N-uptake Rates Research Articles

Flooding affects uptake and distribution of carbon and nitrogen in citrus seedlings

Soil flooding has been widely reported to affect large areas of the world. In this work, we investigated the effect of waterlogging on citrus carbon and nitrogen pools and partitioning. Influence on their uptake and translocation was also studied through 15N and 13C labeling to provide insight into the physiological mechanisms underlying the responses. The data indicated that flooding severely reduced photosynthetic activity and affected growth and biomass partitioning. Total nitrogen content and concentration in the plant also progressively decreased throughout the course of the experiment. After 36 days of treatment, nitrogen content of flooded plants had decreased more than 2.3-fold compared to control seedlings, and reductions in nitrogen concentration ranged from 21 to 55% (in roots and leaves, respectively). Specific absorption rate and transport were also affected, leading to important changes in the distribution of this element inside the plant. Additionally, experiments involving labeled nitrogen revealed that 15N uptake rate and accumulation were drastically decreased at the end of the experiment (93% and 54%, respectively).13CO2 assimilation into the plant was strongly reduced by flooding, with δ13C reductions ranging from 22 to 37% in leaves and roots, respectively. After 36 days, the relative distribution of absorbed 13C was also altered. Thus, 13C recovery in flooded leaves increased compared to controls, whereas roots exhibited the opposite pattern. Interestingly, when carbohydrate partitioning was examined, the data revealed that sucrose concentration was augmented significantly in roots (37–56%), whereas starch was reduced. In leaves, a marked increase in sucrose was detected from the first sampling onwards (36–66%), and the same patter was observed for starch. Taken together, these results indicate that flooding altered carbon and nitrogen pools and partitioning in citrus. On one hand, reduced nitrogen concentration appears to be a consequence of impaired uptake and transport. On the other hand, the observed changes in carbohydrate distribution suggest that translocation from leaves to roots was reduced, leading to significant starch accumulation in leaves and further decreases in roots.

Inorganic and organic nitrogen uptake by phytoplankton and heterotrophic bacteria in the stratified Mid-Atlantic Bight

Little is known about the relative importance of inorganic and organic nitrogen (N) sources in fueling production of phytoplankton versus heterotrophic bacteria on the continental shelf. This issue was addressed during two diel experiments conducted in the Mid-Atlantic Bight at the Long-term Ecosystem Observatory, LEO-15, off southern New Jersey. Uptake of 15N-labeled ammonium (NH 4 +), nitrate (NO 3 −), and nitrite (NO 2 −), and dual-labeled ( 15N and 13C) urea and dissolved free amino acids was measured in water taken from the surface and bottom mixed layers approximately every 4 h over two 24 h periods in July 2002. Two methods were used to quantify 15N uptake rates: (1) traditional filtration into various phytoplankton and bacterial size classes, and (2) flow cytometric (FCM) sorting of autotrophic cells based on the presence of chlorophyll autofluorescence. Due to a strong pycnocline, the nutrient composition was quite distinct between the surface and bottom mixed layers. Dissolved organic N (DON) comprised >99% of the total dissolved N (TDN) pool in surface waters, whereas the bottom-water TDN pool was roughly divided between NH 4 +, NO 3 −, and DON. Urea was the dominant N form used by all fractions at the surface, and although phytoplankton >3 μm was responsible for most of the urea uptake, bacterial use was detected using stable isotopes and also suggested by ureC sequence analysis. The majority of ureC sequences recovered from the 0.2–0.8 μm fraction belonged to members of the Alphaproteobacteria (46%), whereas those of the 0.8–3.0 μm size class consisted primarily of Cyanobacteria (70%). In contrast to the surface, N uptake in the bottom layer was dominated by NH 4 +. The bacterial fraction was responsible for 20–49% of the size-fractionated NH 4 + and NO 3 − uptake in surface samples and 36–93% at the bottom. These results suggest that organic N, such as urea, is a viable source of N nutrition to phytoplankton forced to compete with heterotrophic bacteria for limited inorganic N.

