A great diversity of plants and fungi engage in mycorrhizal associations. In natural habitats, and in an ecologically meaningful time span, these associations have evolved to improve the fitness of both plant and fungal symbionts. In systems managed by humans, mycorrhizal associations often improve plant productivity, but this is not always the case. Mycorrhizal fungi might be considered to be parasitic on plants when net cost of the symbiosis exceeds net benefits. Parasitism can be developmentally induced, environmentally induced, or possibly genotypically induced. Morphological, phenological, and physiological characteristics of the symbionts influence the functioning of mycorrhizas at an individual scale. Biotic and abiotic factors at the rhizosphere, community, and ecosystem scales further mediate mycorrhizal functioning. Despite the complexity of mycorrhizal associations, it might be possible to construct predictive models of mycorrhizal functioning. These models will need to incorporate variables and parameters that account for differences in plant responses to, and control of, mycorrhizal fungi, and differences in fungal effects on, and responses to, the plant. Developing and testing quantitative models of mycorrhizal functioning in the real world requires creative experimental manipulations and measurements. This work will be facilitated by recent advances in molecular and biochemical techniques. A greater understanding of how mycorrhizas function in complex natural systems is a prerequisite to managing them in agriculture, forestry, and restoration.

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable N isotopes, 15N and 14N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ15N] deviations from the standard atmospheric N2) depend on isotopic signatures of inputs and outputs, the input–output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization–N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors vary. For example, because nitrification discriminates against 15N in the substrate more than does N mineralization, NH4+ can become isotopically heavier than the organic N from which it is derived.

Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ15N of a specific N pool is not a constant, and δ15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring δ15N in small and dynamic pools of N in the soil–plant system, and the complexity of the N cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of 15N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ15N might offer the advantage of giving insights into the N cycle without disturbing the system by adding 15N tracer.

Such attempts require, however, that the complex factors affecting δ15N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NOx, NH3, N2-fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil-N used (organic N, NH4+, NO3−), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ15N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non-N2-fixing species should differ more than 5‰ from N derived by N2-fixation, and that several non-N2-fixing references are used, when data on δ15N are used to estimate N2-fixation in poorly described ecosystems.

As well as giving information on N source effects, δ15N can give insights into N cycle rates. For example, high levels of N deposition onto previously N-limited systems leads to increased nitrification, which produces 15N-enriched NH4+ and 15N-depleted NO3−. As many forest plants prefer NH4+ they become enriched in 15N in such circumstances. This change in plant δ15N will subsequently also occur in the soil surface horizon after litter-fall, and might be a useful indicator of N saturation, especially since there is usually an increase in δ15N with depth in soils of N-limited forests.

Generally, interpretation of 15N measurements requires additional independent data and modelling, and benefits from a controlled experimental setting. Modelling will be greatly assisted by the development of methods to measure the δ15N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level 15N studies will be necessary to further develop the interpretation of variations in δ15N in soil–plant systems. Another promising approach is to study ratios of 15N[ratio ]14N together with other pairs of stable isotopes, e.g. 13C[ratio ]12C or 18O[ratio ]16O, in the same ion or molecules. This approach can help to tackle the challenge of distinguishing isotopic source effects from fractionations within the system studied.

