Biomass production and nitrate metabolism of Atriplex hortensis L. (C3 plant) and Amaranthus retroflexus L. (C4 plant) in cultures at different levels of nitrogen supply

Summary Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C₃ plant) and Amaranthus retroflexus L. (C₄ plant) were maintained under open air conditions in standard soil at low and high nitrogen supply levels.A comparison of shoot dry weight and shoot length in the various series shows that the growth of the aboveground parts of both species was severely reduced under low N conditions. In both pure and mixed cultures the differences resulting from low N vs. high N conditions was less pronounced with Atriplex (C₃ plant) than with Amaranthus (C₄ plant). The root dry weight of the two species was not reduced so much under low N conditions as was the shoot dry weight. The low N plants were found to contain a larger proportion of their biomass in the roots than did the high N plants. In general the root proportion of Atriplex was greater than that of Amaranthus. The contents of organic nitrogen and nitrate and the nitrate reductase activity (NRA) per g dry weight of both species decreased continually throughout the experiments. With the exception of young plants, the low N plants always had tower contents of organic nitrogen and nitrate and nitrate reductase activities than did the high N plants. The highest values of NRA were measured in the leaf laminae. The eaves also exhibited the highest concentrations of organic nitrogen. The highest nitrate concentrations, however, were observed in the shoot axis, and in most cases the lowest nitrate values were found in the laminae. At the end of ne growing season this pattern was found to have been reversed with Atriplex, but not with Amaranthus. Thus Atriplex was able to maintain a higher NRA in the laminae than Amaranthus under low N conditions.The transpiration per leaf area of the C₄ plant Amaranthus during the course of a day was substantially lower than that of the C₃ plant Atriplex. There were no significant differences in transpiration between the low N and high N series of Amaranthus. The low N plants of Atriplex, however, clearly showed in most cases higher transpiration rates than the corresponding high N plants. These different transpiration rates of the high N and the low N Atriplex plants were also reflected in a distinct ¹³C discrimination.The sum of these results points to the conclusion that the C₃ plant Atriplex hortensis can maintain a better internal inorganic nitrogen supply than the C₄ plant Amaranthus retroflexus under low N conditions and an ample water supply, due to the larger root proportion and the more pronounced and flexible transpiration of the C₃ plant.Dedicated to Prof. Dr. Karl Mägdefrau, Deisenhofen, on the ocasion of his 80th birthday     

Biomass production and nitrogen contents of the CAM plants Kalanchoe daigremontiana and K. tubiflora in cultures with different nitrogen and water supply

Summary Two CAM plant species (Kalanchoe daigremontiana and K. tubiflora) were cultivated in pure cultures with two different levels of nitrogen and water supply. A comparison of the plant dry weight showed that the productivity was severely reduced under low nitrogen or/and water conditions. Additionally the proportion of the different organs contributing to the total biomass, and thus the ratio of root/shoot dry weight shifted substantially. At the same time the production of leaf buds in ratio to leaf biomass was increased. Concentrations of organic nitrogen and nitrate in the shoots were drastically reduced under low nitrogen or/and water conditions, but organic nitrogen concentration in the roots remained relatively high. The leaf conductance over a day was investigated only for K. daigremontiana, and decreased with reduced water and nitrogen supply. The results indicate that CAM plants do not have the predicted high nitrogen use efficiency. Under environmental stress they change from biomass production towards provisions for life preservation.     