Relationships between nitrogen and phosphorus forms and ratios and the development of dinoflagellate blooms in the East China Sea

During late spring and early summer of 2005, large-scale (> 15 000 km(2)), mixed dinoflagellate blooms developed along the the coast of the East China Sea. Karenia mikimotoi was the dominant harmful algal bloom species in the first stage of the bloom (late May) and was succeeded by Prorocentrum donghaiense approximately 2 wk later. Samples were collected from different stations along both north-south and west-east transects, from the Changjiang River estuary to the south Zhejiang coast, during 3 cruises of the Chinese Ecology and Oceanography of Harmful Algal Blooms Program, before and during the bloom progression. Nitrogen isotope tracer techniques were used to measure rates of NO3-, NH4+, urea, and glycine uptake during the blooms. High inorganic nitrogen (N), but low phosphorus (P) loading from the Changjiang River led to high dissolved inorganic N:dissolved inorganic P ratios in the sampling area and indicate the development of P limitation. The rates of N-15-uptake experiments enriched with PO43- were enhanced compared to unamended samples, suggesting P limitation of the N-uptake rates. The bloom progression was related to the change in availability of both organic and inorganic N and P. Reduced N forms, especially NH4+, were preferentially taken up during the blooms, but different bloom species had different rates of uptake of organic N substrates. K mikimotoi had higher rates of urea uptake, while P. donghaiense had higher rates of glycine uptake. Changes in the availability of reduced N and the ratios of N:P in inorganic and organic forms were suggested to be important in the bloom succession. Nutrient ratios and specific uptake rates of urea were similar when compared to analogous blooms on the West Florida Shelf.

Effects of Long‐Term Free‐Air Ozone Fumigation on δ15N and Total N in Fagus sylvatica and Associated Mycorrhizal Fungi

Patterns of nitrogen (N) isotope composition (delta(15)N) and total N contents were determined in leaves, fine roots, root-associated ectomycorrhizal fungi (ECM) of adult beech trees (FAGUS SYLVATICA), and soil material under ambient (1 x O(3)) and double ambient (2 x O(3)) atmospheric ozone concentrations over a period of two years. From fine root to leaf material delta(15)N decreased consecutively. Under enhanced ozone concentrations total N was reduced in fine roots and delta(15)N showed a decrease in roots and leaves. In the soil and in most types of mycorrhizae, delta(15)N and total N were not altered due to ozone fumigation. The number of vital ectomycorrhizal root tips increased and the mycorrhizal community structure changed in 2 x O(3). Simultaneously, the specific rate of inorganic N-uptake by the roots was reduced under the double ozone regime. From these results it is assumed that 2 x O(3) changes N-nutrition of the trees at the level of N-acquisition, as indicated by enhanced mycorrhizal root tip density, altered mycorrhizal species composition, and reduced specific N-uptake rates.

Nutrient foraging via physiological and morphological plasticity in three plant species

Physiological and morphological plasticity of roots enhance plant nutrient uptake in spatiotemporally heterogeneous soil environments. We examined these two types of plasticity using three plant species (Solidago altissima (L.) Raf., Pinus taeda L., and Liquidambar styraciflua L.). We grew plants in pots (one plant per pot) with equal quantity of fertilizer applied either evenly over the pot surface (H) or on one-quarter of the pot surface (T). A high-concentration 15N-labeled ammonium nitrate solution was injected twice over 48 h before harvest at a random location in H pots, and in either unfertilized or fertilized portion of T pots. Physiological plasticity of N uptake was observed in S. altissima and L. styraciflua. The highest 15N uptake rate for L. styraciflua occurred in H pots (medium level), and that for S. altissima occurred in fertilized portions of T pots (rich level). When low-concentration 15N was added to S. altissima, no differences in uptake were noted among treatments, possibly because of interroot competition. In S. altissima and P. taeda, either morphological or physiological plasticity was strong. In L. styraciflua, both types of plasticity were strong. Total 15N uptake was enhanced when 15N was added to the fertilized patches. Physiological plasticity contributed >70% of enhanced 15N uptake in S. altissima and L. styraciflua.

Carbon allocation and nitrogen acquisition in a developing Populus deltoides plantation.

We established Populus deltoides Bartr. stands differing in nitrogen (N) availability and tested if: (1) N-induced carbon (C) allocation could be explained by developmental allocation controls; and (2) N uptake per unit root mass, i.e., specific N-uptake rate, increased with N availability. Closely spaced (1 x 1 m) stands were treated with 50, 100 and 200 kg N ha(-1) year(-1) of time-release balanced fertilizer (50N, 100N and 200N) and compared with unfertilized controls (0N). Measurements were made during two complete growing seasons from May 1998 through October 1999. Repeated nondestructive measurements were carried out to determine stem height and diameter, leaf area and fine-root dynamics. In October of both years, above- and belowground biomass was harvested, including soil cores for fine-root biomass. Leaves were harvested in July 1999. Harvested tissues were analyzed for C and N content. Nondestructive stem diameter and and fine-root dynamic measurements were combined with destructive harvest data to estimate whole-tree biomass and N content at the end of the year, and to estimate specific N-uptake rates during the 1999 growing season. Shoot growth response was greater in fertilized trees than in control trees; however, the 100N and 200N treatments did not enhance growth more than the 50N treatment. Root biomass proportions decreased over time and with increasing fertilizer treatment. Fertilizer-induced changes in allocation were explained by accelerated development. Specific N-uptake rates increased during the growing season and were higher for fertilized trees than for control trees.

