Cultivar differences in the rate of nitrate uptake by intact wheat plants as related to growth rate

Six Argentinian wheat ( Triticum aestivum L.) cultivars grown in nutrient solutions in controlled environment were compared for their nitrate uptake rates on a root dry weight basis. Up to 3-fold differences were observed among the cultivars at 16, 20 and 24 days from germination, either when measured by depletion from the nutrient solution in short-term experiments, or by total N accumulation in the tissue during 8 days.
No differences in total N concentration in root or shoots were found among cultivars. Although the different cultivars showed significant differences in shoot/root ratio and nitrate reductase activity (EC 1.6.6.1) in the roots, none of these parameters was correlated with the nitrate uptake rate. However, nitrate uptake was found to be positively correlated (r = 0.99) with the shoot relative growth rate of the cultivars. The three cultivars with the highest nitrate uptake rates and relative growth rates showed a positive correlation between root nitrate concentration and uptake. However, this correlation was not found in the cultivars with the lowest growth and uptake rates.
Our results indicate that the difference in nitrate uptake rate among these cultivars may only be a consequence of their differences in growth rate, and it is suggested that at least two mechanisms regulate nitrate uptake, one working when plant demand is low and another when plant demand is high.     

Relatively large nitrate efflux can account for the high specific respiratory costs for nitrate transport in slow-growing grass species

In this paper we address the question why slow-growing grass species appear to take up nitrate with greater respiratory costs than do fast-growing grasses when all plants are grown with free access to nutrients. Specific costs for nitrate transport, expressed as moles of ATP per net mole of nitrate taken up, were 1.5 to 4 times higher in slow-growing grasses than in fast-growing ones (Scheurwater et al., 1998, Plant, Cell & Environ. 21, 995–1005). The net rate of nitrate uptake is determined by two opposing nitrate fluxes across the plasma membrane: influx and efflux. To test whether differences in specific costs for nitrate transport are due to differences in the ratio of nitrate influx to net rate of nitrate uptake, nitrate influx and the net rate of nitrate uptake were measured in the roots of two fast-growing ( Dactylis glomerata L. and Holcus lanatus L.) and two slow-growing (Deschampsia flexuosa L. and Festuca ovina L.) grass species at four points during the diurnal cycle, using ¹⁵NO₃ ^-. Efflux was calculated by subtraction of net uptake from influx; it was assumed that efflux of nitrogen represents the flux of nitrate. Transfer of the plants to the solution containing the labelled nitrate did not significantly affect nitrate uptake in the present grass species. The net rate of nitrate uptake was highest during the middle of the light period in all species. Diurnal variation in the net rate of nitrate uptake was mostly due to variation in nitrate influx. Variation in nitrate efflux did not occur in all species, but efflux per net mole of nitrate taken up was higher during darkness than in the light in the slow-growing grasses. The two fast-growing species, however, did not show diurnal variation in the ratio of efflux to net nitrate uptake. Integrated over 24 hours, the slow-growing grasses clearly exhibited higher ratios of influx to net uptake than the fast-growing grass species. Our results indicate that the higher ratio of nitrate influx to net nitrate uptake can account for higher specific costs for nitrate transport in slow-growing grass species compared with those in their fast-growing counterparts, possibly in combination with greater activity of the non-phosphorylating alternative respiratory path. Therefore, under our experimental conditions with plants grown at a non-limiting nitrate supply, nitrate uptake is less efficient (from the point of ATP consumption) in slow-growing grasses than in fast-growing grass species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of one night with "low light'on uptake, reduction and storage of nitrate in spinach