Seasonal variations in nitrate content,total nitrogen,and nitrate reductase activities of macrophytes from a chalk stream in Upper Bavaria

Summary 11 macrophytic species from a groundwater influenced chalk stream in Upper Bavaria were investigated during a period of one year in order to determine differences in the endogenous nitrate content, in total nitrogen content and in nitrate reductase activity (NRA). Nitrate concentrations of different plants taken from the same site of the river varied by a factor of approximately 10³. A maximum of 1,958 mol NO ₃ ^- g^-1 dry w. could be measured in the petioles of Nasturtium officinale, which accounts for 12% of plant dry w. Very high values were also found in Callitriche obtusangula and Veronica angallis-aquatica. In comparison to the ambient water, mean accumulation rates of up to 131 could be found. In Fontinalis antipyretica, the plant poorest in nitrate, the ratio was only 1.24:1. Elodea canadensis belonged to a group of plants having very low nitrate concentrations. Since NRA was very low too, it is assumed that nitrogen nutrition of this species depends rather on ammonia than on nitrate. With a few exceptions nitrate content of different plant organs varied markedly. In general they were lowest in leaves and highest in shoot axes. Appreciable amounts of nitrate were also found in the roots of plants. No correlation could be found between endogenous nitrate content and NRA. In contrast to endogenous nitrate content and NRA, total nitrogen concentrations of the plants did not differ significantly.     

The response ofPlantago lanceolata L. andP. major L. spp.major to nitrate depletion

To study aspects of the ecology of grassland species, in a comparative experiment, plants ofP. lanceolata andP. major were grown in pots in a greenhouse, and subjected to a gradual nitrate depletion for several weeks. Control plants were weekly supplied with nitrate. Growth, leaf appearance and disappearance, concentrations of cations and inorganic anions, soluble and insoluble reduced nitrogen concentrations,in vivo nitrate reductase activity (NRA) and the concentration of non-structural carbohydrates in several parts of the plants were followed. Depletion of nitrate caused a reduction of shoot growth, both in biomass and number of leaves. Withering of leaves increased. Accumulation of root dry matter was little (P. lanceolata), or not (P. major) affected. The concentration of reduced nitrogen in all tissues also decreased, both that of the soluble and that of the insoluble fraction. As a result, nitrogen use efficiency (NUE, g dry matter produced per mmol N incorporated) increased by nitrate depletion. NRA was higher in the roots than in the leaves, and decreased with increasing nitrate depletion. In control plants, nitrate became also limiting. This resulted in decreasing nitrate concentrations in leaves and roots. In the leaves, the decrease in nitrate concentration was preceded by a decrease in NRA. The decrease of the nitrate concentration was parallelled by an increase in the concentration of soluble sugar. No major differences in the response towards nitrate depletion were observed between the two species. Grassland Species Research Group, publication no. 129     

Leaf gas exchange characteristics and water- and nitrogen-use efficiencies of dominant grass and tree species in a West African savanna

Whereas leaf gas exchange properties are important to assess carbon and water fluxes in ecosystems worldwide, information of this type is scarce for savanna species. In this study, gas exchange characteristics of 2 C₄ grass species (Andropogon canaliculatus and Hyparrhenia diplandra) and 2 C₃ tree species (Crossopteryx febrifuga and Cussonia arborea) from the West-African savanna of Lamto (Ivory Coast) were investigated in the field. Measurements were done in order to provide data to allow the parameterization of biochemically-based models of photosynthesis (for C₄ and C₃ plant metabolic types) and stomatal conductance ; and to compare gas exchange characteristics of coexisting species. No systematic difference was found between grass and tree species for reference stomatal conductance, under standard environmental conditions, or stomatal response to incident light or vapour pressure deficit at leaf surface. Conversely, grass species displayed higher water (1.5-2 fold) and nitrogen (2-5 fold) photosynthetic use efficiencies (WUE and NUE, ratio of net photosynthesis to transpiration and leaf nitrogen, respectively). These contrasts were attributed to the CO₂ concentrating mechanism of C₄ plants. When looking within plant life forms, no important difference was found between grass species. However, significant contrasts were found between tree species, Cussonia showing higher NUE and reference stomatal conductance than Crossopteryx. These results stress the need to account for functional diversity when estimating ecosystem carbon and water fluxes. In particular, our results suggest that the tree/grass ratio, and also the composition of the tree layer, could strongly affect WUE and NUE at the ecosystem scale in West African savannas.     

