氮素水平对茶树叶片氮代谢关键酶活性及非结构性碳水化合物的影响

以福鼎大白茶(FD)、保靖黄金茶1号(HJ1)、白毫早(BHZ)为材料,设置不施氮N_0(0 g)、低氮N1(11 g)、中氮N_2(22 g)和高氮N_3(33 g)4个氮素水平的盆栽实验,研究了铵态氮对3个品种茶树的根系活力、氮代谢关键酶及非结构性碳水化合物(NSC)的影响。结果表明:随着施氮水平的提高,N_2、N_3处理的茶树根系活力较对照N0显著增加(P0.05),但二者间无显著差异;叶片谷氨酰胺合成酶(GS)、谷氨酸合成酶(GOGAT)活性总体呈上升趋势;与对照比较,茶树叶片全氮和可溶性蛋白含量增加,其中HJ1在N_2和N_3处理后显著增加(P0.05);在3个茶树品种中,非结构性碳水化合物中可溶性总糖含量均呈上升趋势,淀粉含量具有品种特异性,施氮处理后3个茶树品种氮代谢关键酶活性及NSC含量变化存在差异,以HJ1的氮同化关键酶GS、GOGAT酶活性较高、根系活力较强,氮代谢产物显著增加,表明其具有较高的氮同化速率。施氮后HJ1的总NSC的含量及碳氮比的变化幅度较另外2个品种小,能够更好的保持碳氮平衡,游离氨基酸含量增幅较高,品质更优。因此,通过茶树氮代谢关键酶活性及非结构性化合物的研究能为茶树品种的品质评价以及提高茶树的品质和氮素利用效率提供依据。     

Modulating infestation rate of white fly (<Emphasis Type="Italic">Bemicia tabaci</Emphasis>) on okra (<Emphasis Type="Italic">Hibiscus esculentus</Emphasis> L.) by nitrogen application

Pests are major factor affecting the crop yield, quality, and esthetic value. Although pest resistance is genetically controlled, it is mediated through physiological and biochemical processes interrelated with the nutritional status of the plant. Nutrition of plant may determine its resistance or susceptibility to pests. An experiment was conducted to determine whether and up to what extent varying concentrations of N modulate the infesting rate of whitefly on Okra (Hibiscus esculentus). Twenty-one-day-old okra plants were subjected to five concentrations of 4, 8, 12, 16, and 20 mM N for 30 days. Number of eggs, nymph were counted on each plant after every 4 days. Growth and yield of okra plants increased with increasing supply of N but it decreased slightly at the highest level of N (20 mM). Leaf soluble proteins, amino acids, and soluble sugars were determined on fresh weight basis. From these results, it is clear that increasing N level increased the biosynthesis or accumulation of protein, free amino acid, and sugars that might have attracted more whitefly for feeding.     

Differential response of carbon and nitrogen metabolism to fluoride application in fruiting structures of chickpea (Cicer arietinum)

The effect of sodium fluoride (10 and 50 mol·m⁻³) on the activities of sucrose metabolizing enzymes, transaminases and glutamine synthetase in relation to the transformation of free sugars to starch and protein in the fruiting structures (pod wall, seed coat, cotyledons) of chickpea was studied by culturing detached reproductive shoots in a liquid medium. Addition of fluoride to the culture medium drastically reduced starch content of the cotyledons and caused a marked build-up of total free sugars comprised mainly of reducing sugars in the pod wall and seed coat, and sucrose in the cotyledons. Concomitantly, the activity of soluble invertase was stimulated in the pod wall but reduced in the cotyledons. However, soluble protein content of both the pod wall and the cotyledons increased in conjunction with an increase in the activities of glutamate-oxaloacetate transaminase, glutamate-pyruvate transaminase and glutamine synthetase. Disruption of starch biosynthesis under the influence of fluoride and the resulting accumulation of free sugars possibly resulted in their favoured utilization in nitrogen metabolism. Labelling studies with [U-¹⁴C]-sucrose showed that the ¹⁴C incorporation into total free sugars was enhanced by fluoride in the pod wall but reduced in the seed coat and cotyledons, possibly due to an inhibitory effect on their translocation to the developing seeds.     

