Search

Information about several papers discussed at the Forestry Workshop of the Plant and Animal Genome XV Conference, held in January 2007 in San Diego, California is presented. The event focused on the genomics and evolution of forest trees, including topics on adaptive polymorphisms, quantitative trait locus analysis and tree life style. It featured presentations by Emanuele De Paoli, Gail Taylor and Gerald Tuskan.

Information about several papers discussed at the polyploidy workshop of the Plant and Animal Genome XV Conference, held in January 2007 in San Diego, California is presented. The event covered several topics such as ancient polyploidy events in Glycine, expression evolution of duplicate genes, gene expression changes, hybridization barriers in Arabidopsis and tissue-specific and stress-induced expression patterns of duplicate genes in cotton and hybrid Populus.

Information about several papers discussed at the Fungal Genomics Workshop: Plant and Animal Genome XV Conference, held in January 2007 in San Diego, California is presented. The event focused on comparative genomics and global functional analyses in relation to fungi and oomycetes. It featured presentations by Sophien Kamoun, Gert Kema and Gary Payne.

• This paper establishes relationships between two aspects of ribosomal DNA (rDNA) biology: epigenetic silencing of rDNA loci; and homogenization leading to concerted evolution. • Here, we examined rDNA inheritance and expression patterns in three natural Nicotiana allopolyploids (closest living descendants of diploid parents are given), N. rustica ( N. paniculata ×  N. undulata), N. tabacum ( N. sylvestris ×  N. tomentosiformis) and N. arentsii ( N. undulata ×  N. wigandioides), and synthetic F1 hybrids and allopolyploids. • The extent of interlocus rDNA homogenization decreased in the direction N. arentsii > N. tabacum >  N. rustica. The persistence of parental rDNA units in one of the subgenomes was associated with their transcription inactivity and likely heterochromatization. Of synthetic hybrids and polyploids only N. paniculata ×  N. undulata showed strong uniparental transcriptional silencing of rDNA triggered already in F1. • Epigenetic patterns of expression established early in allopolyploid nucleus formation may render units susceptible or resistant to homogenization over longer time-frames. We propose that nucleolus-associated transcription leaves rDNA units vulnerable to homogenization, while epigenetically inactivated units, well-separated from the nucleolus, remain unconverted. [ABSTRACT FROM AUTHOR]

Information about several papers discussed at the event Population Genetics and Genomics of Forest Trees: From Gene Function to Evolutionary Dynamics and Conservation, held in Madrid, Spain in October 2006 is presented. The ecological and evolutionary genetics of forest trees and associated organisms comprise a microcosm within this field. A major trend emerging in forest genetics research is the establishment of research programmes directed at the level of biological communities rather than single tree species.

Information about several papers discussed at The Biology of Transpiration: From Guard Cells to Globe, held at Snowbird Mountain Resort in Utah from October 10-14, 2006 is presented. At the event, two lecturers described various components of the pathway which involves a succession of related transcription factors necessary for the division and differentiation of stomatal cells. A number of speakers focused on the characterization of individual signalling components that affect the regulation of stomatal aperture.

• This paper investigates the dependence of monoterpene emissions at the canopy scale on total leaf area and leaf distribution. Simulations were carried out for a range of hypothetical but realistic forest canopies of the evergreen Quercus ilex (holm oak). • Two emission models were applied that either did (SIM-BIM2) or did not (G93) account for cumulative responses to temperature and light. Both were embedded into a canopy model that considered spatial and temporal variations of foliage properties. This canopy model was coupled to a canopy climate model (CANOAK) to determine the micrometeorological conditions at the leaf scale. • Structural properties considerably impacted monoterpene emission. The sensitivities to changes in total leaf area and to leaf area distribution were found to be of similar magnitude. The two different models performed similarly on a whole-year basis but showed clear differences during certain episodes. • The analysis showed that structural indices have to be carefully evaluated for proper scaling of emission from leaves to canopy. Further research is encouraged on seasonal dynamics of emission potentials. [ABSTRACT FROM AUTHOR]

