A critical assessment of the value of petiole sap analysis in optimizing the nitrogen nutrition of the potato crop

The importance of standardizing on sampling procedures was shown by examining diurnal variation of petiole sap nitrate-N concentration, [NO₃ ^--N], and its variation within the plant. There was important variability in petiole sap [NO₃ ^--N] about the general decline in values through the season, in time, between cultivars, between nitrogen fertilizer treatments, and between replicates. We have shown the lack of a consistent relation between petiole sap [NO₃ ^--N] and rates of uptake of nitrogen either in the period around the time of sampling, or integrated over the whole growing season. Finally, we have examined the literature and have been unable to find reports of critical, independent tests of the use of petiole sap [NO₃ ^-] to guide the application of fertilizer supplements. We conclude that a new, critical attitude should be taken to testing petiole sap [NO₃ ^-] if the technique is to be used properly and to its seeming potential. Comparisons were made between measurements using a hand-held system and an auto-analyser.     

Threshold values of canopy reflectance indices and chlorophyll meter readings for optimal nitrogen nutrition of tomato

Sustainable and optimal economic N management requires correct and timely on‐farm assessment of crop N status to detect N deficiency or excess. Optical sensors are promising tools to assess crop N status throughout a crop or at critical times. The ability of optical sensor measurements of canopy reflectance (Crop Circle ACS 470) and leaf chlorophyll (SPAD 502 chlorophyll meter) to assess crop N status was evaluated weekly throughout an indeterminate tomato crop. Strong linear relationships with the optical sensor measurements were obtained, throughout most of the crop, for both (a) crop N content for ranges of 1.5–4.5%, and (b) the nitrogen nutrition index (NNI) for ranges of 0.4–1.3. The relationships of the optical sensor measurements to crop NNI were generally equal to or slightly better than with crop N content. Indices based on reflectance in the red, the normalised difference vegetation index (NDVI) and the red vegetation index (RVI), were the best predictors of crop N status in terms of goodness of fit, earliness and maintenance of relationships throughout the crop. SPAD chlorophyll readings and reflectance indices based on reflectance in the green, the normalised difference vegetation index on greenness (GNDVI) and the green vegetation index (GVI), were good indicators of crop N status for most of the crop, but with lower goodness of fit in the latter part of the crop. The linear relationships between sensor indices and readings with NNI or crop N content, each week, demonstrated the potential for using proximal canopy reflectance indices such as NDVI and RVI, and chlorophyll meter for monitoring crop N status of indeterminate tomato crops. Threshold values for optimal crop N nutrition for canopy reflectance indices and for chlorophyll meter readings were derived for each day of measurement from the relationships between optical sensor measurements and NNI by solving for NNI = 1. The threshold values obtained for each index and type of measurement varied during the crop cycle. The approach developed for determining threshold values from NNI can facilitate on‐farm use of optical sensors for monitoring crop N status, by enabling assessment of whether crop N status is excessive, deficient or adequate.     

Isotopic composition and concentration of total nitrogen and nitrate in xylem sap under near steady-state hydroponics

After root uptake, nitrate is effluxed back to the medium, assimilated locally, or translocated to shoots. Rooted black cottonwood (Populus trichocarpa) scions were supplied with a NO₃⁻-based (0.5 mM) nutrient medium of known isotopic composition (δ¹⁵N), and xylem sap was collected by pressure bombing. To establish a sampling protocol, sap was collected from lower and upper stem sections at 0.1–0.2 MPa above the balancing pressure, and after increasing the pressure by a further 0.5 MPa. Xylem sap from upper stem sections was partially diluted at higher pressure. Further analysis was restricted to sap obtained from intact shoots at low pressure. Total-, NO₃⁻-N and, by difference, organic-N concentrations ranged from 6.1–11.0, 1.2–2.4, and 4.6–9.4 mM, while discrimination relative to the nutrient medium was −6.3 to 0.5‰, −23.3 to −11.5‰ and − 1.3 to 4.9‰, respectively. There was diurnal variation in δ¹⁵N of total- and organic-N, but not NO₃⁻. The difference in δ¹⁵N between xylem NO₃⁻ and organic-N suggests that discrimination by nitrate reductase is near 25.1 ± 1.6‰. When this value was used in an isotope mass balance model, the predicted xylem sap NO₃⁻-N to total-N ratio closely matched direct measurement.     