Root damage affects nitrogen uptake and growth of young Fuji/M.26 apple trees

SummaryEffects of root damage during the transplant process on growth and nitrogen (N) uptake were studied with one-year-old bench-grafted Malus domestica Borkh ‘Fuji’ on M.26 rootstock apple nursery plants. Plants were potted after grafting and grown outside for one season. At the end of the season uniform trees were selected and randomly divided into four groups. One group of plants were moved into a 2°C cold room with soil and container intact (IR Treatment). Plants in other groups were removed from pots and stored as bareroot in the same cold room for three months. In the spring, bareroot plants were either: (1) transplanted with about 10% of the root system damaged during transplant (TP Treatment and Control-CK); or (2) root pruned by 25% (by volume) prior to transplant (RP treatment). Five trees from each treatment received 1 g of 15NH415NO3 at 12, 41 and 76 d after repotting. Control (CK) trees received no N. Trees were harvested 10 d after each N application, and plant growth and total N and 15N content of different tissues were determined. Root pruning reduced plant total biomass and root biomass at the first two harvests, but the plants from the RP treatment had highest total plant biomass and root biomass at the third harvest. There was no significant difference in the new stem and leaf growth among IR, RP and CK treatments at harvests but the TP treatment reduced new shoot biomass. Plants with intact roots (IR) had the higher total N content while control plants (CK) had the lowest. Root pruning reduced 15N uptake rate at the first two harvests but promoted it at the third harvest. Our results suggest that plant growth and nutrient uptake was suppressed by root pruning/damage during transplanting only in the early season, and the negative effects on growth and N uptake were offset later in the season by compensative root regeneration.

Effects of Drought on the Competitive Interference of an Early Successional Species (Rubus fruticosus) on Fagus sylvatica L. Seedlings: 15N Uptake and Partitioning, Responses of Amino Acids and other N Compounds

Abstract: We assessed the role of water availability as a factor regulating the ability of beech seedlings to cope with competitive interference for nitrogen resources by an early successional species (Rubus fruticosus). A glasshouse experiment was performed with two levels of interference (beech with and without R. fruticosus) and three levels of irrigation (high, intermediate, none). 15N uptake and partitioning of both species, and composition of N pools in leaves, roots and phloem of beech, were determined. Under all irrigation regimes, 15N uptake by beech seedlings decreased when grown together with R. fruticosus. R. fruticosus had higher 15N uptake rates than beech, under all water supply levels. When irrigation was reduced, a substantial decrease in 15N uptake of beech seedlings and a concurrent increase in 15N uptake by R. fruticosus were observed. Interference by R. fruticosus and low irrigation also affected the 15N partitioning in beech seedlings and resulted in reduced allocation of 15N to the roots. The combination of competitive interference and lack of irrigation led to an increase in soluble non‐protein N in roots and leaves of beech, due to protein degradation. This response was attributed to an increase in levels of amino acids serving as osmoprotectants under these conditions. The concentration of proline in leaves of beech was negatively correlated to shoot water potential. A competition‐induced reduction of total N in leaves of beech under high and intermediate irrigation was found. These results illustrate (1) the advantage of R. fruticosus in terms of N uptake when compared to young beech, particularly under inadequate water supply, and (2) the changes in N composition of beech seedlings in order to cope with reduced soil water and interference by R. fruticosus.