I am fascinated by the sensory ecology of animal interactions with plants and how colours are in plants, so two other notable research papers are 'How colorful are fruits? Limited color diversity in fleshy fruits on local and global scales' by Stournaras I et al. i ([5]) and 'Adaptation of flower and fruit colours to multiple, distinct mutualists' by Renoult I et al. i ([4]). Iridescence, nanomaterials, optical biomaterials, physics of biology, seed-dispersers, sensory ecology, structural colour, vision. [Extracted from the article]

• The consistent association of Paleotropical Dipterocarpaceae with ectomycorrhizal (ECM) fungi suggests that ECM status is an ancestral character in the family. Despite its distinctive morphology, Pakaraimaea dipterocarpacea, a Neotropical Dipterocarpaceae endemic to the Guayana Region, is phylogenetically related to the Paleotropical Dipterocarpaceae. The confirmation of P. dipterocarpacea ECM status would indicate that Paleotropical Dipterocarpaceae and P. dipterocarpacea probably had a common ECM ancestor. • Mycorrhizal colonization of P. dipterocarpacea was assessed, and ECMs were recorded using histological and molecular methods. • P. dipterocarpacea was highly colonized by typical ECMs, and several ECM fungal taxa belonging to Clavulinaceae, Sebacinaceae, Cortinariaceae and Amanitaceae were identified. • This paper provides the first documented evidence of ECM in a neotropical genus of Dipterocarpaceae and indicates that ECMs possibly evolved in Gondwana in ancestors of Dipterocarpaceae before the separation of South America from Africa by the Atlantic, c. 135 million years ago. The observation of Sebacinaceae and Clavulinaceae suggests that broad host range fungi are important components of P. dipterocarpacea ECM communities. [ABSTRACT FROM AUTHOR]

• Attenuated total reflectance (ATR) spectra of plant leaves display complex absorption features related to organic constituents of leaf surfaces. The spectra can be recorded rapidly, both in the field and in the laboratory, without special sample preparation. • This paper explores sources of ATR spectral variation in leaves, including compositional, positional and temporal variations. Interspecific variations are also examined, including the use of ATR spectra as a tool for species identification. • Positional spectral variations generally reflected the abundance of cutin and the epicuticular wax thickness and composition. For example, leaves exposed to full sunlight commonly showed more prominent cutin- and wax-related absorption features compared with shaded leaves. Adaxial vs. abaxial leaf surfaces displayed spectral variations reflecting differences in trichome abundance and wax composition. Mature vs. young leaves showed changes in absorption band position and intensity related to cutin, polysaccharide, and possibly amorphous silica development on and near the leaf surfaces. • Provided that similar samples are compared (e.g. adaxial surfaces of mature, sun-exposed leaves) same-species individuals display practically identical ATR spectra. Using spectral matching procedures to analyze an ATR database containing 117 individuals, including 32 different tree species, 83% of the individuals were correctly identified. [ABSTRACT FROM AUTHOR]

• Eucalyptus is one of the world's main sources of biomass. The genus includes species representing the principle hardwood trees used for pulp and paper. Here, we aimed to identify genes specifically expressed in differentiating secondary xylem compared with phloem. • We constructed a xylem vs phloem subtractive library (Xp) that generated 263 unique sequences. By transcript profiling of xylem, phloem, vascular cambium and leaves using macroarrays, we classified the 263 unigenes into distinct tissue-specific groups. Reverse transcription–polymerase chain reaction (RT-PCR) confirmed the differential expression of representative expressed sequence tags (ESTs). • A total of 87 unigenes were preferentially expressed in xylem. They were involved in functional categories known to play roles in xylogenesis, such as hormone signaling and metabolism, secondary cell wall thickening and proteolysis. Some of these genes, including unknown genes, may be considered xylem-specific and they are likely to control important functions in xylogenesis. • These data shed light on the cellular functions of xylem cells and, importantly, provide us with a portfolio of Eucalyptus xylem genes that may be major players in the control of wood formation and quality. [ABSTRACT FROM AUTHOR]

