The causes and consequences of biodiversity are central themes in ecology. Perhaps one reason for much of the current interest in biodiversity is the belief that the loss of species (by extinction) or their gain (by invasion) will significantly influence ecosystem function. Arbuscular mycorrhizal (AM) fungi are components of most terrestrial ecosystems and, while many research programs have shown that variability among species or isolates of AM fungi does occur (Giovannetti & Gianinazzi-Pearson, 1994), the basis for this variability and its consequences to the function of communities and ecosystems remains largely unexplored. Smith et al. (pp. 357–366 in this issue) now show clearly that ecologically significant functional diversity exists among AM fungal species in the regions of the soil from which they absorb phosphate, and their results suggest that such diversity may have significant ecological consequences.

It is becoming increasingly apparent that the long-distance signalling associated with many developmental processes is complex and that novel hormone-like signals may play substantial roles. The past decades have seen several substances (e.g. brassinosteroids, systemin and other polypeptides, mevalonic and jasmonic acids, polyamines, oligosaccharides, flavonoids, and quinones) vie for a place among the classical plant hormones (e.g. Spaink, 1996). Recent microinjection and grafting studies have also shown that RNA may act as a long-distance signal (Jorgensen et al., 1998; Xoconostle-Cázares et al., 1999). In this issue, Hannah et al. describe long-distance signalling and the regulation of root–shoot partitioning in dwarf lethal or dosage-dependent lethal (DL) mutants of common bean (Shii et al., 1980, 1981), and present evidence indicating that substances in addition to classical plant hormones (e.g. cytokinins) may be involved.

As in the report by Hannah et al., much of the evidence for roles of unidentified long-distance signals in the control of plant development is indirect. The possibility that a small number of long-distance signals might control a multitude of developmental processes arises through the potential for differences in tissue sensitivity, fluctuations in hormone levels and differences in the nature of responses of different tissues to the same hormone. Consequently, particular hormones may influence numerous processes seemingly simultaneously, yet independently. Even so, long-distance signalling is involved in processes as diverse as root–shoot balance, senescence, branching, flowering, nodulation, stress responses and nutrient uptake. Through comparison of even a few different developmental processes, progress can be made to reveal the true complexity of plant development. Using this approach it is also clear that many unknown signals may be involved.

This Special Issue of New Phytologist contains the latest information and new ideas about how root dynamics might alter in the face of a globally changing environment. The importance of this topic is clear: changes in the production and turnover of roots in forests and grasslands in response to rising atmospheric CO2 concentrations, elevated temperatures, altered precipitation, or nitrogen deposition could be a key link between plant responses and longer-term changes in soil organic matter and ecosystem carbon balance.

The introductory review (Norby & Jackson, 2000), which draws together the different contributions to the volume, asks three central questions:

[bull ] Do elevated atmospheric CO2, nitrogen deposition, and climatic change alter the dynamics of root production and mortality?

[bull ] How do physiological responses of roots to global change factors impact whole-plant and ecosystem metabolism?

[bull ] What are the implications of root dynamics for soil microbial communities and the fate of carbon in soil?

Ecosystem-level observations of root production and mortality in response to global change factors are just starting to emerge. The challenge to root biologists is to overcome the profound methodological and analytical problems and assemble a more comprehensive data set from which ecosystem responses can be explained. The commissioned reviews and research papers in this volume attempt to meet that challenge. Following the introductory review, three papers provide a framework for subsequent analyses by presenting a global perspective on root turnover, a review of morphological and physiological attributes of roots, and a discussion of concepts of carbon allocation in plants. This is followed by a series of papers describing experimental studies on the effects of elevated CO2 and climatic change in various ecosystems. Three papers consider the physiological responses of roots to global change factors, followed by three papers reviewing mycorrhizal interactions and soil biology, and the implications for carbon sequestration in soil. The final paper returns to a global perspective with an analysis of how roots are handled in models of global change. Throughout these articles there is information on topics such as methodology for studying root dynamics, the major gaps in our knowledge, and the idea that leaves are a good analogy for roots.