Nitrate reduction and nitrate content in ash trees (Fraxinus excelsior L.): distribution between compartments,site comparison and seasonal variation

Summary Nitrate reductase activity (NRA), nitrate content and biomass components of leaflets, leaf stalks, old stem, current-year stem and roots of ash trees (Fraxinus excelsior L.) growing in their natural habitats were investigated. In addition, NRA, total nitrogen and nitrate concentration were analyzed in the leaves and roots of ash trees from four different field sites. The highest NRA per gram biomass and also per total compartment biomass was found in the leaflets, even though root biomass was much higher than total leaflet biomass. The highest nitrate concentrations were found in the leaf stalks. Correlations between nitrate availability in the soil and NRA in leaves were not significant due to high variability of the actual soil nitrate concentrations. The seasonal variation in foliar NRA, nitrate concentration and total nitrogen concentration is much smaller in F. excelsior than reported for herbaceous species and is mainly caused by changes in the actual soil nitrate availability and by senescence of the leaves.     

A study on nitrate reductase activity (NRA) of geophytes from Mediterranean environment

Nitrate reductase activity (NRA) in different compartments of 14 Mediterranean geophytes (bulbous, tuberous and rhizomatous) and actual mineral nitrogen (NO₃⁻ and NH₄⁺) in their soils were investigated. The nitrate reduction capacities of each species were determined as NRA per total plant material. Differences among compartments for NRA were significant in all species. The highest NRA was found in leaves of tuberous species (Anemone coronaria, Cyclamen coum) and of most bulbous species (Allium flavum, Allium guttatum, Bellevelia sarmatica, Galanthus plicatus, Leucojum aestivum, Ornithogalum nutans, Tulipa sylvestris). Therefore, in this group of species the contribution of the leaves to total plant NRA was the highest. The other bulbous species (Allium scorodoprasum, Crocus chrysanthus, Fritillaria bithynica, Muscari neglectum) and one rhizomatous taxon (Iris suaveolens) have a different NRA distribution within the plants. In these species the highest values of NRA were found in different organs. For example, in Allium scorodoprasum the highest NRA was in tunics, and in flowers in M. neglectum. Although leaves are the main compartments reducing nitrate in most of the studied geophytes, other compartments also contribute to total plant nitrate reduction.Our results show that the nitrate reduction capacity is different among geophyte species. Even if it roughly reflects the nitrogen supply in a habitat, differences in nitrate reduction capacities of different species collected from same sites indicate that the nitrate reducing capacity is species-specific.     

Ammonium and nitrate nutrition inPlantago lanceolata L. andPlantago major L. ssp.major

With the aims (1) to test whether the different natural occurrence of twoPlantago species in grasslands is explained by a different preference of the species for nitrate or ammonium; (2) to test whether the different occurrence is explained by differences in the flexibility of the species towards changes in the nitrogen form; (3) to find suitable parameters as a tool to study ammonium and nitrate utilization of these species at the natural sites in grasslands, plants ofPlantago lanceolata andP. major ssp.major were grown with an abundant supply of nitrate, ammonium or nitrate+ammonium as the nitrogen source (0.5 mM). The combination of ammonium and nitrate gave a slightly higher final plant weight than nitrate or ammonium alone. Ammonium lowered the shoot to root ratio inP. major. Uptake of nitrate per g root was faster than that of ammonium, but from the mixed source ammonium and nitrate were taken up at the same rate. In vivo nitrate reductase activity (NRA) was present in both shoot and roots of plants receiving nitrate. When ammonium was applied in addition to nitrate, NRA of the shoot was not affected, but in the root the activity decreased. Thus, a larger proportion of total NRA was present in the shoot than with nitrate alone. In vitro glutamate dehydrogenase activity (GDHA) was enhanced by ammonium, both in the shoot and in the roots.In vitro glutamine synthetase activity (GSA) was highest in roots of plants receiving ammonium. Both GDHA and GSA were higher inP. lanceolata than inP. major. The concentration of ammonium in the roots increased with ammonium, but it did not accumulate in the shoot. The concentration of amino acids in the roots was also enhanced by ammonium. Protein concentration was not affected by the form of nitrogen. Nitrate accumulated in both the shoot and the roots of nitrate grown plants. When nitrate in the solution was replaced by ammonium, the nitrate concentration in the roots decreased rapidly. It also decreased in the shoot, but slowly. It is concluded that the nitrogen metabolism of the twoPlantago species shows a similar response to a change in the form of the nitrogen source, and that differences in natural occurrence of these species are not related to a differential adaptation of nitrogen metabolism towards the nitrogen form. Suitable parameters for establishing the nitrogen source in the field are thein vivo NRA, nitrate concentrations in tissues and xylem exudate, and the fraction of total reduced nitrogen in the roots that is in the soluble form, and to some extent thein vitro GDHA and GSA of the roots. Grassland Species Research Group. Publ. no 118.     