Nitrogen deposition and forest nitrogen cycling along an urban–rural transect in southern China

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ¹⁵N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha^?1 yr^?1, while the throughfall covered a larger range of 11.7–65.1 kg N ha^?1 yr^?1. Foliar N concentration, NO₃^? leaching to stream, and soil NO₃^? concentration were low and NO₃^? production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO₃^? leaching to stream, and negatively with pH values in soil and stream water. Foliar δ¹⁵N was between ?6.6‰ and 0.7‰, and was negatively correlated with soil NO₃^? concentration and NO₃^? leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ¹⁵N values. We proposed several potential mechanism that could contribute to the δ¹⁵N pattern, including (1) increased plant uptake of ¹⁵N‐depleted soil NO₃^?, (2) foliage uptake of ¹⁵N‐depleted NH₄⁺, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.     

MACROALGAL RESPONSES TO NITROGEN SOURCE AND AVAILABILITY: AMINO ACID METABOLIC PROFILING AS A BIOINDICATOR USING GRACILARIA EDULIS (RHODOPHYTA)1

The use of macroalgae as biological indicators of dissolved nutrient source and availability in the water column was investigated. Total tissue nitrogen (N) content, pigments, and amino acids of the red alga Gracilaria edulis (Gmelin) Silva were compared to N source and availability in laboratory and field incubations to identify responses that would serve as bioindicators of N. Field-collected algae were preincubated (6–8 wk) in low-nutrient seawater to deplete their luxury reserves ofN. Incubations were then conducted for periods of 3 d in laboratory aquaria (N-spiked seawater) and in the field using macroalgal incubation chambers. After incubation in different N sources (NH₄⁺, NO₃^?, and urea) in laboratory aquaria, photosynthetic pigments (phycoerythrin and chlorophyll a) and total tissue N increased, in response to increasing [NH₄⁺] but not to [NO₃^?] or [urea]. Incubation in two ranges of [NH₄⁺], one from 0 to 80 μM and the other from 0 to 800 μM, in laboratory aquaria increased the total amino acid pool. Citrulline concentrations were the most responsive to [NH₄⁺] (r²= 0. 84). NH₄⁺ source treatments produced increases in citrulline, phenylalanine, serine, and free NH₄⁺ and decreases in alanine; NO₃^? treatments produced increases in glutamic acid, citrulline, and alanine; and urea treatments produced increases in free NH₄⁺ and decreases in phenylalanine and serine. The observed variations in amino acid content facilitated the development of an index for each N source based on relative concentrations of various amino acids (i. e. metabolic profiling). Gracilaria edulis was incubated along a field N gradient in the Brisbane River (three sites) and Moreton Bay (four sites), Queensland, Australia. Both phycoerythrin and tissue N appeared to respond equally to NH₄⁺ and NO₃^? availability in the field. N source indices, based on amino acid concentration, were effective predictors of both [NH₄⁺] and [NO₃^?] over a wide range of concentrations along the field gradient. Macroalgal physiological responses, particularly amino acid content, to changes in source and availability of N appear to be useful as sensitive bioindicators of N.     

Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings

The effect of NO₂ fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO₂ to the plant N budget was measured during a 48 h fumigation with 100mm³m^?3 NO₂. Plants were pre-treated with various nutrient solutions containing NO₂ and NH₄⁺, NO₃^? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH₄⁺ in the solution for 6d led to an increased capacity for NO₃^? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO₃^? uptake. In fumigated plants, NO₂ uptake accounted for 20–40% of NO₃^? uptake. Root NO₃^? uptake in plants supplied with NH₄⁺plus NO₃^? solutions was decreased by NO₂ fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO₂ and NH₄⁺ and were fumigated or not with 100mm³m^?3 NO₂ for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and ¹⁵N labelling. Root NH₄⁺ uptake was approximately three times higher than NO₃^? uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO₃^? nor NH₄⁺ accumulated in the shoot, showing that all the absorbed NO₂ was assimilated. Root NO₃^? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO₂-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed.     

Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

THE RELATIVE IMPORTANCE OF WATER MOTION ON NITROGEN UPTAKE BY THE SUBTIDAL MACROALGA ADAMSIELLA CHAUVINII (RHODOPHYTA) IN WINTER AND SUMMER1

The influence of seawater velocity (1.5–12 cm · s^?1) on inorganic nitrogen (N) uptake by the soft‐sediment perennial macroalga Adamsiella chauvinii (Harv.) L. E. Phillips et W. A. Nelson (Rhodophyta) was determined seasonally by measuring uptake rate in a laboratory flume. Regardless of N tissue content, water velocity had no influence on NO₃^? uptake in either winter or summer, indicating that NO₃^?‐uptake rate was biologically limited. However, when thalli were N limited, increasing water velocity increased NH₄⁺ uptake, suggesting that mass‐transfer limitation of NH₄⁺ is likely during summer for natural populations. Uptake kinetics (V_max, K_s) were similar among three populations of A. chauvinii at sites with different mean flow speeds; however, uptake rates of NO₃^? and NH₄⁺ were lower in summer (when N status was generally low) than in winter. Our results highlight how N uptake can be affected by seasonal changes in the physiology of a macroalga and that further investigation of N uptake of different macroalgae (red, brown, and green) during different seasons is important in determining the relative influence of water velocity on nutrient uptake.     

RAPID NITRATE UPTAKE RATES AND LARGE SHORT‐TERM STORAGE CAPACITIES MAY EXPLAIN WHY OPPORTUNISTIC GREEN MACROALGAE DOMINATE SHALLOW EUTROPHIC ESTUARIES1

We quantified the effects of initial macroalgal tissue nitrogen (N) status (depleted and enriched) and varying pulses of nitrate (NO₃^?) concentration on uptake and storage of nitrogen in Ulva intestinalis L. and Ulva expansa (Setch.) Setch. et N. L. Gardner using mesocosms modeling shallow coastal estuaries in Mediterranean climates. Uptake of NO₃^? (μmol · g dry weight [dwt]^?1 · h^?1) was measured as loss from the water after 1, 2, 4, 8, 12, and 24 h and storage as total tissue nitrogen (% dwt) and nitrate (ppm). Both species of algae exhibited a high affinity for NO₃^? across all N pulses and initial tissue contents. There was greater NO₃^? removal from the water for depleted than enriched algae across all time intervals. In the low‐N‐pulse treatment, U. intestinalis and U. expansa removed all measurable NO₃^? within 8 and 12 h, respectively, and in the medium and high treatments, removal was high and then decreased over time. Maximum mean uptake rates of nitrate were greater for U. expansa (～300 μmol · g dwt^?1 · h^?1) than U. intestinalis (～100 μmol · g dwt^?1 · h^?1); however, uptake rates were highly variable over time. Overall, U. expansa uptake rates were double those of U. intestinalis. Maximum tissue NO₃^? for U. expansa was >1,000 ppm, five times that of U. intestinalis, suggesting that U. expansa has a greater storage capacity in this cellular pool. These results showed that opportunistic green algae with differing tissue nutrient histories were able to efficiently remove nitrate from the water across a wide range of N pulses; thus, both are highly adapted to proliferate in estuarine environments with pulsed nutrient supplies.     

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

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

BIOENERGETIC PROCESSES ARE MODIFIED DURING NITROGEN STARVATION AND RECOVERY IN PHORMIDIUM LAMINOSUM (CYANOPHYCEAE)1

In the non-N₂-fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH-I-pCl₁), N starvation induced an increase in the rate of respiration and a decrease in the rate of O₂ evolution. When NO₃^? was added to illuminated N-starved cells, O₂ evolution immediately increased to levels shown by NO₃^? grown cells, even though N-starved cells had lost most of their in vitro photosynthetic activities. Stimulation of noncyclic electron flow was maximal under light-saturating conditions and after 2–3 days of N starvation. The respiratory rate of N-starved cells was stimulated by the addition of NO₃^? or NH₄⁺ and partially inhibited at very low irradiances, even in the presence of DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). Results indicate that N-starved cells obtain the energy supply for N assimilation through a process different from that used by N-sufficient cells. N-starved cells were able to take up NO₃^? in the dark and when illuminated in the presence of DCMU under anaerobiosis. Following NO₃^? addition, the photosynthetic yield of the in vivo noncyclic electron transport slightly increased, whereas it decreased after NH₄⁺ addition. Addition of NO₃^? or NH₄⁺ favored photoinhibition of photosystem II, the effect being faster after NH₄⁺ addition.     