• Thlaspi caerulescens is a natural zinc (Zn), cadmium (Cd) and nickel (Ni) hyperaccumulator and an emerging plant model species to study heavy metal hyperaccumulation and tolerance. This paper describes the analysis of the first expressed sequence tag (EST) collection from T. caerulescens. This collection is a new resource to unravel the molecular basis of plant metal homeostasis, tolerance and hyperaccumulation. • In total, 4289 ESTs were generated originating from Zn-exposed root and shoot tissues, leading to 3709 T. caerulescens assembled partial cDNA sequences (unigenes). • In comparison to Arabidopsis or other publicly available plant sequences, a fraction of c. 8% of the T. caerulescens unigenes (TcUGs) had no significant similarity with any known DNA sequence and, so far, these sequences are T. caerulescens specific. Three per cent of the TcUGs correspond to Arabidopsis thaliana orthologues that, as yet, have not been found to be expressed. • The T. caerulescens transcriptome generally relates very well to the A. thaliana transcriptome, although, compared with other closely related species, a relatively large number of T. caerulescens-specific transcripts were found. T. caerulescens also expresses a relatively large number of genes which are expressed at a very low level in A. thaliana. [ABSTRACT FROM AUTHOR]

• Conifer foliage structures affect branch and bud temperature by altering the development and convective resistance of the thermal boundary layer. This paper examines foliage effects on forced convection in branches and buds of Picea glauca (Moench) Voss and Pinus contorta Dougl. Ex. Loud., two species that represent the range of variation in foliage structure among conifers. • Forced convection is characterized by a power law relating Nusselt (heat transfer) and Reynolds (boundary layer development) numbers. Data were collected in a laminar flow wind tunnel for free stream velocities of 0.16–6.95 m s−1. Scaling parameters were compared against literature values for silver cast branch replicas, a bed of real foliage, cylinders, and tube banks. • Foliage structures reduced Nusselt numbers (heat transfer) relative to cylinders, which are typically used to approximate leafless branches and buds. Significantly different scaling relationships were observed for all foliage structures considered. • Forced convection scaling relationships varied with foliage structure. The scaling relationships reported here account for variation within populations of branches and buds and can be used to characterize forced convection in a forest canopy. [ABSTRACT FROM AUTHOR]

Bombus, Caryophyllaceae, mutualism, nursery pollination, seed predation, Silene, Dianthus, Hadena Pollination and seed predation by moths on Silene and allied Caryophyllaceae: evaluating a model system to study the evolution of mutualisms. Nursery pollination and seed predation by nocturnal moths Three papers in this issue, including a comprehensive review, focus on "nursery" pollination by nocturnal moths whose larvae grow protected within developing fruits of caryophyllaceous hosts. Keywords: Bombus; Caryophyllaceae; Dianthus; Hadena; mutualism; nursery pollination; seed predation; Silene EN Bombus Caryophyllaceae Dianthus Hadena mutualism nursery pollination seed predation Silene 637 640 4 04/16/21 20060215 NES 060215 In a recent article inaugurating the new evolution section of I New Phytologist i , Verne Grant (2004) suggests that innovative shifts in our understanding of speciation and other processes often arise from the merging and reintegration of ideas already present within existing yet isolated paradigms. [Extracted from the article]

Tim has been an Associate Editor of New Phytologist since 2019, and was previously a Co-editor of New Phytologist from 2010. So it is hard to pick a favourite, but I really like all Taylor Feild's papers about early angiosperm evolution (see for example the Tansley review Feild & Arens, [1]). Callitris is another favourite conifer because it has unbelievably bulletproof xylem, and Parasitaxus because it is such a freak of a red parasitic conifer hidden in the hills of New Caledonia. [Extracted from the article]

This article comments on various issues related to biogeography. Several factors likely contribute to the lack of understanding of the biogeography of plant chemistry. Among these important factors are climate, geomorphology, vegetation type and site history. There has been an increase or renewal of interest in the biogeography and the stoichiometry of ecological chemistry. The Rhizosphere 2004 Congress held in Munich, Germany, was organized to mark the centenary of the publication of a paper by Lorenz Hiltner in which he introduced the term rhizosphere that was defined as the soil compartment influenced by the root.