Relative importance of macrophyte leaves for nitrogen uptake from flood water in tidal salt marshes

Nitrogen limits plant growth in most salt marshes. As foliar N-uptake makes a significant contribution to the overall N-requirements of submerged plant species such as (e.g.) seagrasses, we tested if foliar N-uptake was also significant in Spartina anglica Hubbard, a species that dominates the lowest, regularly flooded areas of salt marshes in the SW Netherlands. Foliar N-uptake was compared for plants from 2 estuaries with contrasting N-loads in their water column. N-uptake was quantified by (1) flooding detached leaves in test tubes, (2) spraying leaves still attached to the plants, and (3) flooding whole plants, with solutions containing either 15NO3- or 15NH4+. We found that detaching the leaves from the plant underestimated NH4+ uptake by between 30 and 50%. Higher salinity also reduced foliar N-uptake. Uptake rates were higher for NH4+ than for NO3-, as has been found for many submerged and terrestrial angiosperms and marine algae. Methodology also had a major effect on the uptake rate, with flooding of intact plants yielding higher uptake rates than spraying attached leaves. However, in general, foliar N-uptake rates were low at the NO3- and NH4+ concentrations that are actually present in the tidal waters during the growth season, and may at most contribute to around 10% of the growth requirement. This percentage is much less than for seagrasses, but in line with data for some terrestrial systems. We conclude that in contrast to seagrasses, foliar N-uptake does not form a significant contribution to the overall N-requirements of S. anglica. This low N-uptake capacity of the S. anglica leaves appears to be a consequence of adaptations to survive tidal flooding. [KEYWORDS: Foliar uptake · Nitrate · Ammonium · Tidal marsh · 15N labelling]

Soil temperature and plant growth stage influence nitrogen uptake and amino acid concentration of apple during early spring growth.

In spring, nitrogen (N) uptake by apple roots begins about 3 weeks after bud break. We used 1-year-old 'Fuji' Malus domestica Borkh on M26 bare-root apple trees to determine whether the onset of N uptake in spring is dependent solely on the growth stage of the plant or is a function of soil temperature. Five times during early season growth, N uptake and total amino acid concentration were measured in trees growing at aboveground day/night temperatures of 23/15 degrees C and belowground temperatures of 8, 12, 16 or 20 degrees C. We used (15NH4)(15NO3) to measure total N uptake and rate of uptake and found that both were significantly influenced by both soil temperature and plant growth stage. Rate of uptake of 15N increased with increasing soil temperature and changed with plant growth stage. Before bud break, 15N was not detected in trees growing in the 8 degrees C soil treatment, whereas 15N uptake increased with increasing soil temperatures between 12 and 20 degrees C. Ten days after bud break, 15N was still not detected in trees growing in the 8 degrees C soil treatment, although total 15N uptake and uptake rate continued to increase with increasing soil temperatures between 12 and 20 degrees C. Twenty-one days after bud break, trees in all temperature treatments were able to acquire 15N from the soil, although the amount of uptake increased with increasing soil temperature. Distribution of 15N in trees changed as plants grew. Most of the 15N absorbed by trees before bud break (approximately 5% of 15N supplied per tree) remained in the roots. Forty-six days after bud break, approximately one-third of the 15N absorbed by the trees in the 12-20 degrees C soil temperature treatments remained in the roots, whereas the shank, stem and new growth contained about two-thirds of the 15N taken up by the roots. Total amino acid concentration and distribution of amino acids in trees changed with plant growth stage, but only the amino acid concentration in new growth and roots was affected by soil temperature. We conclude that a combination of low soil temperature and plant developmental stage influences the ability of apple trees to take up and use N from the soil in the spring. Thus, early fertilizer application in the spring when soil temperatures are low or when the aboveground portion of the tree is not actively growing may be ineffective in promoting N uptake.

The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species, Saxifraga caespitosa and Cerastium alpinum.

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH4+ or NO3−. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g−1 root DM s−1 on glycine at 15 °C and 7.5 nmol g−1 root DM s−1 on NH4+ at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C.

Potential glutamine synthetase and glutamate dehydrogenase activities in the oligo-mesotrophic Lake Pavin: annual comparison with an indirect estimation of inorganic nitrogen utilization

The annual distribution of potential glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities was studied in the oligo-mesotrophic Lake Pavin (France, Massif Central). The validity of these enzymatic activies as an index of N-assimilation in field studies was tested by a comparison with the coupling of distributions of inorganic nitrogen uptake rate based on 14 C incorporation into proteins together with several abiotic and biotic parameters. The potential GS and GDH activities recorded during the annual phytoplankton succession were in the same order of magnitude as those reported from the very scanty literature on this subject. Moreover, specific potential GS and GDH activities were significantly correlated with the external NO 3 - -N concentration suggesting that these enzymatic activities reflected nitrate utilization by algae. Specific activities of GDH were about 2.5 fold higher than GS specific activities suggesting that the GDH pathway may play a leading part in inorganic nitrogen assimilation in the oligo-mesotrophic Lake Pavin. However, N-assimilation rates based on the enzymatic approach were lower than those obtained through indirect isotopic determination of N-uptake rates. The reasons for such a discrepancy are discussed.