Leaf senescence in a non-yellowing mutant of Festuca pratensis: implications of the stay-green mutation for photosynthesis, growth and nitrogen nutrition

Mutation of the nuclear gene sid disables chlorophyll degradation during leaf senescence in the pasture grass Festuca pratensis. This study investigated the effect of the mutation on photosynthesis and on leaf and whole plant growth under a range of nitrogen regimes. When plants were cultivated in a static hydroponic system, the chlorophyll content of fourth leaves of the stay-green mutant Bf993 remained virtually unchanged from full expansion to complete senescence, while tissue of the wild-type (cv. Rossa) became completely yellow. The retention of chlorophyll in Bf993 was not associated with maintenance of photosynthetic activity as shown by rates of light-saturated CO₂ fixation and apparent quantum efficiency. Higher levels of total N in senescing leaves of Bf993 than in Rossa indicated reduced nitrogen remobilization in the mutant. When using a range of [NH₄NO₃], dry matter production and tillering Mere lower for Bf993 at all but the highest [NH₄NO³, which was supra-optimal for the wild type. In contrast to the static system, where fluctuations in N supply occurred, growth and [NO₃^?] uptake were similar in mutant and wild type when [NO₃^?] was continuously maintained by a flowing solution culture system. The results are discussed in relation to the role of N supply and the effect of the stay-green mutation on N recycling.     

Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition

Studies that quantify plant δ¹⁵N often assume that fractionation during nitrogen uptake and intra-plant variation in δ¹⁵N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T-5) at NH₄⁺ or NO^?₃ concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ¹⁵N was not significantly different from source δ¹⁵ N for plants grown on either nitrogen form. No intra-plant variation in δ¹⁵N was observed for plants grown with NH⁺₄. In contrast. δ¹⁵N of leaves was as much as 5.8% greater than that of roots for plants grown with NO^?₃. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH⁺₄ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO^?₃ assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ¹⁵N of NO^?₃ to become enriched relative to organic nitrogen; the δ¹⁵N of NO^?₃ was 11.1 and 12.9% greater than the δ¹⁵N of organic nitrogen in leaves and roots, respectively. Leaf δ¹⁵N may therefore be greater than that of roots because the NO^?₃ available for assimilation in leaves originates from a NO^?₃ pool that was previously exposed to nitrate assimilation in the root.     

Relationships among nitrogen accumulation, nitrogen assimilation and plant growth in chrysanthemums

Relationships among growth, N accumulation and assimilation were investigated in Chrysanthemum morifolium Ramat cv. Fiesta in experiments testing the effects of varying levels of NO^–3₃supply and of increasing NH⁺₄ added to a constant level of NO^–3₃ Flowing solution culture systems were used to provide NO^?₃at concentrations of 0.03 to 5.0 mol m^–3 and NH⁺₄ levels from 0.05 to 0.3 mmol m^–3 added to 0.1 mol m^–3NO^?₃. Rates of growth, N absorption, accumulation, distribution and utilization were estimated by regression analysis of data obtained from sequential plant harvests, and rates of NO^?₃ and NH^?₄ net uptake were estimated from solution depletion. A sustained ambient NO^?₃ concentration of 0.03 mol m^–3 was evidently adequate to support growth, since relative growth rates were not affected by increasing NO^?₃ supply from 0.03 to 1.0 mol m^–3, nor from 0.25 to 5.0 mol m^–3, in separate experiments. Shoot growth rates were stimulated by NH₄ added to NO^?₃ one experiment, but not when the experiment was repeated under ambient conditions less favorable to growth. Relative accumulation rates for total N increased with increasing NO^?₃ and with NH⁺₄added to NO^?₃ A constant proportion of NO^?₃ taken up was reduced when NO^?₃ alone was supplied. Both the proportion of total N taken up as NO^?₃ and the proportion of NO^?₃ reduced decreased with increasing NH⁺₃ added to NO^?₃ NH⁺₄ uptake apparently must exceed a threshold of about 30% of the total uptake to inhibit NO^?₃ uptake. Utilization of N in chrysanthemum was apparently limited by redistribution since relative accumulation rates for total N were equal to or greater than relative growth rates, in contrast to results reported for several other species. Results of this study and other information support the postulate that NH⁺₄ added to NO^?₃might stimulate growth by increasing transport of reduced N from roots to shoots, thus increasing the supply of reduced N available to support growth of shoot meristems.     