Summary 185

I. INTRODUCTION 186

II. EFFECT OF ZINC ON PRODUCTION OF REACTIVE OXYGEN SPECIES 186

1. Superoxide-generating NADPH oxidase 186

2. Zinc deficiency potentiates iron-mediated free radical production 189

(a) Iron accumulation in zinc-deficient plants 189

(b) Iron-induced production of free radicals 189

3. Zinc deficiency-enhanced photooxidation 191

(a) Decrease in photosynthesis 191

(b) Light-induced leaf chlorosis 192

(c) Decrease in indole-3-acetic acid 192

III. MEMBRANE DAMAGE BY REACTIVE OXYGEN SPECIES 193

1. Impairments in membrane structure 193

2. Phospholipids and –SH groups 195

3. Alterations in ion absorption 195

(a) Membrane-bound ATPases 195

(b) Nutrient uptake 197

(c) Changes in activity of ion channels 197

IV. DETOXIFICATION OF REACTIVE OXYGEN SPECIES 198

1. Superoxide dismutases 198

2. H2O2-scavenging enzymes 198

V. INVOLVEMENT OF ZINC IN PLANT STRESS TOLERANCE 199

VI. CONCLUSIONS 199

Acknowledgements 200

References 200

Zinc deficiency is one of the most widespread micronutrient deficiencies in plants and causes severe reductions in crop production. There are a number of physiological impairments in Zn-deficient cells causing inhibition of the growth, differentiation and development of plants. Increasing evidence indicates that oxidative damage to critical cell compounds resulting from attack by reactive O2 species (ROS) is the basis of disturbances in plant growth caused by Zn deficiency. Zinc interferes with membrane-bound NADPH oxidase producing ROS. In Zn-deficient plants the iron concentration increases, which potentiates the production of free radicals. The Zn nutritional status of plants influences photooxidative damage to chloroplasts, catalysed by ROS. Zinc-deficient leaves are highly light-sensitive, rapidly becoming chlorotic and necrotic when exposed to high light intensity. Zinc plays critical roles in the defence system of cells against ROS, and thus represents an excellent protective agent against the oxidation of several vital cell components such as membrane lipids and proteins, chlorophyll, SH-containing enzymes and DNA. The cysteine, histidine and glutamate or aspartate residues represent the most critical Zn- binding sites in enzymes, DNA-binding proteins (Zn-finger proteins) and membrane proteins. In addition, animal studies have shown that Zn is involved in inhibition of apoptosis (programmed cell death) which is preceded by DNA and membrane damage through reactions with ROS.

Root turnover is a critical component of ecosystem nutrient dynamics and carbon sequestration and is also an important sink for plant primary productivity. We tested global controls on root turnover across climatic gradients and for plant functional groups by using a database of 190 published studies. Root turnover rates increased exponentially with mean annual temperature for fine roots of grasslands (r2 = 0.48) and forests (r2 = 0.17) and for total root biomass in shrublands (r2 = 0.55). On the basis of the best-fit exponential model, the Q10 for root turnover was 1.4 for forest small diameter roots (5 mm or less), 1.6 for grassland fine roots, and 1.9 for shrublands. Surprisingly, after accounting for temperature, there was no such global relationship between precipitation and root turnover. The slowest average turnover rates were observed for entire tree root systems (10% annually), followed by 34% for shrubland total roots, 53% for grassland fine roots, 55% for wetland fine roots, and 56% for forest fine roots. Root turnover decreased from tropical to high-latitude systems for all plant functional groups. To test whether global relationships can be used to predict interannual variability in root turnover, we evaluated 14 yr of published root turnover data from a shortgrass steppe site in northeastern Colorado, USA. At this site there was no correlation between interannual variability in mean annual temperature and root turnover. Rather, turnover was positively correlated with the ratio of growing season precipitation and maximum monthly temperature (r2 = 0.61). We conclude that there are global patterns in rates of root turnover between plant groups and across climatic gradients but that these patterns cannot always be used for the successful prediction of the relationship of root turnover to climate change at a particular site.

Root turnover is important to the global carbon budget as well as to nutrient cycling in ecosystems and to the success of individual plants. Our ability to predict the effects of environmental change on root turnover is limited by the difficulty of measuring root dynamics, but emerging evidence suggests that roots, like leaves, possess suites of interrelated traits that are linked to their life span. In graminoids, high tissue density has been linked to increased root longevity. Other studies have found root longevity to be positively correlated with mycorrhizal colonization and negatively correlated with nitrogen concentration, root maintenance respiration and specific root length. Among fruit trees, apple roots (which are of relatively small diameter, low tissue density and have little lignification of the exodermis) have much shorter life spans than the roots of citrus, which have opposite traits. Likewise, within the branched network of the fine root system, the finest roots with no daughter roots tend to have higher N concentrations, faster maintenance respiration, higher specific root length and shorter life spans than secondary and tertiary roots that bear daughter roots. Mycorrhizal colonization can enhance root longevity by diverse mechanisms, including enhanced tolerance of drying soil and enhanced defence against root pathogens. Many variables involved in building roots might affect root longevity, including root diameter, tissue density, N concentration, mycorrhizal fungal colonization and accumulation of secondary phenolic compounds. These root traits are highly plastic and are strongly affected by resource supply (CO2, N, P and water). Therefore the response of root longevity to altered resource availability associated with climate change can be estimated by considering how changes in resource availability affect root construction and physiology. A cost–benefit approach to predicting root longevity assumes that a plant maintains a root only until the efficiency of resource acquisition is maximized. Using an efficiency model, we show that reduced tissue Nconcentration and reduced root maintenance respiration, both of which are predicted to result from elevated CO2, should lead to slightly longer root life spans. Complex interactions with soil biota and shifts in plant defences against root herbivory and parasitism, which are not included in the present efficiency model, might alter the effects of future climate change on root longevity in unpredicted ways.