Growth and composition of maize kernels cultured in vitro with varying supplies of carbon and nitrogen

This study employed in vitro seed culture to determine how C and N supply influence the growth (i.e. starch accumulation) and protein composition of maize (Zea mays L.) endosperm. Immature kernels were grown to maturity on liquid medium containing various concentrations of C (sucrose at 234 millimolar [low] and 468 millimolar [high]) and N (amino acid mixture ranging in N from 0 to 144 millimolar). Low C supply limited starch, but not N, accumulation in the endosperm. With high C, endosperm starch and protein content increased concomitantly as N supply increased from 0 to 13.4 millimolar. Endosperm growth was unaffected by additional N until concentrations exceeding approximately 72 millimolar reduced starch accumulation. A similar inhibition of starch deposition occurred with lower N concentrations when kernels were grown with low C. Endosperm total N content reached a point of saturation with approximately 36 millimolar N in the medium, regardless of C supply. Zein synthesis in the endosperm responded positively across all N levels, while glutelin content remained static and albumin/globulin proteins were reduced in amount when N supply was greater than 36 millimolar. A reciprocal, inverse relationship was observed in mature endosperm tissue between the concentrations of free amino acids and soluble sugars. Our data suggest that under N stress starch and protein accumulation in the endosperm are interdependent, at least in appearance, but are independent otherwise.     

Stage-specific nitrogen metabolism in developing carrot somatic embryos

The physiology of individual somatic embryo developmental stages otDaucus carota L. was examined by in vivo nuclear magnetic resonance (NMR) spectroscopy, amino acid analysis and ¹⁴C-labeling. ¹⁵N NMR spectroscopy was used to examine the uptake and incorporation of ¹⁵N isotopically labeled inorganic nitrogen sources. NMR spectra of proembryogenic masses (PEMs) contained resonances for histidine, amino sugars, glutamine, arginine, urea, alanine. α-amino nitrogen, serine, aliphatic amines and several unknowns. Similar resonances were found in various embryo developmental stages. However, resonances for arginine and aliphatic amines peaked during globular and torpedo stages and substantially decreased in germinating stage embryos. The dominant resonances observed in non-embryogenic cells and germinating embryos were glutamine and α-amino nitrogen. Amino acid analysis of the various embryo stages showed that glutamate, glutamine and arginine were the major contributors to the soluble amino acid profiles. During development, glutamate and glutamine continued to increase in concentration whereas arginine and its related metabolites (i.e. ornithine and y-aminobutyric acid [GABA]) were biphasic; increasing in globular and torpedo stage embryos and decreasing in germinating embryos. Carbon-14 labeling indicated that labeled glutamine pools in non-embryogenic and germinating embryos were greatest compared to other embryo stages, whereas labeled GABA pools were greatest in globular and torpedo stage embryos. Taken together, these data indicate that the physiology of each embryo developmental stage is distinct. They also suggest that during somatic embryo development, a switch takes place in metabolism whereby the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway is predominant in non-embryogenic cells and germinating stage embryos. Furthermore, during early to mid-embryo development (PEMs, globular and torpedo stage embryos), metabolism utilizing the omithine cycle is enhanced and predominant.     

Responses of the green algal foliose lichen Platismatia glauca to increased nitrogen supply

Nitrogen (N) availability and light exposure were manipulated under field conditions to study responses to altered resource supply in the green algal lichen Platismatia glauca. The lichen was fertilized with different concentrations and frequencies of ammonium, nitrate or glutamine under different light regimes for 2-3 months. Responses were followed from the intact thallus to the cellular level. Despite significant differences in overall light exposure, light conditions were not significantly different among treatments when the lichens were wet and active. Ammonium was the preferred N source, followed by glutamine and then nitrate. Thallus N concentration as well as the chlorophyll a (Chl a) concentration increased 3-4-fold at the highest ammonium concentration, while the mycobiont ergosterol concentration remained unaltered. Growth was significantly enhanced by the enhanced N supply, with the increase in dry weight varying from 3 to 30%. Variation in Chl a concentration explained 31% of this variation, suggesting a causal link to the increased growth rate. Platismatia glauca responded to increased N availability by increasing its growth rate and carbon assimilation capacity through increased investments in the photobiont cells. This suggests a tight regulation of resource investments and metabolic pathways between the symbionts of this lichen.     