BIOCHEMICAL STRATEGIES FOR GROWTH OF GRACILARIA TIKVAHIAE (RHODOPHYTA) IN RELATION TO LIGHT INTENSITY AND NITROGEN AVAILABILITY1

The main effects and interactions between light (I_o, full incident sunlight to 0.07 I_o) and NO₃^? loading (0.4 to 4.3 mmol · g dry weight^?1· d^?1) on growth rate, photosynthesis and biochemical constituents of Gracilaria tikvahiae McLachlan were studied using a factorial design experiment in outdoor, continuous-flow seawater cultures. Incipient nitrogen limitation in the low NO₃^? loading, I_o and 0.57 I_o treatments occurred after 2.5 weeks of growth under the experimental conditions and resulted in decreased tissue NO₃^? and R-phycoerythrin. Tissue NO₃^? and R-phycoerythrin accounted for up to ca. 15 and 20%, respectively, of the total N in G. tikvahiae suggesting a N reserve role for these N pools. Under light and NO₃^? limitation, growth rate was a parabolic function of the C:N ratio. As light limitation increased, growth rate and the C:N ratio decreased as levels of Chl-a, R-phycoerythrin, percent N and percent protein increased. As NO₃^? limitation increased, growth rate and levels of Chl-a, R-phycoerythrin, percent N and percent protein all decreased with parallel increases in the C:N ratio. In contrast to the inverse relationship between pigment content and light, ribulose bisphosphate carboxylase (RuBPCase) activity (on both a protein and dry weight basis) varied directly with light. This biochemical acclimation of G. tikvahiae to light and N availability appears to be a process directed towards maximizing photo synthetic capacity and growth.     

PHOTOSYNTHETIC FUNCTION IN DUNALIELLA TERTIOLECTA (CHLOROPHYTA) DURING A NITROGEN STARVATION AND RECOVERY CYCLE

Phytoplankton can be exposed to periods of N starvation with episodic N resupply. N starvation in Dunaliella tertiolecta (Butcher) measured over 4 days was characterized by slow reduction in cell chl and protein content and chl/carotenoid ratio and a decline in photosynthetic capacity and maximum quantum yield of photosynthesis (F_v/F_m). In the early stages of N starvation, cell division was maintained despite reduction in cellular chl. Chl content was more sensitive than carotenoids to N deprivation, and cellular chl a was maintained preferentially over chl b under N starvation. NO₃^? resupply stimulated rapid and complete recovery of F_v/F_m (from 0.4 to 0.7) within 24 h and commencement of cell division after 10 h, although N‐replete levels of cell chl and protein were not reestablished within 24 h. Recovery of F_v/F_m was correlated with increases in cell chl and protein and was more related to increases in F_m than to changes in F₀. Recovery of F_v/F_m was biphasic with a second phase of recovery commencing 4–6 h after resupply of NO₃^?. Uptake of NO₃^? from the external medium and the recovery of F_v/F_m, cell chl, and protein were inhibited when either cytosolic or chloroplastic protein synthesis was inhibited by cycloheximide or lincomycin, respectively; a time lag observed before maximum NO₃^? uptake was consistent with synthesis of NO₃^? transporters and assimilation enzymes. When both chloroplastic and cytosolic translation was inhibited, F_v/F_m declined dramatically. Dunaliella tertiolecta demonstrated a capacity to rapidly reestablish photosynthetic function and initiate cell division after N resupply, an important strategy in competing for limiting inorganic N resources.     