Tradeoffs between nitrogen- and water-use efficiency in dominant species of the semiarid steppe of Inner Mongolia

In water-limited environments, photosynthetic carbon gain and loss of water by transpiration are in a permanent tradeoff as both are contrarily regulated by stomata conductance. In semiarid steppe grasslands water limitation may covary with nitrogen limitation. Steppe grassland species are capable of optimizing their use of limiting resources, water and nitrogen, but regulation is still poorly understood. In a two-year experiment with addition of water (irrigation simulating a wet year) and nitrogen (0, 25, and 50 kg urea-N?ha^?1) we assessed intrinsic water use efficiency (WUE_i), nitrogen use efficiency (NUE), and related plant functional traits (PFTs) of four dominant C₃ species (Leymus chinensis, Agropyron cristatum, Stipa grandis, and Artemisia frigida). Water and N fertilizer supplementation significantly increased plant primary production, and N effect was more pronounced under irrigated conditions. Parallel with the responses of plant production, a strong tradeoff between WUE_i and NUE was detected: water supply increased NUE but decreased WUE_i, whereas N addition slightly increased WUE_i at the expense of NUE. This tradeoff occurred at the leaf level, and involved the responses of leaf N concentration and specific leaf area. WUE_i of species changed among treatments in a predictable manner by the parameter of leaf N content per area. Dominant plant species commonly achieved a higher utilization efficiency of the more limiting resource via altering PFTs, which was an important mechanism of adaptation to variable resource limitation in semiarid grasslands.     

CRASSULACEAN ACID METABOLISM IN THE GESNERIACEAE

The occurrence of the Crassulacean acid metabolism (CAM) was studied in four epiphytic species of the Gesneriaceae: two neotropical species, Codonanthe crassifolia and Columnea linearis, and two paleotropical species, Aeschynanthus pulcher and Saintpaulia ionantha. Gas exchange parameters, enzymology, and leaf anatomy, including mesophyll succulence and relative percent of the mesophyll volume occupied by airspace, were studied for each species. Codonanthe crassifolia was the only species to show nocturnal CO₂ uptake and a diurnal organic acid fluctuation. According to these results, Codonanthe crassifolia shows CAM-cycling under well-watered conditions and when subjected to drought, it switches to CAM-idling. Other characteristics, such as leaf anatomy, mesophyll succulence, and PEP carboxylase and NADP malic enzyme activity, indicate attributes of the CAM pathway. All other species tested showed C₃ photosynthesis. The most C₃-like species is Columnea linearis, according to the criteria tested in this investigation. The other two species show mesophyll succulence and relative percent of the leaf volume occupied by airspace within the CAM range, but no other characters of the CAM pathway. The leaf structure of certain genera of the Gesneriaceae and of the genus Peperomia in the Piperaceae are similar, both having an upper succulent, multiple epidermis, a medium palisade of one or a few cell layers, and a lower, succulent spongy parenchyma not too unlike CAM photosynthetic tissue. We report ecophysiological similarities between these two distantly related families. Thus, the occurrence of CAM-cycling may be more common among epiphytic species than is currently known.     

