum var. Sava, with special focus on: (a) whether tubers are capable of primary N assimilation; (b) whether N assimilation is stimulated by NO₃^–; and (c) whether primary N assimilation in tubers is important for tuber growth. NO₃^– reduction via nitrate reductase (NR; EC 1.6.6.1) and NH₄⁺ assimilation via glutamine synthetase (GS; EC 6.3.1.2) occurred predominantly in the shoots, but up to 20% was contributed by the tubers under low‐NO₃^– conditions. NR activation was highest in tubers (up to 90%) and declined in all organs with increasing NO₃^– supply. NR and GS activity responded with a decline in tubers and roots as opposed to an increase in the shoots. This corresponded to relative organ growth: growth of tubers and roots was stimulated relative to that of shoots at a limiting NO₃^– supply. Absolute growth of all organs was stimulated by NO₃^–, whereas tuber number declined. The concentration of N compounds increased with NO₃^– supply in all organs: NO₃^– increased most dramatically in the shoots (81‐fold), free amino acids most markedly in the tubers (three‐fold). The amount of patatin and of the 22 kDa protein complex in the tuber reached a minimum when the amount of Rubisco in the shoot reached maximum as a response to NO₃^– supply. Tuber sucrose and starch increased by 40%, whereas glucose and fructose declined two‐fold as plant N status increased. It is concluded that tubers are potentially N autotroph organs with capacity for de novo synthesis of amino acids. Primary N assimilation in tubers, however, declines with increasing NO₃^– supply and is not of major importance for tuber growth.     

Nitrate and ammonium influxes in soybean (Glycine max) roots: direct comparison of 13N and 15N tracing

We compared influxes and internal transport in soybean plants (Glycine max cv. Kingsoy) of labelled N from external solutions where either ammonium or nitrate was labelled with the stable isotope¹⁵N and the radioactive isotope¹³N. The objective was to see whether mass spectrometric determinations of tissue ¹⁵N content were sufficiently sensitive to measure influxes accurately over short time periods. Our findings were as follows. (1) There was a close quantitative correspondence between estimates of N influx of individual plants using ¹⁵N or ¹³N measurements with either NO_3/^? or NH₄⁺ at 4 or 2 mol^?3, respectively in the external solution. (2) Transport to the shoot of N from NO₃ absorbed over a 5–15 min period could be monitored when the external NO₃^? concentration ranged from 0–05 to 4 mol m^?3. NH₄⁺ as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with ¹³N. (3) Influx of NO₃^? into root tissue could be measured by ¹⁵N enrichment after 5–10 min at concentrations approaching the probable K_M of the high-affinity transport system. (4) There was some indication of isotope discrimination, especially with respect to the movement of labelled N to the shoot, when NO₃^? is the N source. For many purposes, ¹⁵N tracing can be used satisfactorily to estimate influxes of both NO₃^? and NH₄⁺ in soybean roots. Use of the short-lived radio nuclide ¹³N remains the method of choice for more refined measurements of internal distribution and assimilation.     

Influence of elevated CO2 and nitrogen nutrition on photosynthesis and nitrate photo-assimilation in maize (Zea mays L.)

Measurements of CO₂ and O₂ gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO₂ inhibits nitrate (NO₃ ^– ) photo‐assimilation in the C₄ plant, maize (Zea mays L.). The assimilatory quotient (AQ), the ratio of net CO₂ assimilation to net O₂ evolution, decreases as NO₃ ^– photo‐assimilation increases so that the difference in AQ between the ammonium‐ and nitrate‐fed plants (ΔAQ) provided an in planta estimate of NO₃ ^– photo‐assimilation. In fully expanded maize leaves, NO₃ ^– photo‐assimilation was detectable only under high light and was not affected by CO₂ treatments. Furthermore, CO₂ assimilation and O₂ evolution were higher under NO₃ ^– than ammonia (NH₄⁺) regardless of CO₂ levels. In conclusion, NO₃ ^– photo‐assimilation in maize primarily occurred at high light when reducing equivalents were presumably not limiting. Nitrate photo‐assimilation enhanced C₄ photosynthesis, and in contrast to C₃ plants, elevated CO₂ did not inhibit foliar NO₃^– photo‐assimilation.     