Glutamine Synthetase activity in limnetic phytoplankton

The seasonal distribution of potential Glutamine Synthetase (GS) activities was studied in eutrophic Lake Aydat (France, Massif Central). The validity of this index of N-assimilation in field studies was tested by a comparison with the coupling of distributions of inorganic nitrogen uptake rate based on14C incorporation into proteins and the amino acids to protein ratio together with several abiotic and biotic parameters. The potential GS activities recorded during the seasonal succession of phytoplankton (characterized by the dominance of diatoms) were in the same order of magnitude of those reported from the very poor literature on this subject. Over high N-NO 3 - availability, the potential GS activities recorded were close to indirect estimations of N-uptake rates. During nitrogen depletion, high potential GS activities (which coincided with a development of small diatoms) were recorded suggesting that cell metabolism adapted to N-nutrient constraints. In addition, potential GS activity evolved concomitantly to other indicators of N-uptake and N-status. However, N-assimilation rates based on the enzymatic approach were lower than those obtained through indirect isotopic determination of uptakes rates. We suggest therefore that GS was not the unique pathway of ammonium assimilation in field populations.

Nitrate and nitrite uptake by free-living and immobilized N-started cells ofPhormidium laminosum

N-starved free-living and polyvinyl-immobilized cells ofPhormidium laminosum (strain OH-1-pCl1) have been investigated in relation to their nitrate and nitrite uptake characteristics. N-deficient cells showed higher inorganic N-uptake rates than N-sufficient ones. The photosynthetic activities of the cells decreased progressively with the time of N-starvation. N-starved cells produced high amounts of exopolysaccharides, which appear to assist the immobilization process. Inorganic N-uptake by N-starved cells occurred in both light and dark under aerobic conditions. In anaerobiosis light was required for the uptake, confirming that the necessary energy might perhaps be derived from the respiratory electron transport chain under aerobiosis. Ammonium inhibited nitrate uptake but did not affect the uptake of nitrite. Initial nitrate and nitrite uptake rates were temperature-dependent and yielded hyperbolic curves when plotted against the N source concentration, indicating the existence of saturable transport system(s).

The interaction of defoliation and nutrient uptake in Sporobolus kentrophyllus, a short-grass species from the serengeti plains.

Sporobolus kentrophyllus, a grazing-tolerant C4 grass from the southeastern Serengeti Plains, was grown in solution culture to examine the effects of clipping on the uptake, preference and subsequent transport of varying nitrogen forms. Clipping reduced offtake mass, crown mass ane root mass, resulting in a 58% decline in plant mass. Proportional biomass allocation to roots decreased with clipping, while tillering rates increased. Clipping also increased the nitrogen concentrations of all tissues, and plant nitrogen uptake (nitrogen accumulated throughout the experiment per gram root). The 15N concentrations (% atom excess) of all tissues were higher in clipped compared with unclipped plants, and the average 15N uptake rate of clipped plants was twice that of unclipped plants. The relative 15N allocation to aboveground mass, a measure of canopy sink strength, was higher in clipped plants. Plants fed 15N-ammonium or 15N-nitrate during the 15N pulse experiment had greater 15N tissue concentrations compared with urea-fed plants, and 15N uptake rates were higher in ammonium-fed and nitrate-fed plants, compared with urea-fed plants. The relative magnitudes of these differences were higher when plants were clipped. Clipped plants had higher uptake rates for potassium, phosphorus and sodium, while differences between clipping treatments for calcium, iron, and magnesium were indistinguishable. Rapid uptake rates for species on the southeastern Serengeti plains, particularly during grazing periods, have important implications for nutrient cycling in this system.

Heterosis for rate of nitrogen uptake in F1rice hybrids

Abstract Rate of nitrogen uptake and dry or fresh weight in 8 F1 rice hybrids at the vegetative stage were compared with the corresponding parental values. Dry and fresh weight, amount and rate of nitrogen uptake per plant showed heterosis (ratio of F1 hybrid to mean parental value), unlike the rate of nitrogen uptake per unit weight. Heterosis for dry weight was not correlated with that for the 15N uptake rate per unit plant weight. Thus, the higher nitrogen uptake of the F1 hybrids was presumably not due to the higher rate of nitrogen uptake, but to their larger weight.

