I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

Na+ Tolerance and Na+ Transport in Higher Plants

Publisher Summary In this chapter, the advances that have been made in understanding the ecology of the mineral nutrition of wild plants from terrestrial ecosystems have been reviewed. This chapter is organized along three lines. First, the issues of nutrient-limited plant growth and nutrient uptake, with special emphasis on the importance of the uptake of nutrients in organic form—both by mycorrhizal and by non-mycorrhizal plants—and the importance of symbiotic nitrogen fixation is treated. In addition, the influence of allocation patterns on mineral nutrient uptake is described. Next, a few of the nutritional aspects of leaf functioning and how nutrients are used for biomass production by the plant are explored. That is done by studying the nutrient use efficiency (NUE) of plants and the various components of NUE. Finally, the feedback of plant species to soil nutrient availability by reviewing patterns in litter decomposition and nutrient mineralization is investigated. The chapter concludes with a synthesis of the various aspects of the mineral nutrition of wild plants. The chapter ends with a conceptual description of plant strategies with respect to mineral nutrition.

The Mineral Nutrition of Wild Plants Revisited: A Re-evaluation of Processes and Patterns

Atmospheric nitrogen (N) deposition is a recognized threat to plant diversity in temperate and northern parts of Europe and North America. This paper assesses evidence from field experiments for N deposition effects and thresholds for terrestrial plant diversity protection across a latitudinal range of main categories of ecosystems, from arctic and boreal systems to tropical forests. Current thinking on the mechanisms of N deposition effects on plant diversity, the global distribution of G200 ecoregions, and current and future (2030) estimates of atmospheric N-deposition rates are then used to identify the risks to plant diversity in all major ecosystem types now and in the future. This synthesis paper clearly shows that N accumulation is the main driver of changes to species composition across the whole range of different ecosystem types by driving the competitive interactions that lead to composition change and/or making conditions unfavorable for some species. Other effects such as direct toxicity of nitrogen gases and aerosols, long-term negative effects of increased ammonium and ammonia availability, soil-mediated effects of acidification, and secondary stress and disturbance are more ecosystem- and site-specific and often play a supporting role. N deposition effects in mediterranean ecosystems have now been identified, leading to a first estimate of an effect threshold. Importantly, ecosystems thought of as not N limited, such as tropical and subtropical systems, may be more vulnerable in the regeneration phase, in situations where heterogeneity in N availability is reduced by atmospheric N deposition, on sandy soils, or in montane areas. Critical loads are effect thresholds for N deposition, and the critical load concept has helped European governments make progress toward reducing N loads on sensitive ecosystems. More needs to be done in Europe and North America, especially for the more sensitive ecosystem types, including several ecosystems of high conservation importance. The results of this assessment show that the vulnerable regions outside Europe and North America which have not received enough attention are ecoregions in eastern and southern Asia (China, India), an important part of the mediterranean ecoregion (California, southern Europe), and in the coming decades several subtropical and tropical parts of Latin America and Africa. Reductions in plant diversity by increased atmospheric N deposition may be more widespread than first thought, and more targeted studies are required in low background areas, especially in the G200 ecoregions.

/pdf/global-assessment-of-nitrogen-deposition-effects-on-2c2d4sn04i.pdf

Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis.

Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mM NaCl or more, constitute about 1% of the worlds flora. Some halophytes show optimal growth in saline conditions; others grow optimally in the absence of salt. However, the tolerance of all halophytes to salinity relies on controlled uptake and compartmentalization of Na+, K+ and Cl- and the synthesis of organic compatible solutes, even where salt glands are operative. Although there is evidence that different species may utilize different transporters in their accumulation of Na+, in general little is known of the proteins and regulatory networks involved. Consequently, it is not yet possible to assign molecular mechanisms to apparent differences in rates of Na+ and Cl- uptake, in root-to-shoot transport (xylem loading and retrieval), or in net selectivity for K+ over Na+. At the cellular level, H+-ATPases in the plasma membrane and tonoplast, as well as the tonoplast H+-PPiase, provide the transmembrane proton motive force used by various secondary transporters. The widespread occurrence, taxonomically, of halophytes and the general paucity of information on the molecular regulation of tolerance mechanisms persuade us that research should be concentrated on a number of model species that are representative of the various mechanisms that might be involved in tolerance.