During the night, shoot nitrate concentration in spinach (Spinacia oleracea L. cv. Vroeg Reuzenblad) increased due to increased uptake of nitrate by the roots. When the plants were subjected to a one night “low light’period at 35 μmol m^?2 s^?1, the shoot nitrate concentration did not increase and was reduced by 25% compared to control plants in the dark. The major contribution to this decrease was located in the leaf blades, where the nitrate concentration was decreased by 60%, while the petiole nitrate concentration decreased by only 9%. Nitrate accumulated in the leaf blade vacuoles during a dark night, but this was not the case during the “low light’period. This decrease in vacuolar nitrate concentration, compared to control plants in the dark, was not caused by increased amounts of leaf blade nitrate reductase (NR; EC 1.6.6.1). During a “low light’night period, the cytoplasmic soluble carbohydrate concentration was increased compared to the control plants in the dark. Calculations showed in situ NR activity to be higher than in the control plants in the dark. This increase in NR activity, however, was not large enough to account for the total difference found in the shoot nitrate concentration. Net uptake of nitrate by the roots was increased during the initial hours of the dark night, while vacuolar nitrate concentration in the leaf blades increased at the same time. During the “low light’night period, however, net uptake of nitrate by the roots did not increase, and vacuolar nitrate concentration did not change. We conclude that nitrate uptake by the roots and vacuolar nitrate concentration in the leaf blades are tightly coupled. The decreased shoot nitrate concentration is mainly caused by a reduction in net uptake of nitrate by the roots. During the “low light’night period, carbohydrates and malic acid partly replaced vacuolar nitrate. A “low light’period one night prior to harvest provides a valuable tool to reduce shoot nitrate concentrations in spinach grown in greenhouses in the winter months.     

Nutrition and growth of coniferous seedlings at varied relative nitrogen addition rate

The growth of two provenances of Pinus sylvestris L. were compared with two provenances of Picea abies (L.) Karst. and with Pinus contorta Dougl. when grown in solution cultures with low nutrient concentrations. Nitrogen was added at different exponentially increasing rates, and the other nutrients were added at a rate high enough to ensure free access of them to the seedlings. During an initial period of the culture (a lag phase), when the internal nutrient status was changing from optimum to the level of the treatment, deficiency symptoms appeared. The needles yellowed and the root/shoot ratio increased. The initial phase was followed by a period of exponential growth and steady-state nutrition. The needles turned green again, and the root/shoot ratio stabilized at a level characteristic of the treatment. These patterns were the same as previously reported for other tree species. The relative growth rate during exponential growth was numerically closely equal to the relative nitrogen addition rate. The maximum relative growth rates were about 6 to 7.5% dry weight increase day^-1. This is a much lower maximum than for broad-leaved species (about 20 to 30% day^-1) under similar growth conditions. The internal nitrogen concentrations of the seedlings and the relative growth rates were stable during the exponential period. Close linear relationships were found between these parameters and the relative addition rate up to maximum growth. During steady state the relative growth rates of the different plant parts were equal. However, there were large differences between genotypes in absolute root growth rate at the same seedling size because of differences in root/shoot ratio. Lodgepole pine had the highest root growth rate, whereas that of Norway spruce, especially the southern provenance, was remarkably low. Yet, Norway spruce had a high ability to utilize available nutrients. In treatments with free nutrient access, growth allocation to the shoot had a high priority in all genotypes, but there was still a marked tendency for luxury uptake of nutrients. Nitrogen productivity (growth rate per unit of nitrogen) was lower than in broadleaved species and highest in lodgepole pine. The relevance of the dynamic factors, i.e. maximum relative growth rate, nutrient uptake rate, nitrogen productivity, growth allocation and root growth rate, are discussed with regard to conifer characteristics and selection value.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Influence of nitrate concentration at the root surface on yield and nitrate uptake of kohlrabi (Brassica oleracea gongyloides L.) and spinach (Spinacia oleracea L.)

The objective of this study was to determine if plant roots have to take up nitrate at their maximum rate for achieving maximum yield. This was investigated in a flowing-solution system which kept nutrient concentrations at constant levels. Nitrate concentrations were maintained in the range 20 to 1000 μM. Maximum uptake rate for both species was obtained at 100 μM. Concentrations below 100 μM resulted in decreases in uptake rate per cm root (inflow) for both spinach and kohlrabi by 1/3 and 2/3, respectively. However, only with kohlrabi this caused a reduction in N uptake and yield. Thus indicating that this crop has to take up nitrate at the maximum inflow. Spinach, however, compensated for lower inflows by enhancing its root absorbing surface with more and longer roots hairs. Both species increased their root length by 1/3 at low nitrate concentrations.     