I. INTRODUCTION 360

II. PHOTOINHIBITION AND ACTIVE OXYGEN 360

III. OXYGEN AS AN ELECTRON ACCEPTOR 362

1. Oxygen ‘poises’ electron transport and carbon assimilation 362

2. The role of oxygen in ATP synthesis 364

3. How fast is O2reduction at PSI? 364

4. Chloroplastic processing of H2O2 366

IV. REDOX REGULATION OF PHOTOSYNTHETIC METABOLISM 368

1. The thioredoxin system 368

2. Manipulating the expression of thiol-regulated enzymes 369

3. Modifying sensitivity to thiol regulation 369

V. PHOTORESPIRATION 369

1. The pathway and its genetic manipulation 369

2. Engineering plants that photorespire less? 371

3. Is photorespiration important in energy dissipation? 372

4. Production and processing of photorespiratory H2O2 373

5. Catalase and foliar H2O2levels 374

6. Catalase and non-photorespiratory H2O2generation 375

VI. RESPIRATION 376

1. ‘Photosynthetic’ respiration 376

2. AOS in the mitochondrion 376

3. AOX: regulation and significance to photosynthesis 377

VII. PHOTOSYNTHESIS AND REDOX SIGNAL TRANSDUCTION 378

1. The need for sensors, signals and transducers 378

2. Signal transduction at the local level 378

3. Remote signalling and responses leading to acclimation of photosynthesis? 379

4. Interactions between AOS, NO., and antioxidants 380

VIII. CONCLUSIONS 380

Acknowledgements 381

References 381

The gradual but huge increase in atmospheric O2 concentration that followed the evolution of oxygenic photosynthesis is one consequence that marks this event as one of the most significant in the earth's history. The high redox potential of the O2/water couple makes it an extremely powerful electron sink that enables energy to be transduced in respiration. In addition to the tetravalent interconversion of O2 and water, there exist a plethora of reactions that involve the partial reduction of O2 or photodynamic energy transfer to produce active oxygen species (AOS). All these redox reactions have become integrated during evolution into the aerobic photosynthetic cell. This review considers photosynthesis as a whole-cell process, in which O2 and AOS are involved in reactions at both photosystems, enzyme regulation in the chloroplast stroma, photorespiration, and mitochondrial electron transport in the light. In addition, oxidants and antioxidants are discussed as metabolic indicators of redox status, acting as sensors and signal molecules leading to acclimatory responses. Our aim throughout is to assess the insights gained from the application of mutagenesis and transformation techniques to studies of the role of O2 and related redox components in the integrated regulation of photosynthesis.

Dark septate root endophytes (DSE) are conidial or sterile fungi (Deuteromycotina, Fungi Imperfecti) likely to be ascomycetous and colonizing plant roots. They have been reported for nearly 600 plant species representing about 320 genera and 100 families. DSE fungi occur from the tropics to arctic and alpine habitats and comprise a heterogeneous group that functionally and ecologically overlaps with soil fungi, saprotrophic rhizoplane-inhabiting fungi, obligately and facultatively pathogenic fungi and mycorrhizal fungi. Numerous species of undescribed sterile and anamorphic taxa may also await discovery. Although DSE are abundant in washed root and soil samples from various habitats, and are easily isolated from surface-sterilized roots of ecto-, ectendo-, endo- and non-mycorrhizal host species, their ecological functions are little understood. Studies of DSE thus far have yielded inconsistent results and only poorly illustrate the role of DSE in their natural habitats. These inconsistencies are largely due to the uncertain taxonomic affinities of the strains of DSE used. In addition, because different strains of a single anamorph taxon seem to vary greatly in function, no clear generalizations on their ecological role have been drawn. This paper reviews the current literature on DSE and the ecology and discusses the need for and direction of future research.