Emission of nitrous oxide (N2O) from transgenic tobacco expressing antisense NiR mRNA

The emission of N₂ and N₂O from intact transgenic tobacco (clone 271) expressing antisense nitrite reductase (NiR) mRNA, and wild-type plants grown aseptically, on NO₃^–, NO₂^– or NH₄⁺ -containing medium was investigated. ¹⁵N contents of gas sampled from gas-sealed pots, in which the plants were grown on ¹⁵N-containing medium, were analyzed by gas chromato- graphy and mass spectrometry (GC–MS). No emission of N₂ was detected in either of the gas samples from plant clone 271 or the wild-type grown on NO₃^–-containing medium. N₂O emission from clone 271 grown on NO₃^–-containing medium was detected, but not from the wild-type plants. The N₂O emission rate of clone 271 was 106 ng N₂O mg^–1 incorporated N week^–1 and the N₂O emission was inhibited by tungstate (a nitrate reductase inhibitor). No emission of N₂O was found from clone 271 or wild-type plants grown on medium containing NH₄⁺. Emission of N₂O also was detected from clone 271 grown on NO₂^–-containing medium and its emission rate increased with increasing NO₂^– levels in plants. We speculate that NO₃^– is reduced to NO₂^– and that a part of NO₂^– is metabolized to N₂O in clone 271.     

Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5)

Tomato growth was examined in solution culture under constant pH and low levels of NH₄⁺ or NO₃^?. There were five nitrogen treatments: 20 mmoles m^?3 NH₄⁺, 50 mmoles m^?3 NO₃^?, 100 mmoles m^?3 NH₄⁺ 200 mmoles m^?3 NO₃^?, and 20 mmoles m^?3 NH₄₊+ 50 mmoles m^?3 NO₃^?. The lower concentrations (20 mmoles m^?3 NH₄⁺ and 50 mmoles m^?3 NO₃^?) were near the apparent K_m for net NH₄⁺ and NO₃^? uptake; the higher concentrations (100 mmoles m^?3 NH₄⁺ and 200 mmoles m^?3 NO₃^?) were near levels at which the net uptake of NH₄⁺ or NO₃^? saturate. Although organic nitrogen contents for the higher NO₃^? and the NH₄⁺+ NO₃^? treatments were 22.2–30.3% greater than those for the lower NO₃^? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH₄⁺ or NO₃^? concentrations are held constant over the root surface at near the apparent K_m concentration. Relative growth rates for the two NH₄⁺ treatments were much higher than have been previously reported for tomatoes growing with NH₄⁺ as the sole nitrogen source. Initial growth rates under NH₄⁺ nutrition did not differ significantly (P≥ 0.05) from those under NO₃^? or under combined NH₄⁺+ NO₃^?. Growth rates slowed after 10–15 d for the NH₄⁺ treatments, whereas they remained more constant for the NO₃^? and mixed NH₄⁺+ NO₃^? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH₄⁺ nutrition may have resulted from a reduction in Ca²⁺, K⁺, and/or Mg²⁺ absorption.     

Blue-light-induced pH changes associated with NO3−, NO2− and Cl− uptake by the green alga Monoraphidium braunii

In M. braunii, the uptake of NO₃⁻ and NO₂⁻ is blue-light-dependent and is associated with alkalinization of the medium. In unbuffered cell suspensions irradiated with red light under a CO₂-free atmosphere, the pH started to rise 10s after the exposure to blue light. When the cellular NO₃⁻ and NO₂⁻ reductases were active, the pH increased to values of around 10, since the NH₄⁺ generated was released to the medium. When the blue light was switched off, the pH stopped increasing within 60 to 90s and remained unchanged under background red illumination. Titration with H₂SO₄ of NO₃⁻ or NO₂⁻ uptake and reduction showed that two protons were consumed for every one NH₄⁺ released. The uptake of Cl⁻ was also triggered by blue light with a similar 10 s time response. However, the Cl⁻ -dependent alkalinization ceased after about 3 min of blue light irradiation. When the blue light was turned off, the pH immediately (15 to 30 s) started to decline to the pre-adjusted value, indicating that the protons (and presumably the Cl⁻) taken up by the cells were released to the medium. When the cells lacked NO₃⁻ and NO₂⁻ reductases, the shape of the alkalinization traces in the presence of NO₃⁻ and NO₂⁻ was similar to that in the presence of Cl⁻, suggesting that NO₃⁻ or NO₂⁻ was also released to the medium. Both the NO₃⁻ and Cl⁻-dependent rates of alkalinization were independent of mono- and divalent cations.     