Many human activities, such as ore mining and smeltering, sewage sludge treatment and fossil fuel consumption, result in toxic soil concentrations of ‘heavy metals’ (Al, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Ti, Zn and others) (Gadd, 1993). There are also natural soils, such as serpentine, with levels of heavy metals that inhibit or preclude the growth of many plants and soil micro-organisms. However, certain plants and microorganisms do grow in these metalliferous sites. Understanding the physiology, ecology and evolution of tolerance to elevated soil metal concentrations is important in an applied setting, and is also of interest in theoretical biology. Applied importance relates to the improvement of forest health in areas subject to increasing pollution, rehabilitation of severely polluted sites by phytostabilization of metals, and metal removal using hyperaccumulating plants (Krämer, 2000; Ernst, 2000). Areas of theoretical interest include the evolution of local adaptation (Sork et al., 1993) and how it is shaped by the combined influences of natural selection, gene flow and genetic architecture, as well as metal influences on various species interactions (Pollard, 2000). A paper appears on pages 367–379 in this issue by Jan Colpaert and coworkers which adroitly combines the disparate fields of physiology, genetics and ecology to answer several outstanding questions concerning heavy metal tolerance in mycorrhizal fungi.

Mycorrhizal fungi, which interact mutualistically with the majority of plant species, are well known for improving the P status of their hosts (Smith & Read, 1997). Some mycorrhizal fungi are also able to mobilize N and P from organic substrates and to provide plants with improved micronutrient and water acquisition, pathogen resistance, and a variety of other benefits (Smith & Read, 1997). One of these additional benefits is the amelioration of toxicity in metalliferous soils.

Changes in the production and turnover of roots in forests and grasslands in response to rising atmospheric CO2 concentrations, elevated temperatures, altered precipitation, or nitrogen deposition could be a key link between plant responses and longer-term changes in soil organic matter and ecosystem carbon balance. Here we summarize the experimental observations, ideas, and new hypotheses developed in this area in the rest of this volume. Three central questions are posed. Do elevated atmospheric CO2, nitrogen deposition, and climatic change alter the dynamics of root production and mortality? What are the consequences of root responses to plant physiological processes? What are the implications of root dynamics to soil microbial communities and the fate of carbon in soil? Ecosystem-level observations of root production and mortality in response to global change parameters are just starting to emerge. The challenge to root biologists is to overcome the profound methodological and analytical problems and assemble a more comprehensive data set with sufficient ancillary data that differences between ecosystems can be explained. The assemblage of information reported herein on global patterns of root turnover, basic root biology that controls responses to environmental variables, and new observations of root and associated microbial responses to atmospheric and climatic change helps to sharpen our questions and stimulate new research approaches. New hypotheses have been developed to explain why responses of root turnover might differ in contrasting systems, how carbon allocation to roots is controlled, and how species differences in root chemistry might explain the ultimate fate of carbon in soil. These hypotheses and the enthusiasm for pursuing them are based on the firm belief that a deeper understanding of root dynamics is critical to describing the integrated response of ecosystems to global change.

32nd Annual Air Pollution Workshop

Auburn University in Auburn, AL, USA, April 2000

Air pollution has profound effects on agriculture, forests and natural ecosystems. The first Air Pollution Workshop was held over 30 years ago, and the most vital issues have always been highlighted within this forum. This year, forest health and passive sampling of air pollutants were two key areas of interest.