Salinity tolerance in halophytes

http://utsc.utoronto.ca/%7Ebritto/publications/amtox.pdf

NH4+ toxicity in higher plants: a critical review

Most higher plants develop severe toxicity symptoms when grown on ammonium (NH) as the sole nitrogen source. Recently, NH toxicity has been implicated as a cause of forest decline and even species extinction. Although mechanisms underlying NH toxicity have been extensively sought, the primary events conferring it at the cellular level are not understood. Using a high-precision positron tracing technique, we here present a cell-physiological characterization of NH acquisition in two major cereals, barley (Hordeum vulgare), known to be susceptible to toxicity, and rice (Oryza sativa), known for its exceptional tolerance to even high levels of NH. We show that, at high external NH concentration ([NH]o), barley root cells experience a breakdown in the regulation of NH influx, leading to the accumulation of excessive amounts of NH in the cytosol. Measurements of NH efflux, combined with a thermodynamic analysis of the transmembrane electrochemical potential for NH, reveal that, at elevated [NH]o, barley cells engage a high-capacity NH-efflux system that supports outward NH fluxes against a sizable gradient. Ammonium efflux is shown to constitute as much as 80% of primary influx, resulting in a never-before-documented futile cycling of nitrogen across the plasma membrane of root cells. This futile cycling carries a high energetic cost (we record a 40% increase in root respiration) that is independent of N metabolism and is accompanied by a decline in growth. In rice, by contrast, a cellular defense strategy has evolved that is characterized by an energetically neutral, near-Nernstian, equilibration of NH at high [NH]o. Thus our study has characterized the primary events in NH nutrition at the cellular level that may constitute the fundamental cause of NH toxicity in plants.

https://www.pnas.org/content/pnas/98/7/4255.full.pdf

Futile transmembrane NH4(+) cycling: a cellular hypothesis to explain ammonium toxicity in plants.

THE high incidence of failure when late-successional conifer species are replanted on disturbed forest sites is a considerable problem1–3. Here we advance a hypothesis that might explain many of these reforestation problems on a physiological basis, within the framework of forest succession. It is known that the chemical speciation of inorganic nitrogen in forest soils changes from predominantly ammonium (NH+4) in late-successional (mature forest) soils to mostly nitrate (NO–3) after disturbances such as clearcut harvesting2–6. The capacity of plant roots to take up and use these two sources of nitrogen is therefore very important for species establishment on successionally different sites. We have used kinetic and compartmental-analysis techniques with the radiotracer 13N to compare the efficiency of nitrogen acquisition from NH+4 and NO–3 sources in seedlings of white spruce, an important late-successional conifer. We found that uptake of NH+4 was up to 20 times greater than that of NO–3 from equimolar solution, cytoplasmic concentration of NH+4 was up to 10 times greater than that of NO–3, and physiological processing of NO–3 was much less than that of NH+4. This reduced capacity to use NO–3 is thought to present a critical impediment to seedling establishment on disturbed sites, where species better adapted to NO-3 would have a significant competitive advantage.

Conifer root discrimination against soil nitrate and the ecology of forest succession

Inorganic nitrogen concentrations in soil solutions vary across several orders of magnitude among different soils and as a result of seasonal changes In order to respond to this heterogeneity, plants have evolved mechanisms to regulate and influx In addition, efflux analysis using (13)N has revealed that there is a co-ordinated regulation of all component fluxes within the root, including biochemical fluxes Physiological studies have demonstrated the presence of two high-affinity transporter systems (HATS) for and one HATS for in roots of higher plants By contrast, in Arabidopsis thaliana there exist seven members of the NRT2 family encoding putative HATS for and five members of the AMT1 family encoding putative HATS for The induction of high-affinity transport and Nrt21 and Nrt22 expression occur in response to the provision of, while down-regulation of these genes appear to be due to the effects of glutamine High-affinity transport and AMT11 expression also appear to be subject to down-regulation by glutamine In addition, there is evidence that accumulated and may act post-transcriptionally on transporter function The present challenge is to resolve the functions of all of these genes In Aspergillus nidulans and Chlamydomonas reinhardtii there are but two high-affinity transporters and these appear to have undergone kinetic differentiation that permits a greater efficiency of absorption over the wide range of concentration normally found in nature Such kinetic differentiation may also have occurred among higher plant transporters The characterization of transporter function in higher plants is currently being inferred from patterns of gene expression in roots and shoots, as well as through studies of heterologous expression systems and knockout mutants

/pdf/the-regulation-of-nitrate-and-ammonium-transport-systems-in-15fuqjepe9.pdf

The regulation of nitrate and ammonium transport systems in plants

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

/pdf/sodium-transport-in-plants-a-critical-review-tbnfms6iaw.pdf

Herbert J. Kronzucker

Papers

NH4+ toxicity in higher plants: a critical review

Futile transmembrane NH4(+) cycling: a cellular hypothesis to explain ammonium toxicity in plants.

Conifer root discrimination against soil nitrate and the ecology of forest succession

The regulation of nitrate and ammonium transport systems in plants

Sodium transport in plants: a critical review