Biomass production and nitrogen content of C3- and C4- grasses in pure and mixed culture with different nitrogen supply

Summary Two C₃ grasses (Hordeum vulgare L., Avena sativa L.) and two C₄ grasses (Panicum miliaceum L., Panicum crus-galli L.) were cultivated in standard soil in the open air in pure cultures and in various mixed cultures at low and high nitrogen fertilization levels. After three months the dry weight, length and nitrogen content of the aboveground and below-ground parts of the plants and the shoot/root ratios were determined. Hordeum vulgare was the most successful species irrespective of the nitrogen fertilization level, and also exhibited in most cases the highest nitrogen concentrations. Panicum miliaceum, on the other hand, was the species least able to compete. The production of biomass was reduced in cultures growing under nitrogen starvation conditions, this phenomenon being more pronounced with respect to the C₄ than to the C₃ species. The decrease in the production of biomass at low N conditions was most drastic with Panicum crus-galli, the species with the lowest nitrogen content and thus assumed to be best adapted to nitrogen starvation conditions. In cultures growing at low nitrogen fertilization levels the shoot/root ratios of all species.shifted in favour of an increasing root proportion. The extent of this shift, however, differed from species to species.     

Stable isotopic composition of carbon and nitrogen and nitrogen content in vascular epiphytes along an altitudinal transect*

The foliar content of nitrogen and the relative abundances of ¹³C and ¹⁵N were analysed in vascular epiphytes collected from six sites along an altitudinal gradient from tropical dry forests to humid montane forests in eastern Mexico. The proportion of epiphyte species showing crassulacean acid metabolism (CAM) (atmospheric bromeliads, thick-leaved orchids, Cactaceae, and Crassulaceae) decreased with increasing elevation and precipitation from 58 to 6%. Atmospheric bromeliads, almost all of which had δ ¹³C values indicating CAM, were more depleted in ¹⁵N (x = ? 10·9‰ ± 2·11) than the C₃ bromeliads which form water-storing tanks ( ? 6·05‰ ± 2·26). As there was no difference in δ ¹⁵N values between C₃ and CAM orchids, the difference in bromeliads was not related to photosynthetic pathways but to different nitrogen sources. While epiphytes with strong ¹⁵N depletion appear to obtain their nitrogen mainly from direct atmospheric deposition, others have access to nitrogen in intercepted water and from organic matter decomposing on branches and in their phytotelmata. Bromeliads and succulent orchids had a lower foliar nitrogen content than thin-leaved orchids, ferns and Piperaceae. Ground-rooted hemi-epiphytes exhibited the highest nitrogen contents and δ ¹⁵N values.     

The effects of nitrogen stress on the stable carbon isotope composition, productivity and water use efficiency of white spruce (Picea glauca (Moench) Voss) seedlings

A, assimilation rate
a, fractionation against ¹³C for CO₂ diffusion through air
b, net fractionation against ¹³C during CO₂ fixation
C_a, ambient CO₂ concentration
C_c, CO₂ concentration at the chloroplast
C_i, intercellular CO₂ concentration
D, vapour pressure deficit
E_n, needle transpiration rate
E_p, whole plant water use
g_w, leaf internal transfer conductance to CO₂
g_s, stomatal conductance to water vapour
L, projected leaf area
NUE, nitrogen use efficiency
PEP, phosphoenolpyruvate
Rubisco, ribulose-1,5-biphosphate carboxylase
TDR, time domain reflectometry
WUE, water use efficiency
Δ, carbon isotope discrimination
δ¹³C, carbon isotope abundance parameter
δ¹³C_a, carbon isotopic composition of atmospheric CO₂
θ, volumetric soil water content