Summary 241

I. INTRODUCTION: CAESIUM IN THE ENVIRONMENT 242

II. UPTAKE OF CAESIUM BY PLANT ROOTS 243

1. Evidence for multiple mechanisms of Cs+uptake by plant roots 243

2. Caesium uptake is affected by the presence of other cations 244

3. Caesium inhibits the uptake of other cations 244

III. MOLECULAR MECHANISMS CATALYSING CAESIUM UPTAKE 245

1. ‘High-affinity’ transport mechanisms 245

2. Inward-rectifying potassium (KIR) channels 245

3. Outward-rectifying potassium (KOR) channels 248

4. Voltage-insensitive cation (VIC) channels 249

5. Ca2+-permeable channels 249

IV. MODELLING CAESIUM INFLUX TO ROOT CELLS 249

1. Predicted Cs+influx through high-affinity mechanisms 250

2. Predicted Cs+influx through cation channels 250

3. Predicted dependence of Cs+influx on [Cs+]ext 252

V. PERSPECTIVE 253

Acknowledgements 254

References 254

Caesium (Cs) is a Group I alkali metal with chemical properties similar to potassium (K). It is present in solution as the monovalent cation Cs+. Concentrations of the stable caesium isotope 133Cs in soils occur up to 25 μg g−1 dry soil. This corresponds to low micromolar Cs+ concentrations in soil solutions. There is no known role for Cs in plant nutrition, but excessive Cs can be toxic to plants. Studies of the mechanism of Cs+ uptake are important for understanding the implications arising from releases of radioisotopes of Cs, which are produced in nuclear reactors and thermonuclear explosions. Two radioisotopes of Cs (134Cs and 137Cs) are of environmental concern owing to their relatively long half-lives, emissions of β and γ radiation during decay and rapid incorporation into biological systems. The soil concentrations of these radioisotopes are six orders of magnitude lower than those of 133Cs. Early physiological studies demonstrated that K+ and Cs+ competed for influx to excised roots, suggesting that the influx of these cations to root cells is mediated by the same molecular mechanism(s). The molecular identity and/or electrophysiological signature of many K+ transporters expressed in the plasma membrane of root cells have been described. The inward-rectifying K+ (KIR), outward-rectifying K+ (KOR) and voltage-insensitive cation (VIC) channels are all permeable to Cs+ and, by analogy with their bacterial counterparts, it is likely that ‘high-affinity’ K+/H+ symporters (tentatively ascribed here to KUP genes) also transport Cs+. By modelling cation fluxes through these transporters into a stereotypical root cell, it can be predicted that VIC channels mediate most (30–90%) of the Cs+ influx under physiological conditions and that the KUP transporters mediate the bulk of the remainder. Cation influx through KIR channels is likely to be blocked by extracellular Cs+ under typical ionic conditions in the soil. Further simulations suggest that the combined Cs+ influxes through VIC channels and KUP transporters can produce the characteristic ‘dual isotherm’ relationship between Cs+ influx to excised roots and external Cs+ concentrations below 200 μM. Thus, molecular targets for modulating Cs+ influx to root cells have been identified. This information can be used to direct future genetic modification of plants, allowing them to accumulate more, or less, Cs and thereby to remediate contaminated sites.

Summary 421

I. INTRODUCTION 421

II. EFFECTS OF OZONE ON REPRODUCTION 423

1. Pollen germination and pollen tube growth 424

2. Floral initiation and development 428

3. Effects on seed and fruit yield and yield components 433

4. Effects of ozone on seed and fruit quality, germination and seedling growth 437

III. INFLUENCE OF REPRODUCTIVE HABIT AND IMPLICATIONS FOR FIELD-GROWN PLANTS 438

IV. CONCLUSIONS AND FUTURE RESEARCH 441

Acknowledgements 442

References 442

Sexual reproductive development is a crucial stage in the life cycle of higher plants as any impairment of the processes involved might have significant implications for the productivity of crop plants and the survival of native species. There is considerable evidence that exposure to ozone, even at current ambient levels in many industrialized countries, reduces grain and fruit yields and adversely affects yield quality. It is also well established that sensitivity to ozone may differ not only between species, but also between cultivars and populations of individual species, and that the impact of exposure is highly dependent on ozone concentration and the duration and timing of exposure. However, few studies have attempted to distinguish between the direct effects of air pollutants on reproductive development, and indirect effects mediated by injury to the vegetative organs and associated changes in the supply of assimilates and other essential resources to support reproductive growth, or the levels of endogenous growth regulators. This review considers the impact of ozone on the reproductive biology of agricultural and native species, and examines its direct effects on specific reproductive processes. The extent to which compensatory responses redress the adverse effects of exposure is also explored, with particular reference to recent studies of Brassica napus (oilseed rape), Brassica campestris (Wisconsin Fast Plants), Plantago major (greater plantain) and Triticum aestivum (wheat).

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.