• This study establishes a topographical framework for functional investigations on the regulation of lipid biosynthesis and its interaction with embryo photosynthesis in developing soybean seed. • Structural observations, combined with molecular and functional parameters, revealed the gradual transformation of chloroplasts into storage organelles, starting from inner regions going outwards. This is evidenced by electron microscopy, confocal laser scanning microscopy, in situ hybridization and histochemical/biochemical data. • As a consequence of plastid differentiation, photosynthesis becomes distributed along a gradient within the developing embryo. Electron transport rate, effective quantum yield and O2 production rate are maximal in the embryo periphery, as documented by imaging pulse-amplitude-modulated fluorescence and O2 release via microsensors. The gradual loss of photosynthetic capacity was accompanied by a similarly gradual accumulation of starch and lipids. Noninvasive nuclear magnetic resonance spectroscopy of mature seeds revealed steep gradients in lipid deposition, with the highest concentrations in inner regions. • The inverse relationship between photosynthesis and lipid biosynthesis argues against a direct metabolic involvement of photosynthesis in lipid biosynthesis during the late storage stage, but points to a role for photosynthetic oxygen release. This hypothesis is verified in a companion paper. New Phytologist (2005) doi: 10.1111/j.1469-8137.2005.01474.x© New Phytologist (2005) [ABSTRACT FROM AUTHOR]

• Based on the topographical analysis of photosynthesis and oil storage, we propose in a companion paper that photosynthetic oxygen release plays a key role in the local energy state, storage metabolism and flux toward lipid biosynthesis in developing soybean seeds. To test this hypothesis, we combined topographical analysis of ATP gradients across tissues, microsensor quantifications of internal O2 levels, assays of energy balance, metabolite profiles and isotope-labelling studies. • Seeds show a marked degree of oxygen starvation in vivo (minimum O2 levels 0.1 kPa, ≈ 1.3 µ m), affecting ATP gradients, overall energy state, metabolite pools and storage activity. • Despite the low light availability, photosynthesis supplies significant amounts of oxygen to the hypoxic seed tissue. This is followed by an increase in local ATP levels, most prominently within the lipid-synthesizing (inner) regions of the embryo. Concomitantly, partitioning of 14C-sucrose to lipids is increased, suggesting higher rates of lipid biosynthesis. • It is concluded that both respiratory and biosynthetic fluxes are dynamically adjusted to photosynthetic oxygen supply. New Phytologist (2005) doi: 10.1111/j.1469-8137.2005.01473.x© New Phytologist (2005) [ABSTRACT FROM AUTHOR]

This article comments on a paper by M. Diabate and colleagues on the role of nodulation and nitrogen fixation in West African legumes. The authors examined 156 species in six different forests and reported on their nodulation. Overall their data are consistent with the known distribution of nodulating behaviour in the Leguminosae. Some of their new reports are on species of genera widely reported as nodulating. Others are on new genera, from sections of the Leguminosae that include nonnodulating genera. Because the criteria on which their nodule identification was based were not given, it is difficult to evaluate these data. The family Leguminosae is generally subdivided into three subfamilies, Caesalpinioideae, Mimosoideae and Papilionoideae, and the extent of nodulation varies widely within these, from uncommon, through common to very common, within subfamilies, considerable correspondance with tribal affinities. Thus it is particularly important to be careful when reporting nodulation in groups where it is uncommon. The article also offered a longitudinal perspective on the significance of nodulation in African rainforest legumes. Clearly, as Diabate and colleagues stress, people need to know much more about the nodulation status of West African legumes if forests are to be properly understood and managed. This will require much careful work on the part of many people, particularly those who live in the countries concerned.