The effect of nitrogen stress on needle δ¹³C, water-use efficiency (WUE) and biomass production in irrigated and dry land white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings, representing 10 controlled crosses, were planted in the field in individual buried sand-filled cylinders. Two nitrogen treatments were imposed, nitrogen stressed and fertilized. The ranking of δ¹³C of the crosses was maintained across all combinations of water and nitrogen treatments and there was not a significant genetic versus environmental interaction. The positive relationships between needle δ¹³C, WUE and dry matter production demonstrate that it should be possible to use δ¹³C as a surrogate for WUE, and to select for increased WUE without compromising yield, even in nitrogen deficient environments. Nitrogen stressed seedlings had the lowest needle δ¹³C in both irrigated and dry land conditions. There was a positive correlation between needle nitrogen content and δ¹³C that was likely associated with increased photosynthetic capacity. There was some indication that decreased nitrogen supply led to increased stomatal conductance and hence lower WUE. There was a negative correlation between intrinsic water use efficiency and photosynthetic nitrogen use efficiency (NUE). This suggests that white spruce seedlings have the ability to maximize NUE when water becomes limited. There was significant genetic variation in NUE that was maintained across treatments. Our results suggest that in white spruce, there is no detectable effect of anaplerotic carbon fixation and that it is more appropriate to use a value of 29‰ (‘Rubisco only’) for the net discrimination against ¹³C during CO₂ fixation. This leads to excellent correspondence between values of C_i/C_a derived from gas exchange measurements or from δ¹³C.     

Comparing nitrate storage and remobilization in two rice cultivars that differ in their nitrogen use efficiency

Soil nitrogen (N) is available to rice crops as either nitrate or ammonium, but only nitrate can be accrued in cells and so factors that influence its storage and remobilization are important for N use efficiency (NUE). The hypothesis that the ability of rice crops to remobilize N storage pools is an indicator of NUE was tested. When two commonly grown Chinese rice cultivars, Nong Ken (NK) and Yang Dao (YD), were compared in soil and hydroponics, YD had significantly greater NUE for biomass production. The ability of each cultivar to remobilize nitrate storage pools 24 h after N supply withdrawal was compared. Although microelectrode measurements of the epidermal sub-cellular nitrate pools in leaves and roots showed similar patterns of vacuolar remobilization in both cultivars, whole-tissue analysis showed very little depletion of storage pools after 24 h. However, leaf epidermal cell cytosolic nitrate activities were significantly higher in YD when compared with NK. Before N starvation and growing in 10 mM nitrate, the xylem nitrate activity in YD was lower than that of NK. After 24 h of N starvation the xylem nitrate had decreased more in YD than in NK. Tissue analysis of stems showed that YD had accumulated significantly more nitrate than NK, and the remobilization pattern suggested that this store is important for both cultivars. Changes in nitrate reductase activity (NRA) and expression were measured. Growing in 10 mM nitrate, NRA was undetectable in roots of both cultivars, and the leaf total NRA of equivalent leaves was similar in NK and YD. When the N supply was withdrawn, after 24 h NRA in NK was reduced to 80% but no decrease was found in YD. The proportion of NRA in an active form in YD was significantly higher than that in NK under both nitrate supply and deprivation conditions. Checking NR gene expression showed that leaf expression of OsNia1 was faster to respond to nitrate deprivation than OsNia2 in both cultivars. These measurements are discussed in relation to cultivar differences and physiological markers for NUE in rice.     

Distribution of nitrate reductase activity in nodulated lucerne plants

Nitrogen fixing plants of lucerne (Medicago sativa L. cv. Aragón) were grown in a glasshouse for three months in the absence of nitrate, and then supplied with 5 mM KNO₃ for a week. In control (non-nitrate fed) plants, nitrate reductase activity (NRA EC 1.6.6.1) was detected only in nodules. After nitrate supply, root NRA showed a transient increase. Shoot NRA increased with time, paralleling changes in nitrate distribution; stem NRA represented nearly 50% of total NRA in plant tissues. Total nitrogen, expressed on a dry weight basis, tended to decrease in shoots upon nitrate supply. Bacteroid NRA (EC 1.7.99.4) showed a great variation depending on Rhizobium meliloti strains, ranging from 5 to 40% of total plant NRA. However, different Rhizobium strains did not give different results in terms of plant growth parameters, nitrate or organic nitrogen content.     