Summary 257

I. DEFINING AND QUANTIFYING OCEANICITY 257

II. ECOLOGICAL CONSEQUENCES OF OCEANICITY 261

III. ECOLOGICAL HISTORY OF OCEANICITY IN WESTERN EUROPE 263

IV. POSITIVE AND NEGATIVE INFLUENCES OF OCEANICITY 267

1. Species versus communities 267

2. Case study – Primula scotica – climatic effects on reproduction 268

3. Case study – cranberry production and anoxia tolerance 268

V. MODIFYING EFFECTS OF OCEANICITY ON PLANT DISTRIBUTION 269

VI. PHYSIOLOGICAL IMPACT OF WARM WINTERS 271

1. Phenology 271

2. Metabolic consequences of warm winters 272

3. Warm winters and mountain-top vegetation 274

VII. TREELINES AND OCEANICITY 276

VIII. CONCLUSIONS 278

Acknowledgements 279

References 279

A cyclic behaviour in the intensity of maritime conditions which varies with the periodic behaviour of the North Atlantic oscillation has recently become apparent in the climatic record of northern Europe. Periodic increases in oceanicity are usually viewed as having a positive effect on plant survival, as milder winters, reduction of temperature extremes, low risk of exposure to frost, and freedom from drought reduce many aspects of environmental stress. However, warmer winters in maritime environments may also have a powerful influence in creating habitats that are unfavourable for many species. The dangers of long periods of soil saturation for overwintering plants, soaking injury to germinating seeds, premature bud burst in spring, depression of treelines by increased cloud cover and high lapse rates, as well as the constant leaching of soils, are all negative aspects of the maritime environment. Woody species in which root dormancy is delayed by mild winters are particularly vulnerable to the consequences of winter flooding. Subsequent re-exposure to oxygen as water tables fall in spring can aggravate flooding damage through post-anoxic injury, leading to severe dieback of anchoring roots. Soil leaching, particularly after human disturbance, can induce nutrient deficiencies and the establishment of oligotrophic communities with reduced productivity. Podzolization and iron pan formation have in the past facilitated the processes of paludification in oceanic regions. The resulting spread of bogs and acid moorlands can further reduce the potential for productivity in both agriculture and natural plant communities. Given the probability that current climatic trends may increase the degree of oceanicity in western and northern regions of Europe, the potentially negative consequences of such a climatic change need to be considered in relation to future ecological changes and their consequences for conservation and land use.

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.