Modelling the relative growth rate as a function of plant nitrogen concentration

The relationship between the relative growth rate (RGR) and the nitrogen concentration of the whole plant (PNC) was analyzed by using experimentally determined relations (1) between the PNC and the fraction of dry matter (LWR) and nitrogen in leaves, (2) between the specific leaf area (SLA) and the leaf nitrogen concentration (LNC) and (3) between the net assimilation rate (NAR) and the LNC on an area basis. A strong dependence of RGR on nitrogen concentration resulted from the increase in NAR, LWR and SLA with increasing PNC. A curvilinear relationship between RGR and PNC gave an optimum curve for nitrogen productivity against PNC.     

Growth and shoot:root partitioning of spinach plants as affected by nitrogen supply

Spinach plants (Spinacia oleracea L.) were grown hydroponically in fixed environmental conditions either at full nitrate availability (11·8mol m^-3) or at a suboptimum relative nitrate addition rate of 0·20d^-1, 0·15d^-1 or 0·10d^-1 respectively, the other nutrients being adequately provided. The relative growth rate (RGR) of the plants varied significantly with the nutrition treatment and decreased during development in all treatments. The concentration of reduced nitrogen in the plants grown at full nitrate availability did not change significantly during the experimental growth period and nitrate accumulation was substantial. After an adaptation period, the concentration of reduced nitrogen in the plants at the suboptimum nitrate addition rates increased during growth and was lowest at the lowest relative nitrate addition rate. Nitrate uptake was almost complete in the suboptimum treatments and nitrate accumulation was negligible as long as the concentration of reduced nitrogen was below 2·0 mmol (g dry weight)^-1. The RGR of all plants was proportional to the concentration of reduced nitrogen in the plant minus a minimal tissue concentration required for growth. However, the proportionality factor was inversely related to the plant mass. This relationship was summarized in an empirical model which explained 98·7% of the variance of the dry weight (log scale) data of all treatments at all harvests. The model was compared with other growth models found in the literature. The shoot/root weight ratio increased from 2 to 4 if nitrate provision was not limiting, and initially, this ratio decreased at suboptimum nitrate provision but increased at higher growth stages. Possible explanations of the dynamics of dry matter partitioning are discussed in relation to models.     

Changes in growth and nutrient uptake in Brassica oleracea exposed to atmospheric ammonia

BACKGROUND AND AIMS: Plant shoots form a sink for NH3, and are able to utilize it as a source of N. NH3 was used as a tool to investigate the interaction between foliar N uptake and root N uptake. To what extent NH3 can contribute to the N budget of the plant or can be regarded as a toxin, was investigated in relation to its concentration and the N supply in the root environment. METHODS: Brassica oleracea was exposed to 0, 4 and 8 microL L(-1) NH3, with and without nitrate in the nutrient solution. Growth, N compounds, nitrate uptake rate, soluble sugars and cations were measured. KEY RESULTS: In nitrate-sufficient plants, biomass production was not affected at 4 microL L(-1) NH3, but was reduced at 8 microL L(-1) NH3. In nitrate-deprived plants, shoot biomass was increased at both concentrations, but root biomass decreased at 8 microL L(-1) NH3. The measured nitrate uptake rates agreed well with the plant's N requirement for growth. In nitrate-sufficient plants nitrate uptake at 4 and 8 microL L(-1) NH3 was reduced by 50 and 66 %, respectively. CONCLUSIONS: The present data do not support the hypothesis that NH3 toxicity is caused by a shortage of sugars or a lack of capacity to detoxify NH3. It is unlikely that amino acids, translocated from the shoot to root, are the signal metabolites involved in the down-regulation of nitrate uptake, since no relationship was found between changes in nitrate uptake and root soluble N content of NH3-exposed plants.     

Effects of nitrate on influx, efflux and translocation of potassium in young sunflower plants

Influx, efflux and translocation of K⁺(⁸⁶Rb) were studied in the roots of sunflower seedlings ( Helianthus annuus L. cv. Uniflorus) treated with 0–4.0 m M NO₃⁻ during a 9 day growth period or a 24 h pretreatment period. Roots treated with high levels of NO₃⁻ absorbed and translocated more K⁺(⁸⁶Rb) than seedlings treated with low levels of NO₃⁻. The content of K⁺ in the shoots was, however, higher in seedlings treated with low levels of NO₃⁻, indicating a low rate of retranslocation of K⁺ in those plants. K⁺(⁸⁶Rb) efflux was highest into the low-NO₃⁻ solutions. All effects on K⁺(⁸⁶Rb)-fluxes were more obvious in high-K plants than in low-K plants. The results are discussed in relation to the Dijkshoorn-Ben Zioni hypothesis for K⁺+ NO₃⁻-uptake and translocation in plants.     