Regulation of Starch and Protein Synthesis in Wheat Grains by Feeding Sucrose and Glutamine to Detached Ears Cultured in vitro

Three experiments of in vitro ear culture were conducted to evaluate how the substrates of C (carbon) and N (nitrogen) supply in liquid medium regulate the grain growth and synthesis of protein and starch in two winter wheat cultivars. Increasing glutamine supply with constant sucrose concentration increased the contents of total protein and protein components of albumin and globulin in grain, and the activity of glutamate pyruvate transaminase (GPT) across most treatments, while markedly reducing the contents of total starch and components of amylose and amylopectin as well as the activities of soluble starch synthase (SSS) and granule bounded starch synthase (GBSS). The opposite patterns were observed in the experiment of increasing sucrose supply at constant glutamine concentration. When simultaneously increasing sucrose and glutamine supply at constant ratio, the contents of total protein, albumin and globulin in grain were slowly enhanced, whereas the contents of total starch, amylose and amylopectin and the activities of SSS, GBSS and GPT increased only to a certain extent and then decreased. Negative correlations were found between the contents of protein or protein components in grains and the relative ratio of sucrose to glutamine concentrations in the culture medium, while positive correlations were seen between the contents of total starch or starch components and the ratio of sucrose to glutamine. These results implied that the composition of protein and starch in wheat grain could be readily manipulated by varying the concentrations of sucrose and glutamine and their ratio in the culture medium.     

Removal of nitrogen during needle senescence in Scots pine (Pinus sylvestris L.)

The concentrations of arginine, protein and total nitrogen (N) and the abundance of¹⁵N were measured in 3-and 4-year-old needles of Scots pine trees fertilized with either 0 (C), 36 (N1) or 73 (N2) kg N ha^-1 year^-1 annually for 22 years (average doses of N). Remaining green needles and needles that were shed were compared and removal of N from total, protein and arginine pools was calculated. Earlier investigations had shown that high arginine concentrations are found in needles of trees that have an excessive N supply (Näsholm and Ericsson 1990). This study aimed to elucidate the fate of the accumulated arginine during needle senescence. It was speculated that a low removal of arginine during senescence would implicate that the primary function of arginine is in N detoxification and not in N storage. Moreover, litter quality would be altered if needles are shed with high concentrations of arginine and this might affect the turnover of N in forest ecosystems. In remaining green needles, the concentration of total N increased with increasing N supply. Protein N concentrations were higher in fertilized trees, but did not differ between the two N treatments. Arginine N was low in C and N1 trees but high in N2 trees. Senescent needles from C and N1 trees had about equal total N concentrations while in N2 trees this concentration was significantly higher. Protein N in senescent needles did not differ between treatments. Arginine N, however, was less than 0.1 mg g^–1 dw in C and N1 trees but was higher than 1.5 mg g^–1 dw in N2 trees. Removal of N was highest in N1 trees followed by C trees while N2 trees removed least N from senescing needles. The high concentration of total N in senescent needles from N2 trees was to a great extent explained by a high arginine concentration.The ¹⁵N value of remaining, green needles was higher (less negative) in N2 trees than in C and N1 trees. The same pattern was found for senescent needles. Comparisons of ¹⁵N values between remaining, green and senescent needles within each treatment showed a significant increase in ¹⁵N for all treatments during senescence possibly indicating losses of N as NH₃ (g) from needles during senescence. It is concluded that arginine, accumulated in response to high N supply, is retranslocated only to a small extent during needle senescence. The ecological and physiological implications of this finding are discussed.     