Nitrate concentration and nitrate reductase activity in the leaves of three legumes and three cereals*

Nitrate concentration and nitrate reductase activity (NRA) were studied in the leaves of soybean (Glycine max), groundnut (Arachis hypogaea and cowpea (Vigna unguiculata) and sorghum (Sorghum bicolor), pearl millet (Pennisetum americanum) and maize (Zea mays) at three nitrogen fertiliser levels in two field experiments. Higher nitrate concentrations were detected in the leaves of groundnut, cowpea and pearl millet than in sorghum and maize. Nitrate content in the leaves and leaf NRA were not related across crop species, nor was a generalised pattern of leaf NRA and leaf nitrate observed within legumes or within cereals. Nitrogen application resulted in higher nitrate availability in the leaves, with varied leaf NRA.     

Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. II. Activity and affinity of the nitrate uptake system in Pisum and Lemna in relation to nitrogen availability and nitrogen demand

Abstract Net nitrate uptake rates were measured and the kinetics calculated in non-nodulated Pisum sativum L. cv. Marma and Lemna gibba L. adapted to constant relative rates of nitrate-N additions (R_A), ranging from 0.03 to 0.27 d^?1 for Pisum and from 0.05 to 0.40 d^?1 for Lemna, V_max of net nitrate uptake (measured in the range 10 to 100 mmol m^?3 nitrate, i.e. ‘system I’) increased with R_A in the growth limiting range but decreased when R_A exceeded the relative growth rate (RGR), K_m was not significantly related to changes in R_A. On the basis of previous ¹³N-flux experiments, it is concluded that the differences in V_max at growth limiting R_A are attributable to differences in influx rates. Linear relationships between V_max and tissue nitrogen concentrations were obtained in the growth limiting range for both species, and extrapolated intercepts relate well with the previously defined minimal nitrogen concentrations for plant growth (Oscarson, Ingemarsson & Larsson, 1989). Analysis of V_max for net nitrate uptake on intact plant basis in relation to nitrogen demand during stable, nitrogen limited, growth shows an increased overcapacity at lower R_A values in both species, which is largely explained by the increased relative root size at low R_A. A balancing nitrate concentration, defined as the steady state concentration needed to sustain the relative rate of increase in plant nitrogen (R_N), predicted by R_A, was calculated for both species. In the growth limiting range, this value ranges from 3.5 mmol m^?3 (R_A 0.03 d^?1) to 44 mmol m^?3 (R_A 0.21 d^?1) for Pisum and from 0.2 mmol m^?3 (R_A 0.05 d^?1) to 5.4 mmol m^?3 (R_A 0.03 d^?1) for Lemna. It is suggested that this value can be used as a unifying measure of the affinity for nitrate, integrating the performance of the nitrate uptake system with nitrate flux and long term growth and demand for nitrogen.     

Physiological responses of intertidal marine brown algae to nitrogen deprivation and resupply of nitrate and ammonium

Intertidal macroalgae Fucus and Laminaria experience seasonally fluctuating inorganic N supply. This study examined the effects of long‐term N deprivation, recovery following N resupply, and effects of elevated ammonium and nitrate exposure on N acquisition in intertidal algae using manipulations of N supply in tank culture. Over 15 weeks of N deprivation, internal N and nitrate reductase activity (NRA) declined, but maximum quantum yield of PSII was unaffected in Fucus serratus and Fucus vesiculosus. Low NRA was maintained despite no external nitrate availability and depletion of internal pools, suggesting a constitutive NRA, insensitive to N supply. Nitrate resupplied to N‐starved thalli was rapidly taken up and internal nitrate pools and NRA increased. Exposure to elevated (50 μM) nitrate over 4 days stimulated nitrate uptake and NRA in Laminaria digitata and F. serratus. Exposure to elevated ammonium suppressed NRA in L. digitata but not in F. serratus. This novel insensitivity of NRA to ammonium in Fucus contrasts with regulation of NRA in other algae and higher plants. Ammonium suppression of NRA in L. digitata was not via inhibition of nitrate uptake and was independent of nitrate availability. L. digitata showed a higher capacity for internal nitrate storage when exposed to elevated ambient nitrate, but NRA was lower than in Fucus. All species maintained nitrate assimilation capacity in excess of nitrate uptake capacity. N uptake and storage strategies of these intertidal macroalgae are adaptive to life in fluctuating N supply, and distinct regulation of N metabolism in Fucus vs Laminaria may relate to position in the intertidal zone.     