This study explored reflectance indices sampled with a ‘leaf reflectometer’ as measures of pigment content for leaves of contrasting light history, developmental stage and functional type (herbaceous annual versus sclerophyllous evergreen). We employed three reflectance indices: a modified normalized difference vegetation index (NDVI), an index of chlorophyll content; the red/green reflectance ratio (RRED[ratio ]RGREEN), an index of anthocyanin content; and the change in photochemical reflectance index upon dark–light conversions (ΔPRI), an index of xanthophyll cycle pigment activity. In Helianthus annuus (sunflower), xanthophyll cycle pigment amounts were linearly related to growth light environment; leaves in full sun contained approximately twice the amount of xanthophyll cycle pigments as leaves in deep shade, and at midday a larger proportion of these pigments were in the photoprotective, de-epoxidized forms relative to shade leaves. Reflectance indices also revealed contrasting patterns of pigment development in leaves of contrasting structural types (annual versus evergreen). In H. annuus sun leaves, there was a remarkably rapid increase in amounts of both chlorophyll and xanthophyll cycle pigments along a leaf developmental sequence. This pattern contrasted with that of Quercus agrifolia (coast live oak, a sclerophyllous evergreen), which exhibited a gradual development of both chlorophyll and xanthophyll cycle pigments along with a pronounced peak of anthocyanin pigment content in newly expanding leaves. These temporal patterns of pigment development in Q. agrifolia leaves suggest that anthocyanins and xanthophyll cycle pigments serve complementary photoprotective roles during early leaf development. The results illustrate the use of reflectance indices for distinguishing divergent patterns of pigment activity in leaves of contrasting light history and functional type.

A key element of most recently proposed plant strategy schemes is an axis of resource capture, usage and availability. In the search for a simple, robust plant trait (or traits) that will allow plants to be located on this axis, specific leaf area is one of the leading contenders. Using a large new unpublished database, we examine the variability of specific leaf area and other leaf traits, the relationships between them, and their ability to predict position on the resource use axis. Specific leaf area is found to suffer from a number of drawbacks; it is both very variable between replicates and much influenced by leaf thickness. Leaf dry-matter content (sometimes referred to as tissue density) is much less variable, largely independent of leaf thickness and a better predictor of location on an axis of resource capture, usage and availability. However, it is not clear how useful dry matter content will be outside northwest Europe, and in particular in dry climates with many succulents.

Forests exchange large amounts of CO2 with the atmosphere and can influence and be influenced by atmospheric CO2. There has been a recent proliferation of literature on the effects of atmospheric CO2 on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long-term focus and involving CO2×stress or environment interactions. The recent data on long-term effects of elevated atmospheric CO2 on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long-term datasets currently available are for juvenile trees.

The current literature indicates a significantly larger average long-term biomass increment under elevated CO2 for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO2 in long-term studies was similar for conifers (62%) and deciduous trees (53%). Recent studies indicate that elevated CO2 causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age-dependent and size-dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO2. In addition, there are many possible avenues to down-regulation, making the predicted canopy CO2 exchange and growth of mature trees and forests in a CO2-rich atmosphere uncertain. Although, physiological down-regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO2. A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands.

Resource interactions can both constrain tree responses to elevated CO2 and be altered by them. Although drought can reduce gas-exchange rates and offset the benefits of elevated CO2, even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO2 than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO2. We conclude that positive effects of CO2 on leaf area can be at least as important in determining canopy transpiration as negative, direct effects of CO2 on stomatal aperture. With respect to nutrition, elevated CO2 has the potential to alter tree–soil interactions that might influence future changes in ecosystem productivity. There is continued evidence that in most cases nutrient limitations diminish growth and photosynthetic responses to elevated CO2 at least to some degree, and that elevated CO2 can accelerate the appearance of nutrient limitations with increasing time of treatment. In many studies, tree biomass responses to CO2 are artefacts in the sense that they are merely responses to CO2-induced changes in internal nutritional status of the tree.

There are numerous interactions between CO2 and factors of the biotic and abiotic environment. The importance of increasing atmospheric CO2 concentrations for productivity is likely to be overestimated if these are not taken into account. Many interactions, however, are simply additive rather than synergistic or antagonistic. This appears to hold true for many parameters under elevated CO2 in combination with temperature, elevated O3, and other atmospheric pollutants. However, there is currently little evidence that elevated CO2 will counteract O3 damage. When the foliage content of C, mineral nutrients and secondary metabolites is altered by elevated CO2, tree×insect interactions are modified. In most trees, mycorrhizal interactions might be less important for direct effects of CO2 than for alleviating general nutrient deficiencies.