The effect of nitrate assimilation deficiency on the carbon and nitrogen status of Arabidopsis thaliana plants

Carbon (C) and nitrogen (N) metabolism are integrated processes that modulate many aspects of plant growth, development, and defense. Although plants with deficient N metabolism have been largely used for the elucidation of the complex network that coordinates the C and N status in leaves, studies at the whole-plant level are still lacking. Here, the content of amino acids, organic acids, total soluble sugars, starch, and phenylpropanoids in the leaves, roots, and floral buds of a nitrate reductase (NR) double-deficient mutant of Arabidopsis thaliana (nia1 nia2) were compared to those of wild-type plants. Foliar C and N primary metabolism was affected by NR deficiency, as evidenced by decreased levels of most amino acids and organic acids and total soluble sugars and starch in the nia1 nia2 leaves. However, no difference was detected in the content of the analyzed metabolites in the nia1 nia2 roots and floral buds in comparison to wild type. Similarly, phenylpropanoid metabolism was affected in the nia1 nia2 leaves; however, the high content of flavonol glycosides in the floral buds was not altered in the NR-deficient plants. Altogether, these results suggest that, even under conditions of deficient nitrate assimilation, A. thaliana plants are capable of remobilizing their metabolites from source leaves and maintaining the C–N status in roots and developing flowers.     

沿干旱梯度樟子松人工林针叶和枝条非结构性碳水化合物及氮含量的变化

选取沿内蒙古科尔沁沙地自然干旱梯度分布的6个樟子松人工林样点(辉南、西丰、付家、章古台、奈曼和乌兰敖都)为研究对象,测定当年生和1年生针叶和枝条中非结构性碳水化合物(NSCs)和氮(N)含量的变化,以探究干旱条件下樟子松的碳供需状态和养分贮存策略。结果表明: 随干旱加剧,樟子松针叶和枝条中的NSCs和可溶性糖含量显著降低。从最湿润样点(辉南)到最干旱样点(乌兰敖都),樟子松当年生和1年生针叶中可溶性糖含量分别由12.8%和12.5%下降到9.0%和9.5%,而当年生枝条中可溶性糖含量由15.6%下降到9.2%。随干旱加剧,樟子松针叶和枝条中淀粉含量变化不显著,当年生和1年生针叶可溶性糖和淀粉的比值降低,当年生和1年生枝条中N含量显著增加。科尔沁沙地樟子松在干旱条件下显著消耗可溶性糖存储,存在“碳饥饿”致死风险。樟子松倾向维持稳定的淀粉含量以及在枝条中积累N以应对长期的干旱胁迫。     

Effects of different LED sources on the growth and nitrogen metabolism of lettuce

Using a light-emitting diode (LED) as the light source, the effects of eight different light treatments [white light (control, W), purple light (P), blue light (B), red light (R), green light (G), yellow light (Y), red–blue light in a 9:1 ratio (9R/1B), and red–blue light in a 4:1 ratio (4R/1B)] on the growth, quality and nitrogen metabolism of lettuce were studied. The results showed that compared with the white light, the purple light, blue light, red light, and the red-blue light combination could all increase the biomass of the aboveground part of lettuce to various degrees, while green light and yellow light inhibited lettuce growth. Under blue light, the contents of soluble protein and flavonoid in lettuce were the highest; under red light, the soluble sugar content was the highest, while the contents of soluble protein, free amino acids, and vitamin C (VC) were relatively higher under the 4R/1B light condition. Compared with white light, the sources of purple, blue, and red lights as well as the red–blue light combination all significantly reduced nitrate accumulation in lettuce, and the activities of the nitrogen (N) metabolism-related enzymes such as nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, and glutamate dehydrogenase were increased to varying degrees. In contrast, the contents of nitrate and ammonium N were significantly accumulated in lettuce under green light, and the activities of relative enzymes were significantly reduced. Therefore, the purple light, blue light, and red–blue combination light sources could promote N assimilation and improve the aboveground biomass accumulation in lettuce by improving the activity of the N metabolism-related enzymes in lettuce. Particularly under the 4R/1B light source, the biomass, soluble protein, VC, and total amino acid content were rather high in lettuce, which indicated that the 4R/1B light source could better effectively improve the nutritional quality and promote the growth of lettuce, while yellow light and green light are not suitable to serve as direct sources in a plant factory. These results provide a certain theoretical basis for the regulation of the light environment in cultivation facilities.     