Sun and shade environments place markedly different constraints on the photosynthetic performance of plants. Leaf-level photosynthetic responses to sun and shade have been extensively investigated, whereas there has been much less research on the functional role of crown architecture in these environments. This paper focuses on the role of architecture in maximizing light capture and photosynthesis in shaded understories and in minimizing exposure to excess radiation in open high light environments. Understanding these contrasting roles of architecture is facilitated by application of a three-dimensional structural–functional model, Y-plant. Surveys of understory plants reveal a diversity of architectures but a strong convergence at only modest light-capture efficiencies because of significant self-shading. Simulations withPsychotriaspecies revealed that increasing internode lengths would increase light-capture efficiencies and whole plant carbon gain. However, the costs of the additional required biomechanical support was high, which, in terms of relative growth rates, would override the advantage provided by higher light-capture efficiencies. In high light environments, leaf angles and self-shading provide structural photoprotection, minimizing potential damage from photoinhbition. Simulations reveal that without these structural protections photoinhibition of photosynthesis is likely to be much greater with daily carbon gain significantly reduced.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01328.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

• Functional–structural plant models that include detailed mechanistic representation of underlying physiological processes can be expensive to construct and the resulting models can also be extremely complicated. On the other hand, purely empirical models are not able to simulate plant adaptability and response to different conditions. In this paper, we present an intermediate approach to modelling plant function that can simulate plant response without requiring detailed knowledge of underlying physiology.• Plant function is modelled using a‘canonical’ modelling approach, which uses compartment models with flux functions of a standard mathematical form, while plant structure is modelled using L-systems.• Two modelling examples are used to demonstrate that canonical modelling can be used in conjunction with L-systems to create functional–structural plant models where function is represented either in an accurate and descriptive way, or in a more mechanistic and explanatory way.• We conclude that canonical modelling provides a useful, flexible and relatively simple approach to modelling plant function at an intermediate level of abstraction.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01330.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

• This paper presents an architectural model of wheat (Triticum aestivum), designed to explain effects of light conditions at the individual leaf level on tillering kinetics. Various model variables, including blade length and curvature, were parameterized for spring wheat, and compared with winter wheat and other Gramineae species.• The architectural model enables simulation of plant properties at the level of individual organs. Parameterization was based on data derived from an outdoor experiment with spring wheat cv. Minaret.• Final organ dimensions of tillers could be modelled using the concept of relative phytomer numbers. Various variables in spring wheat showed marked similarities to winter wheat and other species, suggesting possibilities for a general Gramineae architectural model.• Our descriptive model is suitable for our objective: investigating light effects on tiller behaviour. However, we plan to replace the descriptive modelling solutions by physiological, mechanistic solutions, starting with the localized production and partitioning of assimilates as affected by abiotic growth factors.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01337.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

• The emergence of a regular phyllochron from the dynamic processes of leaf initiation, leaf elongation and whorl construction suggests causal relationships between leaf elongation and leaf emergence. This paper presents a hypothesis as to how the ontogeny of the growth zone of leaves is triggered by emergence events, and implements it in a dynamic model of leaf elongation.• Two different experiments, presenting two contrasted cases of relationships between leaf emergence and kinetics of leaf elongation, were analysed and interpreted with the model in terms of the functioning of the growth zone.• Analysis of elongation kinetics revealed that the hypothesis allows for several contrasted elongation patterns that were observed, and for a regular phyllochron emerging from the variable dynamic of elongation. The model was able to simulate these patterns, and helped to identify the mechanisms underlying the key points of the analysis.• The hypothesis is not demonstrated, but its coherence and robustness are established, which should inform a renewal of the modelling of leaf elongation in architectural models.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01371.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

• This paper compares the responses to ozone in five woody species:Fagus sylvatica(FS),Acer pseudoplatanus(AP),Fraxinus excelsior(FE),Viburnum lantana(VL) andAilanthus altissima(AA). The hypothesis being tested was that the strategies that plants adopt to resist oxidative pressure are species-specific.• The study was carried out on field grown plants in an area in Northern Italy characterized by elevated levels of ozone pollution. The observations were made both at ultrastructural (using light and electronic microscopy) and physiological (using chlorophyllatransient fluorescence and microspectral fluorometry) level.• Common responses were: the hypersensitive response (i.e. the death of palisade mesophyll cells) and the formation of callose layers separating injured from healthy cells. FS and AP were capable of thickening the palisade mesophyll cell walls. This thickening process involved changes in cell wall chemical structure, evidenced by the accumulation of yellow autofluorescence compounds. Species-specific behaviours were observed with the fluorescence analysis, with special reference to the photochemical de-excitation constant (Kp). This value increased in FE and AP, and decreased in AA.• The observed responses are interpreted as adaptative strategies against the ozone stress. The increase of Kp indicates that the reaction centres were working as more effective quenchers.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01385.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