Summary 449

I. INTRODUCTION 450

II. THE PARTNERS 451

1. Cyanobionts and their role 451

2. Hosts and their role 453

3. Location of cyanobionts in their hosts 455

III. INITIATION AND DEVELOPMENT OF SYMBIOSES 458

1. Initiation of symbioses 458

2. Geosiphon pyriforme 458

3. Cyanolichens 459

4. Liverworts and hornworts 460

5. Azolla 460

6. Cycads 461

7. Gunnera 461

IV. THE SYMBIOSES 462

1. Geographical distribution and ecological significance 462

2. Benefits to the partners 462

(a) Benefits to the cyanobionts 462

(b) Benefits to the hosts 463

3. Duration and stability 463

4. Mode of transmission and perpetuation 463

5. Recognition between the partners 464

6. Specificity and diversity 464

7. Symbiosis-related genes 465

8. Modifications of the cyanobiont 466

(a) Growth and morphology 466

(b) Photosynthesis and carbon metabolism 467

(c) Glutamine synthetase 467

(d) Heterocysts 469

(e) N2fixation 470

9. Nutrient exchange 471

(a) Carbon 471

(b) Nitrogen 472

V. EVOLUTIONARY ASPECTS 472

VI. ARTIFICIAL SYMBIOSES 474

VII. FUTURE OUTLOOK AND PERSPECTIVES 475

1. Cryptic symbioses 476

2. Developmental profile of symbiotic tissues 476

3. Sensing and signalling 476

4. Genetic aspects 476

5. Physiological and biochemical aspects of nutrient exchange 477

6. Microaerobiosis 477

7. Potential applications 477

Acknowledgements 477

References 477

Cyanobacteria are an ancient, morphologically diverse group of prokaryotes with an oxygenic photosynthesis. Many cyanobacteria also possess the ability to fix N2. Although well suited to an independent existence in nature, some cyanobacteria occur in symbiosis with a wide range of hosts (protists, animals and plants). Among plants, such symbioses have independently evolved in phylogenetically diverse genera belonging to the algae, fungi, bryophytes, pteridophytes, gymnosperms and angiosperms. These are N2-fixing symbioses involving heterocystous cyanobacteria, particularly Nostoc, as cyanobionts (cyanobacterial partners). A given host species associates with only a particular cyanobiont genus but such specificity does not extend to the strain level. The cyanobiont is located under a microaerobic environment in a variety of host organs and tissues (bladder, thalli and cephalodia in fungi; cavities in gametophytes of hornworts and liverworts or fronds of the Azolla sporophyte; coralloid roots in cycads; stem glands in Gunnera). Except for fungi, the hosts form these structures ahead of the cyanobiont infection. The symbiosis lasts for one generation except in Azolla and diatoms, in which it is perpetuated from generation to generation. Within each generation, multiple fresh infections occur as new symbiotic tissues and organs develop. The symbioses are stable over a wide range of environmental conditions, and sensing–signalling between partners ensures their synchronized growth and development. The cyanobiont population is kept constant in relation to the host biomass through controlled initiation and infection, nutrient supply and cell division. In most cases, the partners have remained facultative, with the cyanobiont residing extracellularly in the host. However, in the water-fern Azolla and the freshwater diatom Rhopalodia the association is obligate. The cyanobionts occur intracellularly in diatoms, the fungus Geosiphon and the angiosperm Gunner a. Close cell–cell contact and the development of special structures ensure efficient nutrient exchange between the partners. The mobile nutrients are normal products of the donor cells, although their production is increased in symbiosis. Establishment of cyanobacterial–plant symbioses differs from chloroplast evolution. In these symbioses, the cyanobiont undergoes structural–functional changes suited to its role as provider of fixed N rather than fixed C, and the level of intimacy is far less than that of an organelle. This review provides an updated account of cyanobacterial–plant symbioses, particularly concerning developments during the past 10 yr. Various aspects of these symbioses such as initiation and development, symbiont diversity, recognition and signalling, structural–functional modifications, integration, and nutrient exchange are reviewed and discussed, as are evolutionary aspects and the potential uses of cyanobacterial–plant symbioses. Finally we outline areas that require special attention for future research. Not only will these provide information of academic interest but they will also help to improve the use of Azolla as green manure, to enable us to establish artificial N2-fixing associations with cereals such as rice, and to allow the manipulation of free-living cyanobacteria for photobiological ammonia or hydrogen production or for use as biofertilizers.

We review four hypotheses for the control of carbon acquisition by roots, and conclude that the functional equilibrium hypothesis can offer a good description of C acquisition by roots relative to shoots, but is deficient mechanistically. The hypothesis that import into roots is solely dependent on export from the shoot, itself determined by features of the shoot alone (the ‘push’ hypothesis), is supported by some but not all the evidence. Similarly, the idea that root demand, a function of the root alone, determines import into it (the ‘pull’ hypothesis), is consonant with some of the evidence. The fourth, general, hypothesis (the ‘shared control’ hypothesis) – that acquisition of C by roots is controlled by a range of variables distributed between root and shoot – accords with both experiment and theory. Top-down metabolic control analysis quantifies the control of C flux attributable to root relative to source leaf. We demonstrate that two levels of mechanistic control, short-term regulation of phloem transport and control of gene expression by compounds such as sugars, underlie distributed control. Implications for the impact of climate change variables are briefly discussed.