Seasonal changes in nitrate use by three woody species: the importance of the leaf-expansion period

Seasonal changes in plant NO₃ ⁻-N use were investigated by measuring leaf nitrate reductase activity (NRA), leaf N concentration, and leaf expansion in one evergreen woody species (Quercus glauca Thunb.) and two deciduous woody species [Acer palmatum Thunb. and Zelkova serrata (Thunb.) Makino]. Leaf N concentration was highest at the beginning of leaf expansion and decreased during the expansion process to a steady state at the point of full leaf expansion in all species. The leaf NRA of all species was very low at the beginning of leaf expansion, followed by a rapid increase and subsequent decrease. The highest leaf NRA was observed in the middle of the leaf-expansion period, and the lowest leaf NRA occurred in summer for all species. Significant positive correlations were detected between leaf NRA and leaf expansion rates, while leaf N concentrations were negatively correlated with leaf area. In the evergreen Q. glauca, the N concentration in current buds increased before leaves opened; concurrently, the N concentration in 1-year-old leaves decreased by 25%. Our results show that the leaf-expansion period is the most important period for NO₃ ⁻-N assimilation by broadleaf tree species, and that decreases in leaf N concentration through the leaf-expansion period are at least partly compensated for by newly assimilated NO₃ ⁻-N in current leaves.     

Effects of acid irrigation and liming on nitrate reduction and nitrate content of Picea abies (L.) Karst. and Oxalis acetosella L.

Nitrate reductase activities (NRA) and nitrate concentration per unit biomass in Picea abies (L.) Karst. roots from four different soil horizons and in leaves and roots of the frequent field-layer species Oxalis acetosella L. were measured on six different irrigation and liming treatments within the Höglwald project, S-Bavaria, Germany. Liming increased and acid irrigation reduced soil nitrate availability when compared to control plots. Nitrate assimilation capacities of the respective plant compartments per unit of soil volume or ground area were calculated from the NRA per unit of biomass and from the biomass distribution on the various treatments.Mean NRA per unit of biomass in Picea abies roots ranged between 0.23 and 0.09 mol NO ₂ ^- g_-1 d.w. h_-1 without significant effects of soil horizon or treatment. Limed and non-limed treatments showed for Picea different root distributions within the soil profile, but root biomass per unit of ground area (295 to 220 g d.w. m^-2) was not affected by the various treatments. Thus, nitrate assimilation capacity of Picea roots per unit of ground area ranged between 19.5 and 11.4 mol NO ₂ ^- m^-2 h_-1 without major treatment effects.In laminae of Oxalis acetosella mean NRA per unit of biomass ranged between 2.91 and 0.27 mol NO ₂ ^- g_-1 d.w. h_-1 and, in contrast to Picea abies, treatment effects were found with NRA on limed plots increased and on acid irrigated plots reduced when compared to control plots. Mean leaf biomass of Oxalis per unit of ground area ranged between 9.57 and 0.66 g d.w. m^-2 and responded in a similar manner to the various treatments. Thus, for the Oxalis leaf NRA per unit of ground area (27.85 to 0.18 mol NO₂ m^-2 h_-1) a cumulative response to the variations in nitrate availability was found.The different responses of Picea abies and Oxalis acetosella to changes in soil nitrate availability are discussed with respect to their suitability to prevent soil nitrate leaching.