Diurnal variations in growth rate and growth substrate levels of spinach (Spinacia oleracea L.) under nitrogen-limiting conditions

Spinach plants were grown in hydroponic culture provided with variable limiting amounts of N. During a complete diurnal cycle, growth of the root and shoot parts, as well as levels of soluble and insoluble sugars and of free amino acids, were monitored. No clear relationship could be detected between the level of N feeding and the levels of free sugars and amino acids. Analysis of variance revealed that the variances in the relative growth rates of plant root and shoot could be correlated with the levels of sugars and amino acids. Root amino acid concentration could be correlated with shoot amino acid concentration and root sugar concentration. No relationship was found between the variances in root and shoot free sugar concentrations.     

Influx,efflux and net uptake of nitrate in Quercus suber seedlings

Nitrate influx, efflux and net nitrate uptake were measured for the slow-growing Quercus suber L. (cork-oak) to estimate the N-uptake efficiency of its seedlings when grown with free access to nitrate. We hypothesise that nitrate influx, an energetically costly process, is not very efficiently controlled so as to avoid losses through efflux, because Q. suber has relatively high respiratory costs for ion uptake. Q. suber seedlings were grown in a growth room in hydroponics with 1 mM NO₃ ^-. Seedlings were labelled with ¹⁵NO₃ ^- in nutrient solution for 5 min to measure influx and for 2 h for net uptake. Efflux was calculated as the difference between influx and net uptake. Measurements were made in the morning, afternoon and night. The site of nitrate reduction was estimated from the ratio of NO₃ ^- to amino acids in the xylem sap; the observed ratio indicated that nitrate reduction occurred predominantly in the roots. Nitrate influx was always much higher than net acquisition and both tended to be lower at night. High efflux occurred both during the day and at night, although the proportion of ¹⁵NO₃ ^- taken up that was loss through efflux was proportionally higher during the night. Efflux was a significant fraction of influx. We concluded that the acquisition system is energetically inefficient under the conditions tested. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of metabolic deprivation on methotrexate transport in L1210 leukemia cells: Further evidence for separate influx and efflux systems with different energetic requirements

Summary Measurements of methotrexate transport in L1210 cells in the presence and absence ofd-glucose reveal that both influx and efflux are depressed in the absence ofd-glucose, whereas the steady-state accumulation of drug is enhanced. The reason for the increase in steady state is that the relative decline in efflux is greater than the decline in influx. Analysis of the concentration dependence of steady-state methotrexate accumulation ind-glucose-deprived cells indicates a linear relationship consistent with a one-carrier active transport model. Similar data in nondeprived cells is highly nonlinear and strongly supports the postulate that under physiological conditions influx and efflux of methotrexate are mediated by separate carrier systems. These results indicate that the efflux system, preferentially transporting methotrexate under normal conditions, cannot operate in the absence ofd-glucose, whereas the influx system is only partially inhibited under conditions of glucose deprivation.     

Intra-specific variation in relative growth rate: impact on competitive ability and performance of Lychnis flos-cuculi in habitats differing in soil fertility