Growth of crops in CO2-enriched atmospheres typically results in significant changes in root growth and development. Increased root carbohydrates stimulate root growth either directly (functioning as substrates) or indirectly (functioning as signal molecules) by enhancing cell division or cell expansion, or both. Although highly variable, the literature suggests that, generally, initiation and stimulation of lateral roots is favored over the elongation of primary roots, leading to more highly branched, shallower root systems. Such architectural shifts can render root systems less efficient, perhaps contributing to the lower specific root activities often reported. Allocation of carbon (C) to roots fluctuates through the life of the plant; root functional and growth responses should therefore not be viewed as static. In annual crops, C allocation to belowground processes changes as vegetative growth switches to reproduction and maturation. Reductions in C allocation to roots over time might cause temporal shifts in root deployment, perhaps affecting root demography. However, significant changes in root turnover (defined here as root flux or mortality relative to total root pool size) as a result of decreased root longevities in crop plants are unlikely. Consideration of changing C allocation to roots, a more thorough understanding of the mechanistic controls on root longevity, and a better characterization of the rooting habits (life histories) of different crop species will further our understanding of how increasing atmospheric [CO2] will affect root demography. This knowledge will lead the way toward a more thorough understanding of the linkage of atmosphere with belowground plant function and also that of plant function with soil biology and structure. Ultimately, successful modeling of global C and nitrogen (N) cycles will require empirical data concerning spatial and temporal deployment of roots for a range of crop species grown under different agricultural management systems.

N2 fixation, measured as acetylene reduction, was studied in laboratory cultures and in natural assemblages (both as a mixed population and as individually picked colonies) of the heterocystous cyanobacteria Aphanizomenon sp. and Nodularia spp. from the Baltic Sea. During a diurnal cycle of alternating light and darkness, these organisms reduced acetylene predominantly during the period of illumination, although considerable activity was also observed during the dark period. In both laboratory cultures and natural populations N2 fixation was saturated below a photon flux density of 600 μm−2 s−1. In cyanobacterial blooms in the Baltic Sea, nitrogenase activity was mostly confined to the surface layers. Samples collected from greater depths did not possess the same capacity for acetylene reduction as samples from the surface itself, even when incubated at the photon flux density prevailing in surface waters. This suggests that, with respect to N2 fixation, Baltic cyanobacteria are adapted to the intensity of illumination that they are currently experiencing.

Axial and radial transport and the accumulation of photoassimilates in carrot taproot were studied using 14C labelling and autoradiography. Axial transport of the 14C labelled assimilates inside the taproot was rapid and occurred mainly in the young phloem found in rows radiating from the cambium. The radial transport of the assimilate inward (to cambium, xylem zone and pith) and outward (to phloem zone and periderm) from the conducting phloem was an order of magnitude slower than the longitudinal transport and was probably mainly diffusive. The cambial zone of the taproot presented a partial barrier in the inward path of the assimilate to the xylem zone. We suggest that this is due to the cambium comprising a strong sink for the assimilate on the basis that our previous work has shown that it contains very low concentrations of free sucrose. By contrast, a high accumulation of nonsoluble 14C was found in the cambium region in good agreement with the active growth of this zone. Autoradiography following the feeding of 14C labelled sugars to excised sections of taproot indicated that only a ring of cells at and/or just within the cambium take up sugars from the apoplast. This indicates that radial movement in the phloem and pith must be symplastic. An apoplastic step between phloem and xylem is possible. The rapid uptake of sugars from the apoplast at this point might represent a mechanism for keeping photoassimilates away from the transpiration stream and re-location back to the leaves.

Crosses between certain genotypes of common bean result in dwarfing of F1 plants and lethal dwarfing in a proportion of the F2 population. This is under the control of the semi-dominant alleles, DL1 and DL2 at two complementary loci which are expressed in the root and shoot respectively. The various DL genotypes can be simulated by grafting. The graft combination DL1DL1dl2dl2/dl1dl1DL2DL2 was found to have a significantly higher root dry matter fraction than either parent. Lethally dwarfed plants (DL1DL1DL2DL2) and the analogous lethal graft combination (dl1dl1DL2DL2/DL1DL1dl2dl2) exhibit failure of root growth and have very low root fractions. Hybrids or graft combinations with failed roots ceased growth and accumulated large amounts of starch throughout their hypocotyls. In sterile culture, both lethal dwarfs and lethal graft combinations were able to grow roots if sucrose was added to the growth medium. This indicates that a failure of sucrose translocation to the roots is probably responsible for failed root growth. Data from screening the DL genotypes of 49 cultivars could be fully explained using the DL system hypothesis, and grafting proved to be efficient for identifying DL genotype. The DL system might be of fundamental importance in root–shoot partitioning. Current evidence favours the hypothesis that failure of root growth is the outcome of excessively high sink strength of shoots compared to roots, which might arise from signalling incompatibilities between the genotypes.