Nitrogen dynamics in a marginal sea-ice zone

Intense blooms of phytoplankton develop at the edge of the seasonal sea ice in the southeastern Bering Sea in spring. These blooms are closely associated with extremely strong water column density stratification due to ice melt. Here we examine the nitrogen requirements of the phytoplankton and evaluate the importance of nitrogen sources to the surface mixed layer. Ammonium and nitrate uptake rates and inorganic nitrogen concentrations within the mixed layer over a 12-day period in May 1982 showed that the amount of nutrient initially present was insufficient to satisfy the requirements of the phytoplankton. During the late winter, inorganic nitrogen levels over the Bering Sea shelf are approximately 18–20 mg-at. NH 4 m −2 and 332–427 mg-at. NO 3-N m −2 in a 25 m surface mixed layer. Phytoplankton specific nitrogen uptake rates measured with 15N averaged 0.0071 h −1 for ammonium and 0.0208 h −1 for nitrate at locations representative of early bloom conditions. The total nitrogen productivity over the 12-day period during which a phytoplankton bloom developed was estimated at 370.8 mg-at. NH 4 m −2 and 588.5 mg-at. NO 3 m −2. Nitrogen was not found to limit the bloom associated with the ice edge and N-uptake rates appeared to be close to saturation values. These uptake rates are used in a budget which provides some general boundary conditions for nitrogen input rates to the mixed layer by different biological and physical mechanisms. The budget was run as an electronic spreadsheet on a personal computer. After examining the various possible sources, we conclude that upwelling associated with the ice edge in conjunction with stationary fronts and in situ ammonification by nanoplankton and bacteria are the significant processes involved in inorganic nitrogen supply. The model presented is an efficient tool for examining physical processes at the ice edge if detailed measurements of biological parameters are available.

Variation in nitrogen response among spring rape ( Brassica napus) cultivars and its relationship to nitrogen uptake and utilization

Growth, development and yield of 40 diverse spring rape ( Brassica napus) cultivars were measured at three different levels of applied nitrogen in the field. There were significant cultivar differences in yield under a limiting N supply and in yield response to higher N application rates. Cultivars with the lowest yields at the lowest N level generally responded more markedly to increase in N application rates than cultivars with higher yields in this treatment. A greater yield response was associated with relatively greater rates of growth and development at higher N levels. Measurements of dry weights and N contents of above-ground plant parts over a period between 42 and 66 days after sowing of the field experiment were used to calculate N-uptake rates and N-utilization (change in dry weight/unit change in N content) indices. Significant cultivar differences were observed only at the lowest N level for N-utilization index and only at the intermediate level for N-uptake rate. If cultivars were grouped on the basis of their geographic origin—Japanese, Canadian, European—those of Japanese origin generally had the highest uptake rates and utilization indices in the respective treatments in which significant differences were detected. Two solution-culture experiments were also conducted in a daylight phytotron at 20/15°C with the same cultivars. Significant cultivar differences in N-uptake rate and N-utilization index were detected with an initial concentration of 1m M N, but not with an initial concentration of 8 m M N. Cultivar differences in root growth and N uptake per unit root dry weight were significant at both concentrations. Again, most of the cultivars with the highest uptake rates or utilization indices were of Japanese origin. Uptake rate was correlated with uptake rate per unit dry weight, but not with initial root dry weight or increase in root weight over the course of the experiment. The significance of geographic variation in N-uptake rate in both field and controlled experiments is discussed with particular reference to the use of this character as a selection criterion in breeding programs utilizing recently developed another culture techniques.

87:6411 Calculations of 15N uptake rates by phytoplankton assimilating one or several nitrogen sources : Collos, Yves, 1987. Int. J. Radiat. Appl. Instrum. (Part A, Appl. Radiat. Isotopes), 38(4):275–282

87:6411 Calculations of 15N uptake rates by phytoplankton assimilating one or several nitrogen sources : Collos, Yves, 1987. Int. J. Radiat. Appl. Instrum. (Part A, Appl. Radiat. Isotopes), 38(4):275–282

Calculations of 15N uptake rates by phytoplankton assimilating one or several nitrogen sources

A general equation is derived, which allows one to compute unbiased values of nitrogen uptake rates in the presence of a single nitrogen compound. In the case of simultaneous uptake of several nitrogen sources, current designs of 15N uptake measurements yield underestimates of specific uptake rates (in units of time −1) of the labeled compound. This is due to a characteristic of stable isotope techniques where, unlike radioactive techniques, a ratio of isotopic abundances is measured. A consequence of this is that reports of inhibition of nitrate or urea uptake by ammonium in studies using the 15N method may be calculation artefacts rather than physiological phenomena.