Plant species from unproductive or adverse habitats are often characterized by a low potential relative growth rate (RGR). Although it is generally assumed that this is the result of selection for specific trait combinations that are associated with a low rate of net biomass accumulation, few studies have directly investigated the selective (dis-)advantage of specific growth parameters under a set of different environmental conditions. Aim of the present study was to quantify the impact of inherent differences in growth parameters among phenotypes of a single plant species, Lychnis flos-cuculi, on their performance under different soil nutrient conditions. Growth analysis revealed significant variation in RGR among progeny families from a diallel cross between eight genotypes originating from a single population. Differences in RGR were due to variation in both leaf area ratio (LAR) and in net assimilation rate (NAR). A genetic trade-off was observed between these two components of growth, i.e. progeny families with high investment in leaf area had a lower rate of net biomass accumulation per unit leaf area. The degree of plasticity in RGR to nutrient conditions did not differ among progeny families. Inherent differences in growth parameters among progeny families had a significant impact on their yield in competition with Anthoxanthum odoratum and Taraxacum hollandicum. In nutrient-rich conditions, progeny families with an inherently high leaf weight ratio (LWR) achieved higher yield in competition, but variation in this trait could not explain differences in competitive yield under nutrient-poor conditions. Inherent differences in growth parameters among progeny families were poorly correlated with differences in survival and average rosette biomass (a good predictor of fecundity) among these progeny families sown in four field sites along a natural gradient of soil fertility. In the more productive sites none of the growth parameters was significantly correlated with rosette biomass, but in the least productive site progeny families with an inherently high specific leaf area (SLA) tended to produce smaller rosettes than low-SLA families. These results are consistent with the view that a selective advantage may accrue from either high or low values of individual RGR components, depending on habitat conditions, and that the selective advantage of low trait values in nutrient-poor environments may results in indirect selection for low RGR in these habitats.     

氮过量吸收对地肤暗呼吸速率及相对生长速率的影响

为研究N过量吸收对植物生长的作用,以耐盐植物地肤(Kochia scoparia)作为研究对象,设置3个不同的施N处理,测量了不同生长时期的N含量、暗呼吸速率、生物量和相对生长速率(RGR)。结果表明:在N过量吸收的情况下,多余的N对暗呼吸速率并没有显著的影响,导致了暗呼吸中N的利用效率变低;单位质量暗呼吸速率与相对生长速率(RGR)有很好的线性相关,并且直线的斜率和截距并不受氮素过量吸收的影响,表明单位质量暗呼吸速率与RGR的关系不受施氮水平的影响;暗呼吸速率与总N的异速关系中,幂指数的大小与施N量相关,施N量越大对应的幂指数越小。     

Effects of NaCl salinity on 15N-nitrate fluxes and specific root length in the halophyte Plantago maritima L.

The effect of salinity on nitrate influx, efflux, nitrate net uptake rate and net nitrogen translocation to the shoot was assessed in a ¹⁵N steady state labelling experiment in the halophyte Plantago maritima L. raised for 14 days on solution supplied with 50, 100 and 200 mol m^–3 sodium chloride or without sodium chloride. Additionally, salinity induced changes in root morphology were determined. Specific root length increased upon exposure to elevated sodium chloride concentrations due to variations in biomass allocation and length growth of the tap root. Changes in root morphology, however, had a minor effect on nitrate fluxes when expressed on a root fresh weight basis. The decreased rate of nitrate net uptake in plants grown on elevated levels of sodium chloride was almost entirely due to a decrease in nitrate influx. Expressed as a proportion of influx, nitrate efflux remained unchanged and was even lower at the highest salinity level. At all sodium chloride concentrations applied the initial rate of nitrogen net translocation to the shoot decreased relative to the rate of nitrate net uptake. It is concluded that under steady state conditions the negative effect of sodium chloride on the rate of nitrate net uptake at non growth-limiting salinity levels was due to the interaction between sodium chloride and nitrate transporters in the root plasma membrane and/or processes mediating the translocation of nitrogen compounds, possibly nitrate, to the shoot.     

Cluster-root formation and carboxylate release in three Lupinus species as dependent on phosphorus supply,internal phosphorus concentration and relative growth rate

Background and Aims

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. The cause of variation in cluster-root formation among cluster-root-forming Lupinus species is unknown. The aim of this study was to investigate if cluster-root formation is, in part, dependent on different relative growth rates (RGRs) among Lupinus species when they show similar shoot P status.

Methods

Three cluster-root-forming Lupinus species, L. albus, L. pilosus and L. atlanticus, were grown in washed river sand at 0, 7·5, 15 or 40 mg P kg⁻¹ dry sand. Plants were harvested at 34, 42 or 62 d after sowing, and fresh and dry weight of leaves, stems, cluster roots and non-cluster roots of different ages were measured. The percentage of cluster roots, tissue P concentrations, root exudates and plant RGR were determined.

Key Results

Phosphorus treatments had major effects on cluster-root allocation, with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg⁻¹ sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20 % of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant.

Conclusions

Cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.