Effects of Nitrate Nutrition on Nitrogen Metabolism in Cassava

Two experiments were conducted independently with plants of cassava (Manihot esculenta Crantz) growing in sand with nutrient solutions with four nitrate concentrations (0.5, 3, 6 or 12 mM). In leaves, nitrate-N was undetectable at the low nitrate applications; total-N, ammonium-N, amino acid-N, reduced-N and insoluble-N all increased linearly, while soluble proteins did it curvilinearly, with increasing nitrate supply. In contrast, soluble-N did not respond to N treatments. Total-N and soluble proteins, but not nitrate-N or ammonium-N, were much higher in leaves than in roots. Plants grown under severe N deficiency accumulated ammonium-N and amino acid-N in their roots. Further, plants were exposed to either 3 or 12 mM nitrate-N, and leaf activities of key N-assimilating enzymes were evaluated. Activities of nitrate reductase, glutamine synthetase, glutamate synthase and glutamate dehydrogenase were considerably lower in low nitrate supply than in high one. Despite the low nitrate reductase activity, cassava leaves showed an ability to maintain a large proportion of N in soluble proteins.     

供氮水平对菠菜营养品质和体内抗氧化酶活性的影响

通过水培实验,研究了供氮水平对菠菜营养品质和抗氧化酶活性的影响.结果表明,供氮水平由4mmol·L^-1增加到8mmol·L^-1,菠菜产量显著增加,叶片中的维生素C(Vc)含量随着供氮浓度由4mmol·L^-1提高到8mmol·L^-1,再提高供氮水平,Vc含量则明显下降.叶片硝酸盐含量随着氮浓度的提高而增加.供氮浓度从4mmol·L^-1增加到8mmol·L^-1,叶片可溶态草酸含量略有下降,再提高供氮水平则明显上升,而草酸总量随供氮水平提高,先显著升高然后略有降低.SOD和POD酶的活性随供氮水平由4mmol·L^-1提高到8mmol·L^-1而增加,再提高供氮水平,酶活性显著下降;CAT活性随供氮水平的增加而降低,叶片MDA含量先降低后显著升高,而游离脯氨酸含量随氮水平的升高而增加.可见供氮水平为8mmol·L^-1时,菠菜叶片具有较高的生物量、Vc含量和抗氧化酶活性,较低的硝酸盐和草酸含量以及较低的MDA和游离脯氨酸含量,表明供氮浓度8mmol·L^-1有利于提高菠菜的产量、营养品质和抗逆能力,是菠菜生长较适宜的供氮水平.     

Diurnal variation of photosynthesis and photoinhibition in tea: effects of irradiance and nitrogen supply during growth in the field.