Since many responses to elevated CO2 and their interactions with stress show considerable variability among species/genotypes, one principal research need is for comparative studies of a large variety of woody species and ecosystems under realistic conditions. We still need more long-term experiments on mature trees and stands to address critical scaling issues likely to advance our understanding of responses to elevated CO2 at different stages of forest development and their interactions with climate and environment. The only tools available at present for coping with the consequences of rising CO2 are management of resources and selection of genotypes suitable for the future climate and environment.

The relationships of foliage assimilation capacity per unit area (PPmax) with leaf dry mass per unit area (LMA) and nitrogen content per unit area (NP) differ between species and within species grown in different habitats. To gain a more mechanistic insight into the dependencies of PPmax on LMA and NP, this literature study based on 597 species from a wide range of earth biomes with woody vegetation examines the relations between leaf photosynthetic capacity and the components of LMA (leaf density (D, dry mass per volume) and thickness (T)), and also the correlations of D and T with leaf nitrogen content and fractional leaf volumes in different tissues. Across all species, PPmax varied 12-fold and photosynthetic capacity per unit dry mass (Pmmax) 16-fold, NP 12-fold, and nitrogen per unit dry mass (Nm) 13-fold, LMA 46-fold, D 13-fold, and T 35-fold, indicating that foliar morphology was more plastic than foliar chemistry and assimilation rates. Although there were strong positive correlations between PPmax and NP, and between Pmmax and Nm, leaf structure was a more important determinant of leaf assimilation capacities. PPmax increased with increasing LMA and T, but was independent of D. By contrast, Pmmax scaled negatively with LMA because of a negative correlation between Pmmax and D, and was poorly related to T. Analysis of leaf nitrogen and tissue composition data indicated that the negative relationship between D and Pmmax resulted from negative correlations between D and Nm, D and volumetric fraction of leaf internal air space, and D and symplasmic leaf fraction. Thus, increases in leaf density bring about (1) decreases in assimilative leaf compounds, and (2) extensive modifications in leaf anatomy that may result in increases in intercellular transfer resistance to CO2. Collectively, (1) and (2) lead to decreased Pmmax, and also modify PPmax versus LMA relationships.

Summary 599

I. Introduction 599

II. Concepts and terminology 600

III. Historical background 600

IV. Studies of experimental hybrids 601

1. Isolating mechanisms 601

2. Prezygotic barriers 602

(a) Gametic barriers to hybridization 602

3. Postzygotic barriers 603

(a) Chromosomal rearrangements 604

(b) Genic sterility or inviability 604

4. Hybrid vigour 605

5. Introgression 606

6. Hybrid speciation 607

V. Experimental manipulations of natural hybrid populations 609

1. Hybrid-zone formation 610

2. Pollinator-mediated selection 610

3. Habitat selection 612

VI. The biology of different classes of hybrids 612

1. Character expression 613

(a) Morphological characters 613

(b) Chemical characters 613

(c) Molecular characters 613

2. The fitness of different classes of hybrids 614

(a) The importance of variance 614

(b) Estimating hybrid fitness 615

3. Interactions with parasites and herbivores 616

4. Patterns of mating 617

(a) Outcrossing rate 617

(b) Hybridization frequency 618

(c) Mate choice 618

VII. Conclusions and future research 619

Acknowledgements 620

References 620

Most studies of plant hybridization are concerned with documenting its occurrence in different plant groups. Although these descriptive, historical studies are important, the majority of recent advances in our understanding of the process of hybridization are derived from a growing body of experimental microevolutionary studies. Analyses of artificially synthesized hybrids in the laboratory or glasshouse have demonstrated the importance of gametic selection as a prezygotic isolating barrier; the complex genetic basis of hybrid sterility, inviability and breakdown; and the critical role of fertility selection in hybrid speciation. Experimental manipulations of natural hybrid zones have provided critical information that cannot be obtained in the glasshouse, such as the evolutionary conditions under which hybrid zones are formed and the effects of habitat and pollinator-mediated selection on hybrid-zone structure and dynamics. Experimental studies also have contributed to a better understanding of the biology of different classes of hybrids. Analyses of morphological character expression, for example, have revealed transgressive segregation in the majority of later-generation hybrids. Other studies have documented a high degree of variability in fitness among different hybrid genotypes and the rapid response of such fitness to selection – evidence that hybridization need not be an evolutionary dead end. However, a full accounting of the role of hybridization in adaptive evolution and speciation will probably require the integration of experimental and historical approaches.