The ability of an organism to alter the environment that it experiences has been termed‘niche construction’. Plants have several ways whereby they can determine the environment to which they are exposed at different life stages. This paper discusses three of these: plasticity in dispersal, flowering timing and germination timing. It reviews pathways through which niche construction alters evolutionary and ecological trajectories by altering the selective environment to which organisms are exposed, the phenotypic expression of plastic characters, and the expression of genetic variation. It provides examples whereby niche construction creates positive or negative feedbacks between phenotypes and environments, which in turn cause novel evolutionary constraints and novel life-history expression.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01357.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

• This paper is the first global study of the extent to which fire determines global vegetation patterns by preventing ecosystems from achieving the potential height, biomass and dominant functional types expected under the ambient climate (climate potential).• To determine climate potential, we simulated vegetation without fire using a dynamic global-vegetation model. Model results were tested against fire exclusion studies from different parts of the world. Simulated dominant growth forms and tree cover were compared with satellite-derived land- and tree-cover maps.• Simulations were generally consistent with results of fire exclusion studies in southern Africa and elsewhere. Comparison of global‘fire off’ simulations with landcover and treecover maps show that vast areas of humid C4 grasslands and savannas, especially in South America and Africa, have the climate potential to form forests. These are the most frequently burnt ecosystems in the world. Without fire, closed forests would double from 27% to 56% of vegetated grid cells, mostly at the expense of C4 plants but also of C3 shrubs and grasses in cooler climates.• C4 grasses began spreading 6–8 Ma, long before human influence on fire regimes. Our results suggest that fire was a major factor in their spread into forested regions, splitting biotas into fire tolerant and intolerant taxa.New Phytologist(2004)doi: 10.1111/j.1469-8137.2004.01252.x© New Phytologist(2004) [ABSTRACT FROM AUTHOR]

The ability to forecast invasion of harmful and beneficial organisms is becoming increasingly important in agricultural and horticultural production systems as well as in natural plant communities. In this paper we examine the spread of a fungus through a population of discrete sites on a lattice, using replicable, yet stochastically variable experimental microcosms. We combine epidemiological concepts to summarise fungal growth dynamics with percolation theory to derive and test the following hypotheses: first fungal invasion into a population of susceptible sites on a lattice can be stopped by a threshold proportion of randomly removed sites; second random removal of susceptible sites from a population introduces a shield which can prevent invasion of unprotected sites; and third the rate at which a susceptible population is invaded reduces with increasing number of randomly protected sites. The broader consequences of thresholds for fungal invasion in natural and agricultural systems are discussed briefly. [ABSTRACT FROM AUTHOR]

Comments on the initial success in the retrospective interpretation of the hydrosystem of trees as shown in the paper "Suitability of Chestnut Earlywood Vessel Chronologies for Ecological Studies," by P. Fonti and I. Garcia Gonzales. Features of dendroecology and dendroclimatology that need to be examined; Utility of the hydrosystem; Factors which can contribute to reconstruction of the climate in Central Europe.

Patterns of intraspecific variation in the number of pollen tubes per style in naturally pollinated plants are poorly known, yet that information is essential for assessing the frequency of occurrence and evolutionary implications of microgametophyte competition in the wild. This paper analyses intraspecific variation in the number of pollen tubes per style for six species of southern Spanish insect-pollinated Lamiaceae (Ballota hirsuta, Lavandula latifolia, Marrubium supinum, Phlomis lychnitis, Rosmarinus officinalis and Teucrium rotundifolium) differing in growth form, phenology, flower size and pollinators. Number of pollen tubes exceeded number of fertilizable ovules in 26–87% of styles, and mean number per pollen tube of other pollen tubes in the same style varied between five and 12. Within-plant variation in pollen tube number was extensive in all species, accounting for 68–92% of total population-level variance. In L. latifolia mean pollen tube number per style differed between populations and between plants within populations, but such differences were not consistent among years. It is concluded that opportunities for microgametophyte competition and selection on competitive ability are considerable in all species studied, although extreme spatial fine-graininess and marked stochasticity in the variation of pollen tube numbers, including temporal inconstancy of individual differences, will greatly reduce the opportunity for selection on sporophytic characters influencing degree of microgametophyte competition.