Decreased ribulose-1,5-bisphosphate carboxylase/oxygenase in transgenic tobacco transformed with 'antisense'rbcS.VIII. Impact on photosynthesis and growth in tobacco growing under extreme high irradiance and high temperature

Wild-type and antisense rbcS tobacco (Nicotiana tabacum) plants were grown in a glasshouse in midsummer in Portugal with an irradiance of 1500–2000 μmol m⁻²s⁻¹ and daytime temperatures of 30–35 °C. The Rubisco content of the transformants was lower by 35, 80 and over 90% than that of the wild-type. Gas exchange was measured over three separate days. There was a near-linear relation between Rubisco content and photosynthetic rate during the period of high irradiance, allowing a flux control coefficient of 0.83–0.89 to be estimated. The relation deviated slightly from linearity, because the internal CO₂ concentration (c;) was higher in the transformants than in the wild-type (190 and 275 μmol mol⁻¹ in plants with 35 and 80% less Rubisco, respectively, compared with 175 μmol mol⁻¹ for wild-type), compensating to some extent for the decreased Rubisco content. This increase in c_i occurred because the stomatal conductance (g) remained unaltered or was even higher in plants with decreased Rubisco, despite the lower rate of CO₂ assimilation. As a consequence, water use efficiency declined. The decreased rate of photosynthesis was not accompanied by a stoichiometric decrease in apparent growth rate. These results are discussed in relation to earlier studies of the plant set in growth cabinets. It is concluded that tobacco can adjust over a wide range of growth conditions to avoid a onesided limitation by Rubisco, but that in extreme environmental conditions this capacity to adapt is exhausted.     

Mechanism of nitrate uptake into Chara corallina cells: lack of evidence for obligatory coupling to proton pump and a new NO3−/NO3− exchange model

Abstract. Nitrate uptake into Chara corallina cells at different external pH (pH_o) after different NO₃⁻ pretreatment conditions has been investigated. Following NO₃⁻ pretreatment (0.2 mol m⁻³ NO₃⁻) there was little effect of pH_o on subsequent net NO₃⁻ uptake into Chara cells. After N deprivation (2 mmol m⁻³ NO₃⁻) there was a pronounced effect of pH_o on nitrate uptake, the maximum rate occurring at pH_o 4.7. There was no consistent relationship between OH⁻ efflux and NO₃⁻ uptake in short term experiments (< 1 h). NO₃⁻ efflux was also sensitive to pH_o, the maximum rate occurring at ∼ pH_o 5.0. An inhibitor of the proton pump, DES, immediately stimulated NO₃⁻ uptake into cells pretreated with NO₃⁻ and prevented the time-dependent decrease in NO₃⁻, uptake into Chara cells that had been previously treated with low N (2 mmol m⁻³ NO₃⁻). NO₃⁻ efflux was found to be very sensitive to DES with K_i= 0.7 mmol m⁻³. At the optimum pH_o for NO₃⁻ uptake the effect of DES on membrane potential (ψ_m) were slight, and only apparent after 30 min. The results are interpreted in context of current models relating NO₃⁻ uptake and H⁺ pump activity. A new model for NO₃⁻ uptake is proposed which involves NO₃⁻/NO₃⁻ exchange at steady state.     

Alterations in nitrogen assimilation and partitioning in nitrogen stressed plants

Although nutrient stress is known to alter partitioning between shoots and roots, the physiological basis for the phenomenon is unresolved. Experiments were conducted to examine assimilation of ¹⁵NO₃ by N-stressed plants and to determine whether apparent changes in assimilation in the root contributed to alterations in whole-plant partitioning of reduced-N. Tobacco plants (Nicotiana tabacum L. cv. NC 2326) were exposed to a low concentration of NO₃^? in solution (80 μM) for 9 days to effect a N-stress response. Exposure of plants to 1000 μM¹⁵NO₃^? for 12 h on selected days revealed that roots of N-stressed plants developed an increased capacity to absorb NO₃^?, and accumulation of reduced-¹⁵N in the root increased to an even greater extent. When plants were exposed to 80 or 1000 μM¹⁵NO₃^? in steady-state, ¹⁵NO₃^? uptake over a 12 h period was noticeably restricted at the lower concentration, but a larger proportion of the absorbed ¹⁵N still accumulated as reduced-¹⁵N in the root. The alteration in reduced-¹⁵N partitioning was maintained in N-stressed plants during the subsequent 3-day “chase” period when formation of insoluble reduced-¹⁵N in the root was quantitatively related to the disappearance of ¹⁵NO₃^? and soluble reduced-¹⁵N. The results indicate that increased assimilation of absorbed NO₃^?, in the root may contribute significantly to the altered reduced-N partitioning which occurs in N-stressed plants.     