SUMMARY 367

I. INTRODUCTION 367

II. NUMBER 368

III. SIZE 379

IV. AIR SPACE IN THE SEEDS 381

V. FLOATATION AND DISPERSAL 383

1. Air 383

(a) Physical considerations 383

(b) Dispersal 387

(c) Birds 415

2. Water 416

(a) Physical considerations 416

(b) Dispersal 416

VI. CONCLUSIONS 417

Acknowledgements 417

References 418

Orchid seeds are very small, extremely light and produced in great numbers. Most range in length from c. 0.05 to 6.0 mm, with the difference between the longest and shortest known seeds in the family being 120-fold. The ‘widest’ seed at 0.9 mm is 90-fold wider than the ‘thinnest’ one, which measures 0.01 mm (because orchid seeds are tubular or balloon-like, ‘wide’ and ‘thin’ actually refer to diameter). Known seed weights extend from 0.31 lg to 24 μg (a 78-fold difference). Recorded numbers of seeds per fruit are as high as 4000000 and as low as 20–50 (80000–200000-fold difference). Testae are usually transparent, with outer cell walls that may be smooth or reticulated. Ultrasonic treatments enhance germination, which suggests that the testae can be restrictive. Embryos are even smaller: their volume is substantially smaller than that of the testa. As a result, orchid seeds have large internal air spaces that render them balloon-like. They can float in the air for long periods, a property that facilitates long-distance dispersal. The difficult-to-wet outer surfaces of the testa and large internal air spaces enable the seeds to float on water for prolonged periods. This facilitates distribution through tree effluates and/or small run-off rivulets that may follow rains. Due to their size and characteristics, orchid seeds may also be transported in and on land animals and birds (in fur, feathers or hair, mud on feet, and perhaps also following ingestion).

The global emission of ammonia (NH3) is about 54 Mt N. The major global sources are excreta from domestic animals and fertilizers, but oceans, biomass burning and crops are also important. About 60% of the global NH3 emission is estimated to come from anthropogenic sources. NH3-N emissions are of the same order as the NOx-N emissions on both global and European scales. Emitted NH3 returns to the surface mainly in the form of dry deposition of NH3 and wet deposition of ammonium (NH4+). In countries with high NH3 emission densities, dry deposition of NH3 from local sources and wet deposition of NH4+ from remote sources dominate the deposition. In countries with low NH3 emission densities only wet deposition of NH4+ from remote sources dominates the deposition. Surface exchange of NH3 is essentially bi-directional, depending on the NH3 compensation point concentration of the vegetation and the airborne concentration. In general, the compensation point is larger for agricultural than semi-natural plants, and varies with plant growth stage. According to basic thermodynamics the leaf tissue or stomatal compensation point of NH3 doubles for each increase of 5°C. However, exchange of NH3 does not only occur through the stomata, but it can also be deposited to leaf surfaces, as well as emitted back to the atmosphere from drying leaf surfaces. Atmospheric transport and deposition models can be used to interpolate NH3 concentrations and depositions in space and time, to calculate import/export balances and to estimate past or future situations. Adverse effects on sensitive ecosystems caused by high N deposition can be reduced by lowering the emissions and, to a limited extent, also by removing sources close to the ecosystem to be protected.

There is considerable uncertainty about how rates of soil carbon (C) and nitrogen (N) cycling will change as CO2 accumulates in the Earth's atmosphere. We summarized data from 47 published reports on soil C and N cycling under elevated CO2 in an attempt to generalize whether rates will increase, decrease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization, because these pools and processes represent important control points for the below-ground flow of C and N. To determine whether differences in C allocation between plant life forms influence soil C and N cycling in a predictable manner, we summarized responses beneath graminoid, herbaceous and woody plants grown under ambient and elevated atmospheric CO2. The below-ground pools and processes that we summarized are characterized by a high degree of variability (coefficient of variation 80–800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration were more rapid under elevated CO2, indicating that (1) greater plant growth under elevated CO2 enhanced the amount of C entering the soil, and (2) additional substrate was being metabolized by soil microorganisms. However, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elevated CO2, contributing to a high degree of variability within and between plant life forms. From this analysis we conclude that there are insufficient data to predict how microbial activity and rates of soil C and N cycling will change as the atmospheric CO2 concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our ability to predict the response of soil microorganisms to rising atmospheric CO2, and that understanding differences in fine-root longevity and biochemistry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO2.