Diurnal changes in the rate of photosynthesis (A) of mature tea (Camellia sinensis (L.) O. Kuntze) bushes grown at high elevation in the field in Sri Lanka, were related to environmental conditions. Bushes were either unshaded, receiving 100% of incident photosynthetically active radiation (PAR), moderately shaded, (65% PAR) or heavily shaded (30% PAR). These treatments were combined with nitrogen fertilizer applications of 0, 360 and 720 kg ha(-1) year(-1). When recently fully expanded leaves were measured under the growing conditions on bright, clear days from dawn to dusk, A was greatest in the morning with increasing radiation between approximately 8 h and 10 h. Stomatal conductances (g(s)) and substomatal carbon dioxide concentrations (C(i)) were then large, leaf temperatures (T(L)) cool, and saturated water vapour deficits (VPD) small. However, as the irradiance, T(L) and VPD increased towards midday, A, g(s), photochemical quenching, and C(i) decreased, and non-photochemical quenching increased. In the late afternoon, irradiance, T(L) and VPD fell, but despite the relatively large increase in g(s) and C(i), A remained low; however, it recovered overnight. The zero-N treatment decreased total-N content of leaves by 50% and A by c. 20% (not significant). Leaves of unshaded plants receiving least N had significantly (P<0.05) smaller A and greater total sugar content than shaded but with abundant N, A and sugars did not differ between shade treatments. Analysis of the responses of A to environment in the morning compared to the afternoon, and of chlorophyll fluorescence, suggests that A was photoinhibited as a consequence of greatly increased PAR, whilst decreasing g(s) (related to changes in PAR, VPD and T(L)) caused C(i) to fall. End-product inhibition of A is not consistent with decreased C(i). Inhibition of A as a result of photoinhibition was minimized, but not eliminated, by abundant N. Interactions between factors regulating A in tea are discussed.     

Effects of sulfur deficiency on non-protein nitrogen,soluble sugars,and N/S ratios in young corn (Zea mays L.) plants

Summary The concentrations of several N fractions, soluble sugars and N/S ratios were determined in Golden Bantam sweet corn and hybrid field corn grown in nutrient solutions and/or soil with various SO₄-S levels provided. When the level of SO₄-S supplied was less than that needed for maximum growth, higher-than-normal concentrations of amide (asparagine) N and lower-than-normal soluble sugar concentrations were found in the plants. An inadequate S supply appears to limit the plants capacity to synthesize protein to a much greater extent than it inhibits the nitrogen uptake mechanism.Total N/total S ratios (grammole basis) varying from over 100 to less than 15 were found in plants grown with different levels of sulfate supplied. When SO₄-N, NO₃-N, and the large amount of amide-N which accumulates under conditions of S deficiency are excluded, the ratio in young corn plants is generally between 22–50. The precision of the N/S ratio as an indicator of S adequacy for growing crops might be improved by a better understanding of the nature of the forms in which N and S occur in plants, and the effects of soil and plant factors other than S level. The percentage of amide-N shows promise as an indicator of S nutrient status. Data available indicates that added S will increase yields of corn plants if their amide-N level exceeds 500 ppm. Amide-N concentration correlates with N/S ratios, and it is much more readily determined than the latter. Its usefulness as a S nutrition indicator should be checked under field conditions.     

Ribulose bisphosphate carboxylase, protein and nitrogen in Scots pine seedlings cultivated at different nutrient levels

Seedlings of Scots pine (Pinus sylvestris L.) of a northern provenance were cultivated in nutrient solution for 10 weeks in a climate chamber. The nutrient solution (renewed by solution exchange) contained 2.5, 10 or 50 mg N I^?1. All other essential elements were added in optimal proportion to the nitrogen. Seedlings cultivated at 10 and 50 mg N I^?1 were similar with respect to all characteristics studied. Seedlings cultivated at 2.5 mg N I^?1 showed a lower growth rate, especially for the shoot, and an altered morphology, with high root:shoot ratios and long, slender roots. The nitrogen concentrations in shoot and needles as well as in whole seedlings were not significantly affected by the nitrogen supply, while the nitrogen concentrations in the roots were somewhat lower at 2.5 mg N I^?1. Ribulose bisphosphate carboxylase (EC 4.1.1.39) activity and the concentrations of carboxylase, total and soluble protein and of chlorophyll in the needles were consistently much lower for seedlings cultivated at 2.5 mg N I^?1, than for seedlings grown at higher nutrient levels. A close correlation was observed between activity and concentration of the carboxylase (r=0.95). Carboxylase activity and protein were more sensitive to a low nutrient supply than was chlorophyll. The data show how activity and concentration of ribulose bisphosphate carboxylase and the concentrations of soluble and total protein and of chlorophyll in needles of pine seedlings can be negatively affected by the nutrient supply, also when the nitrogen concentrations in the needles are close to those observed at optimal nutrient supply. It is suggested that pine seedlings store assimilated non-protein nitrogen in the needles when protein synthesis is under restraint. The nitrogen concentration in needles and seedlings could not be used as a measure of the physiological state of the seedlings.     