The impact of Verne Grant's 1981 edition of Plant Speciation reverberates as strongly today as it did 22 yr ago. His insights into the field of plant evolutionary biology continue to inform current and future research programmes. For example, Grant's work pointed the way for those who have argued that the consequences of natural hybridization are of primary importance in the evolution of many plant and animal species complexes. It is thus now widely accepted that numerous, important outcomes are afforded by crosses between divergent lineages. In the present paper we highlight some of the concepts that appear in Grant's seminal work. We address these in the context of their application by Grant to a plant species complex that we have studied for 15 yr, the Louisiana Irises. We consider Grant's inferences in light of new data and our own conclusions. In the main, we find ourselves supporting Grant's conceptual framework. [ABSTRACT FROM AUTHOR]

Hybridization and polyploidy are well illustrated in the genus Spartina. This paper examines how recent molecular approaches have helped our understanding of the past and recent reticulate history of species, with special focus on allopolyploid speciation. Spartina species are tetraploid, hexaploid or dodecaploid perennials, most of them being native to the New World. The molecular phylogeny indicates an ancient split between the tetraploid and the hexaploid species, with S. argentinensis as sister to the hexaploid lineage. Recent hybridization and polyploidization events involved hexaploid species, resulting from introductions of the east-American S. alterniflora. In California, ongoing hybridizations with its sister species S. foliosa result in introgressant hybrid swarms. In Europe, hybridization with S. maritima resulted in S. ×  neyrautii (France) and S. ×  townsendii (England), with. S. alterniflora as the maternal parent. The allopolyploid S. anglica resulted from chromosome doubling of S. ×  townsendii. This young allopolyploid contains divergent homoeologous subgenomes that have not undergone significant changes since their reunion. Hybridization, rather than genome duplication, appears to have shaped the allopolyploid genome at both the structural and epigenetic levels. [ABSTRACT FROM AUTHOR]

Ecological transitions are at the core of different modes of speciation. These transitions face both genetic and demographic hurdles. This paper focuses on how these hurdles are overcome, allowing ecological speciation and speciation via hybridization and/or polyploidy. Niche shifting is a two-step process. First there is the establishment of ill-adapted populations where ecological opportunity allows. This is followed by the genetic refinement of populations, which allows them to be integrated into novel communities and habitats. These steps are more readily accomplished in unsaturated floras, where competition is less intense. Ecological transitions in saturated floras may be facilitated by disturbance. Invasive species serve as heuristic model systems for understanding the early stages of speciation where niche shifts are involved. [ABSTRACT FROM AUTHOR]

This paper reviews the evidence that three pairs of diploid annual plant species are related as progenitor and recent derivative. The species pairs are Layia glandulosa and Layia discoidea, Clarkia biloba and Clarkia lingulata, and Stephanomeria exigua ssp. coronaria and Stephanomeria malheurensis. The three cases are examples of Verne Grant's model of ‘Quantum Speciation’, in which a derived species is budded off and acquires new traits while the parental species continues more or less as before. The derived species differ from their progenitors in different ways and show different modes of reproductive isolation. However, the number of differences between each derivative and its progenitor appears to be few and relatively simple in genetic terms. Comparison of a recently evolved species with its progenitor can reveal what happens during speciation because overall divergence is minimal and the direction of evolution is clear. Evidence from DNA sequences may be particularly useful to recognize additional examples of progenitor and derivative relationship. [ABSTRACT FROM AUTHOR]