Soil temperature can influence the functioning of roots in many ways. If soil moisture and nutrient availability are adequate, rates of root length extension and root mortality increase with increasing soil temperature, at least up to an optimal temperature for root growth, which seems to vary among taxa. Root growth and root mortality are highly seasonal in perennial plants, with a flush of growth in spring and significant mortality in the fall. At present we do not understand whether root growth phenology responds to the same temperature cues that are known to control shoot growth. We also do not understand whether the flush of root growth in the spring depends on the utilization of stored nonstructural carbohydrates, or if it is fueled by current photosynthate. Root respiration increases exponentially with temperature, but Q10 values range widely from c. 1.5 to > 3.0. Significant questions yet to be resolved are: whether rates of root respiration acclimate to soil temperature, and what mechanisms control acclimation if it occurs. Limited data suggest that fine roots depend heavily on the import of new carbon (C) from the canopy during the growing season. We hypothesize that root growth and root respiration are tightly linked to whole-canopy assimilation through complex source–sink relationships within the plant. Our understanding of how the whole plant responds to dynamic changes in soil temperature, moisture and nutrient availability is poor, even though it is well known that multiple growth-limiting resources change simultaneously through time during a typical growing season. We review the interactions between soil temperature and other growth-limiting factors to illustrate how simple generalizations about temperature and root functioning can be misleading.

There is some evidence that traits of fresh leaves that provide structural or chemical protection (‘defence’) remain operational in the leaf litter and control interspecific variation in decomposition rate in or on the soil. We tested experimentally whether the negative relationship between foliar defence and litter decomposition rate is fundamental, i.e. whether it is seen consistently across higher plant species and life forms, and whether it is repeated in the floras of geographically and climatically distinct areas separated by an ocean. We employed the published results of two outdoor litter bag experiments, in which we simultaneously compared the relative mass losses (‘decomposibility’) of leaf litters of a wide range of plant species. One experiment was in Córdoba, Argentina, and included 48 Argentine species typical of the dry, subtropical landscapes along a steep altitudinal gradient. The other was in Sheffield, UK, and hosted 72 British species typical of the temperate–Atlantic landscape there. We linked the two experiments through a similar experiment in Sheffield that hosted litters of subsets of both the Argentine and British species. We also tested fresh leaves of all species from the same areas for tensile strength (‘toughness’) and relative palatability to generalist herbivorous snails in multi-species ‘cafeteria’ experiments. Both in Argentina and in Great Britain there were highly significant correlations between leaf palatability (r=0.61; 0.73) or leaf tensile strength (r=−0.60; −0.60) and litter mass loss across all species. These relationships could be explained by variation both between and within broad life-form groups. Specific leaf area (area[ratio ]dry mass) of fresh leaves was consistently correlated only with litter mass loss within British life form groups. We illustrated the possible ecosystem consequences of these relationships by comparing functional traits of British species differing in leaf habit. In comparison with deciduous species, evergreens generally had innately slow growth, which corresponded to their longer-lived leaves of lower specific leaf area, higher tensile strength and lower palatability to generalist invertebrate herbivores. Correspondingly, evergreens produced more resistant leaf litter. Thus, slow-growing evergreens might maintain their position in infertile ecosystems through leaf traits that help them to conserve their nutrients efficiently and to keep nutrient mineralization low, thereby not allowing potentially fast-growing deciduous species to outcompete them.

Global warming over the next century is likely to be associated with a change in the extent to which atmospheric and soil temperatures fluctuate, on both a daily and a seasonal basis. The average annual temperature of the Earth's surface is expected to increase, as is the frequency of hot days. In this review, we explore what effects short-term and long-term changes in temperature are likely to have on root respiratory metabolism, and what impacts such changes will have on daily, seasonal and annual CO2 release by roots under field conditions. We demonstrate that Q10 values, and the degree of acclimation, differ between and within plant species. Changes in the temperature sensitivity of respiration with measuring temperature are highlighted. Temperature-dependent changes in adenylate control and substrate supply are likely to control the Q10 and degree of acclimation of root respiration. Limitations in respiration capacity are unlikely to control respiratory flux at most temperatures. The potential role of nonphosphorylating pathways such as the alternative oxidase in controlling Q10 values is highlighted. The possibility that potentially rapid changes in adenylate control might underlie the acclimation response (rather than slow changes in enzyme capacity) has implications for the total amount of CO2 respired by roots daily and annually. Our modelling suggests that rapid acclimation will result in near-perfect homeostasis of respiration rates and minimize annual CO2 release. However, annual CO2 release increases substantially if the speed of full acclimation is lower. Our modelling exercise also shows that high Q10 values have the potential to increase daily and annual CO2 release substantially, particularly if the frequency of hot days increases after global warming.