Uptake of atmospheric 15NO2 and its incorporation into free amino acids in wheat (Triticum aestivum)

Five-week-old wheat plants were exposed, under controlled environmental conditions, to 60 nl 1^?115NO₂ or to purified air. After 48 and 96 h of exposure, leaves, stalks and roots were analysed for ¹⁵N-enrichment in α-amino nitrogen of soluble, free amino acids. In addition, the in vitro nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NIR, EC 1.7.7.1) activities were determined in the leaves. NR activity in the leaves decreased continously during the 96-h exposure to purified air. In the leaves exposed to ¹⁵NO₂, NR activity increased within the first 24 h, then decreased, and reached the level of controls after 96 h. NiR activity in leaves exposed to purified air was almost constant during the 96-h exposure. In leaves exposed to ¹⁵NO₂, NiR activity increased within the first 48 h, then decreased, and reached the level of controls after 72 h, Exposure to ¹⁵NO₂ enhanced the total content of soluble, free amino acids in all tissues analysed. Most of this increase was attributed to Glu in the leaves and to Asn plus Gln the α-amino group of soluble, free amino acids was observed in the leaves, the lowest enrichment in the roots. The main labelled amino compounds were Glu (with 8.0%¹⁵N enrichment compared to the control), γ-aminobutyric acid (GABA; 7.9%), Ala (7.2%). Ser (6.8%), Asp (5.5%) and Gln (4.6%). Appreciable incorporation of ¹⁵ into Asn was not found. After 96 h exposure to ¹⁵NO₂ the ¹⁵N enrichment in the α-amino group of soluble, free amino acids in the leaves declined as compared to the values obtained after 48 h fumigation. The possible pathway and the time course of ¹⁵N incorporation into soluble, free amino acids from the ¹⁵NO₂ absorbed are discussed.     

Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18OO during photosynthesis

The ¹⁸O content of CO₂ is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO₂ and ¹⁸O-enriched leaf water and retrodiffusion of most of this CO₂ back to the atmosphere, leaves effectively discriminate against ¹⁸O during photosynthesis. Discrimination against ¹⁸O ( Δ ¹⁸O) is expected to be lower in C₄ plants because of low c_i and hence low retrodiffusing CO₂ flux. C₄ plants also generally show lower levels of carbonic anhydrase (CA) activities than C₃ plants. Low CA may limit the extent of ¹⁸O exchange and further reduce Δ ¹⁸O. We investigated CO₂–H₂O isotopic equilibrium in plants with naturally low CA activity, including two C₄ (Zea mays, Sorghum bicolor) and one C₃ (Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ ¹⁸O in C₄, as well as in some C₃, plants. Variations in CA activity and in the extent of CO₂–H₂O isotopic equilibrium ( θ _eq) estimated from on-line measurements of Δ ¹⁸O showed large range of 0–100% isotopic equilibrium ( θ _eq = 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO₂ concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ ¹⁸O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ _eq < 1 (incomplete isotopic equilibrium) on ¹⁸O of atmospheric CO₂ can be similar for C₃ and C₄ plants and in both cases it increases with natural enrichment of ¹⁸O in leaf water.     

Reduced sedoheptulose-1,7-bisphosphatase levels in transgenic tobacco lead to decreased photosynthetic capacity and altered carbohydrate accumulation

Transgenic tobacco (Nicotiana tabacum L. cv. Samsun) plants with reduced levels of the Calvin cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase; EC 3.1.3.37) were produced using an antisense construct in which the expression of a tobacco SBPase cDNA clone was driven by the cauliflower mosaic virus (CaMV) promoter. The reduction in SBPase protein levels observed in the primary transformants correlated with the presence of the antisense construct and lower levels of the endogenous SBPase mRNA. No changes in the amounts of other Calvin cycle enzymes were detected using Western blot analysis. The SBPase antisense plants with less than 20% of wild-type SBPase activity were observed to display a range of phenotypes, including chlorosis and reduced growth rates. Measurements of photosynthesis, using both light-dosage response and CO₂ response curves, of T1 plants revealed a reduction in carbon assimilation rates, which was apparent in plants retaining 57% of wild-type SBPase activity. Reductions were also observed in the quantum efficiency of photosystem II. This decrease in photosynthetic capacity was reflected in a reduction in the carbohydrate content of leaves. Analysis of carbohydrate status in fully expanded source leaves showed a shift in carbon allocation away from starch, whilst sucrose levels were maintained in all but the most severely affected plants. Plants with less than 15% of wild-type SBPase activity were found to contain less than 5% of wild-type starch levels. The results of this preliminary analysis indicate that SBPase activity may limit the rate of carbon assimilation. Received: 23 February 1997 / Accepted: 2 May 1997