A comparative study of fluxes and compartmentation of nitrate and ammonium in early-successional tree species

¹³NO₃^– and ¹³NH₄⁺ compartmental analyses were carried out in seedling roots of trembling aspen (Populus tremuloides Michx.), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) at 0·1 and 1·5 mol m^–3 external NO₃^– or NH₄⁺ concentrations ([NO₃^–]_o or [NH₄⁺]_o, respectively). At the lower [NO₃^–]_o, the capacities and efficiencies of acquisition and accumulation of NO₃^–, based upon NO₃^– fluxes and cytoplasmic NO₃^– concentrations ([NO₃^–]_c), were in the order aspen >> Douglas-fir > pine. At 1·5 mol m^–3[NO₃^–]_o, the NO₃^– influx increased 18-fold in pine, four-fold in Douglas-fir and approximately 1·4-fold in aspen; in fact, at 1·5 mol m^–3[NO₃^–]_o, the NO₃^– influx in pine was higher than in aspen. However, at high [NO₃^–]_o, efflux also increased in the two conifers to a much greater extent than in aspen. In aspen, at both [NO₃^–]_o, approximately 30% of the ¹³N absorbed was translocated to the shoot during 57 min of ¹³N loading and elution, compared with less than 10% in the conifers. At 0·1 mol m^–3[NH₄⁺]_o, influx and net flux were in the order: aspen > pine > Douglas-fir but the differences were much less than in NO₃^– fluxes. At 1·5 mol m^–3[NH₄⁺]_o, NH₄⁺ influx, efflux and [NH₄⁺]_c greatly increased in aspen and Douglas-fir and, to a much lesser extent, in pine. In aspen, 29 and 12% of the ¹³N absorbed was translocated to the shoot at 0·1 and 1·5 mol m^–3[NH₄⁺]_o, respectively, compared with 5 to 7% in the conifers at either [NH₄⁺]_o. These patterns of nitrogen (N) uptake, particularly in the case of NO₃^–, and the observed concentration responses of NO₃^– uptake, reflect the availability of N in the ecological niches, to which these species are adapted.     

NUTRITIONAL STUDIES OF TWO RED ALGAE. II. KINETICS OF AMMONIUM AND NITRATE UPTAKE1, 2

Similar NH₄₊ and NO₃^?.uptake kinetic patterns were observed in Neoagardhiella baileyi (Harvey ex Kiitzing) Wyinne & Taylor and Gracilaria foliifera (Forssk?l) Borgesen. NO₃^? was taken up in a rate-sturating fashion described by the Michaelis-Menten equation. NH₄₊ uptake was multicomponent: a saturable component was accompanied by a diffusive or a high K component showing no evidence of saturation (at ≤50 μM [NH₄₊]). Nitrogen starved plantsi(C/N atom ratios > ca. 10) showed higher transient rates of NH₄₊ uptake at a given concentration than plants not N-Iimited. Only plants with high N content exhibited diel changes inNH₄₊ uptake rates, and showed transient rates of NH₄₊ accumulation which did not greatly exceed the capacity to incorporate N in steady-state growth. NH₄₊ was preferred over NO_3?even in plants preconditioned on NO_3?as the sole N. source, NO_3? uptake was suppressed at 5μM [NH₄₊], but simultaneous uptake occurred at unsurpressed rates at lower concentrations. Potential for N accumulation was greater via NH₄₊uptake than via NO_3?uptake. Changing capacity for NH₄₊ uptake with N content appears to be a mechanism whereby excessive accumulation of N was avoided by N-.satiated plants but a large accumulation was possible for N-depleted plants.     

CHANGES IN INTRACELLULAR NITROGEN POOLS AND FEEDBACK CONTROLS ON NITROGEN UPTAKE IN CHAETOMORPHA LINUM (CHLOROPHYTA)1

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH₄⁺ and NO₃^?) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient algae could be ranked as residual organic N (RON) comprised mainly of amino acids and amino compounds > protein N > NO₃^? > NH₄⁺ > chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletion period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within 10 days. Upon N resupply, the RON pool increased 3 × faster than either the inorganic or protein pools, suggesting that protein synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compounds. Maximum uptake rates for both NH₄⁺ and NO₃^? varied inversely with macroalgal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO₃^? showed an initial lag phase, but the initial uptake of NH₄⁺ was enhanced and was present only when the intracellular NH₄⁺ pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool size and maximum assimilation uptake rates for both NH₄⁺ and NO₃^? suggested a feedback control on assimilation uptake by the buildup and depletion of organic compounds. Enhanced uptake and the accumulation of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demand in C. linum.     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.