In this review, we discuss the potential for mycorrhizal fungi to act as a source or sink for carbon (C) under elevated CO2 and nitrogen deposition. Mycorrhizal tissue has been estimated to comprise a significant fraction of soil organic matter and below-ground biomass in a range of systems. The current body of literature indicates that in many systems exposed to elevated CO2, mycorrhizal fungi might sequester increased amounts of C in living, dead and residual hyphal biomass in the soil. Through this process, the fungi might serve as a negative feedback on the rise in atmospheric CO2 levels caused by fossil fuel burning and deforestation. By contrast, a few preliminary studies suggest that N deposition might increase turnover rates of fungal tissue and negate CO2 effects on hyphal biomass. If these latter responses are consistent among ecosystems, C storage in hyphae might decline in habitats surrounding agricultural and urban areas. When N additions occur without CO2 enrichment, effects on mycorrhizal growth are inconsistent. We note that analyses of hyphal decomposition under elevated CO2 and N additions are extremely sparse but are critical in our understanding of the impact of global change on the cycling of mycorrhizal C. Finally, shifts in the community composition of arbuscular and ectomycorrhizal fungi with increasing CO2 or N availability are frequently documented. Since mycorrhizal groups vary in growth rate and tissue quality, these changes in species assemblages could produce unforeseeable impacts on the productivity, survivorship, or decomposition of mycorrhizal biomass.

Temperate forests are recipients of anthropogenic nitrogen (N) deposition. Because growth in these ecosystems is often limited by N availability, elevated N inputs from the atmosphere can influence above- and belowground production in forests. Although fine-root production is the largest component of belowground production in forests, it is unclear whether or how increases in Navailability to forest trees accompanying increased N deposition might influence fine-root growth. Uncertainties as to how fine-root dynamics (i.e. production and turnover) vary in relation to soil N availability contribute to this problem. Although fine-root biomass typically decreases along soil N availability gradients in forests, it is unclear whether fine-root production and turnover also decrease along these gradients. Here, four possible relationships between fine-root turnover, fine-root production, and forest soil N availability are evaluated to develop a general hypothesis about changes in rooting dynamics that might accompany increases in N deposition. The four possible relationships are as follows. (1) Fine-root turnover rates do not systematically change with N availability in forest soils. If this is true, then fine-root production rates decrease with fine-root biomass in relation to soil N availability, and increased N deposition could lead to decreased fine-root production in forests. (2) Decreases in photosynthate allocation belowground along N availability gradients will function to slow fine-root turnover (or increase life span) as N availability increases with N deposition, thereby dramatically decreasing fine-root production. (3) Fine-root production might increase with N availability even though fine-root biomass typically decreases with N availability. This could occur if fine-root metabolism and turnover increase (life span decreases) with soil N supply. Increases in fine-root production accompanying increases in N availability, if large enough, could result in constant proportions of forest production being allocated to fine roots as soil N availability increases with N deposition. (4) Although fine-root turnover and production might both increase as N becomes more available to tree roots, the proportional allocation of total primary production to fine roots could decrease. Identifying the most likely of these four possibilities requires intersite comparisons of forest root dynamics along gradients of soil N availability and N deposition. Collective results of studies that use sequential sampling of fine-root biomass to estimate production suggest that fine-root turnover and production either; do not vary systematically, or that they decrease as N availability increases. By contrast, studies using ecosystem C or N budgets suggest that fine-root turnover and production both increase with N availability and that similar increases might be expected with elevated N deposition. It is argued here that assumptions underlying most biomass-based estimates of fine-root production are more suspect than are assumptions underlying element budget-based estimates. If so, it is likely that N deposition will function to decrease forest fine-root biomass but to stimulate fine-root turnover and production. However, increases in fine- root turnover and production could eventually decrease if chronically elevated N deposition leads to forest stand mortality.