Construction cost of loblolly and ponderosa pine leaves grown with varying carbon and nitrogen availability

We grew loblolly and ponderosa pine seedlings in a factorial experiment with two CO₂ partial pressures (35 and 70 Pa), and two nitrogen treatments (1.0 and 3.5 mol m^?3 NH₄⁺), for one growing season to examine the effects of carbon and nitrogen availability on leaf construction cost. Growth in elevated CO₂ reduced leaf nitrogen concentrations by 17 to 40%, and increased C:N by 22 to 68%. Elevated N availability increased leaf N concentrations and decreased C:N. Non-structural carbohydrates increased in high-CO₂-grown loblolly seedlings, except in fascicles from low N, and in ponderosa primary and fascicle leaves grown in high N. In loblolly, increases in starch were nearly 2-fold greater than the increases in soluble sugars. In ponderosa, only the soluble sugars were affected by CO₂. Leaf construction cost (g glucose g^?1 dm) varied by 9.3% across all treatments. All of the variation in loblolly leaf construction cost could be explained by changes in non-structural carbohydrates. A model of the response of construction cost to changes in the mass of different biochemical fractions suggests that the remainder of the variation in ponderosa, not explained by non-structural carbohydrates, is probably attributable to changes in lignin, phenolic or protein concentrations.     

Effects of nitrogen addition on nitrogen metabolism and carbon reserves in the temperate seagrass Posidonia oceanica

The effect of repeated N additions on a dense, shallow meadow of Posidonia oceanica (L.) Delile in the NW Mediterranean was studied over a year. N was added biweekly both to the sediment and to the water column as ammonium and nitrate. The most obvious result of these additions was an overall increase in N content (% DW) in all tissues of fertilized plants; this increase was maximum in rhizomes, with values of 5% N reached, which confirmed the storage capacities of these organs.Fertilization affected the different N fractions in distinct ways. The free amino acid (FAA) concentration increased the most, particularly in rhizomes and roots, suggesting the function of these compounds for N storage and, probably, translocation. The non-soluble N fraction also increased greatly. The total soluble protein (TSP) and the inorganic N forms concentrations were less sensitive to fertilization, and only increased moderately in a few cases. N assimilation, assessed through in vivo glutamine synthetase (GS) activity, was maximum in leaves after the peak of growth, which coincided with the lowest N values in both control and fertilized plants. Thus assimilation was probably greatest at the period of highest N deficiency. Growth rates did not respond to N enrichment. Another clear effect of N addition was to decrease carbon reserves. In effect, the concentration of total non-structural carbohydrate (TNC) greatly decreased in rhizomes of fertilized plants, coinciding with the increase in FAA. We conclude that increased nitrogen availability can affect plant survival through the decrease in their carbon reserves, crucial for P. oceanica overwintering. This interaction between N and C metabolism helps to explain changes in benthic vegetation after steadily increasing coastal water eutrophication.     

Signaling mechanisms integrating root and shoot responses to changes in the nitrogen supply

During their life cycle, plants must be able to adapt to wide variations in the supply of soil nitrogen (N). Changes in N availability, and in the relative concentrations of NO₃ ⁻and NH₄ ⁺, are known to have profound regulatory effects on the N uptake systems in the root, on C and N metabolism throughout the plant, and on root and shoot morphology. Optimising the plant’s responses to fluctuations in the N supply requires co-ordination of the pathways of C and N assimilation, as well as establishment of the appropriate allocation of resources between root and shoot growth. Achieving this integration of responses at the whole plant level implies long-distance signaling mechanisms that can communicate information about the current availability of N from root-to-shoot, and information about the C/N status of the shoot in the reverse direction. In this review we will discuss recent advances which have contributed to our understanding of these long-range signaling pathways.