Summary • The aim of the paper was to determine nitrogen compounds contributing to leaf cell osmoregulation of spinach (Spinacia oleracea ) submitted to increasing salt stress. • Sodium, free amino acids and glycine betaine contents were determined in the last fully expanded leaf of plants stressed by daily irrigation with saline water (0.17 M NaCl). • After 20 d of treatment, when Na+ content was c . 55 umol g-1 f. wt above the control, and the reduction of stomatal conductance lowered photosynthesis to c . 60% of the control, the free amino acids of the leaves, especially glycine and serine, strongly increased. Proline and glycine betaine also increased significantly. After 27 d of treatment, when the Na+ content was c . 100 umol g-1 f. wt above the control and photosynthesis was 33% of the control, the free amino acid content, especially glycine and serine, declined. Gycine betaine, but not proline, increased further. • Glycine betaine comprised c . 15% of the overall nitrogen osmolytes at mild salt-stress, but represented 55% of the total, when the stress became more severe. The increase of glycine betaine balanced the decline in free amino acids, mainly replacing glycine and serine (the precursors of glycine betaine) in the osmotic adjustment of the cells. Photorespiration, which increased during salt stress, was also suggested to have a role in supplying metabolites to produce compatible osmolytes. [ABSTRACT FROM AUTHOR]

Summary A growing body of empirical work suggests that soil organisms can exert a strong role in plant community dynamics and may contribute to the coexistence of plant species. Some of this evidence comes from examining the feedback on plant growth through changes in the composition of the soil community. Host specific changes in soil community composition can generate feedback on plant growth and this feedback can be positive or negative. Previous work has demonstrated that negative soil community feedback can contribute to the coexistence of equivalent competitors. In this paper, I show that negative soil community feedback can also contribute to the coexistence of strong competitors, maintaining plant species that would not coexist in the absence of soil community dynamics. I review the evidence for soil community feedback and find accumulating evidence that soil community feedback can be common, strongly negative, and generated by a variety of complementary soil microbial mechanisms, including host-specific changes in the composition of the rhizosphere bacteria, nematodes, pathogenic fungi, and mycorrhizal fungi. Finally, I suggest topics needing further examination. [ABSTRACT FROM AUTHOR]

Summary • Reproductive traits are key characteristics for predicting the response of communities and ecosystems to global change. • We used meta-analysis to integrate data on eight reproductive traits from 159 CO 2 enrichment papers that provided information on 79 species. • Across all species, CO 2 enrichment (500–800 µl l -1 ) resulted in more flowers (+19%), more fruits (+18%), more seeds (+16%), greater individual seed mass (+4%), greater total seed mass (+25%), and lower seed nitrogen concentration, (N) (-14%). Crops and undomesticated (wild) species did not differ in total mass response to elevated CO 2 (+31%), but crops allocated more mass to reproduction and produced more fruits (+28% vs +4%) and seeds (+21% vs +4%) than did wild species when grown at high CO 2 . Seed [N] was not affected by high CO 2 concentrations in legumes, but declined significantly in most nonlegumes. • Our results provide robust estimates of average plant reproductive responses to CO 2 enrichment and demonstrate important differences among individual taxa and among functional groups. In particular, crops were more responsive to elevated CO 2 than were wild species. These differences and the substantial decline in seed [N] in many species have broad implications for the functioning of future natural and agro-ecosystems. [ABSTRACT FROM AUTHOR]

Summary • The main aims of this paper are to investigate: if widespread lichen species ( Xanthoria parietina , Physconia grisea and Physcia adscendens , collected in Italy and Poland) exposed to cadmium (Cd), lead (Pb) and zinc (Zn) synthesize phytochelatins; whether phytochelatins are synthesized by the heterotrophic fungus (mycobiont) or by the photosynthetic alga (photobiont), or by both of the partners; and if there are significant differences in phytochelatin production in the same lichen species growing at different latitudes. • In the lichens investigated, Cd, Pb and Zn induced the biosynthesis of phytochelatins (PC 2 , PC 3 and PC 4 ) and some DES-GLYCIL derivatives. However, only the photobiont partners (the green microalgae of the genus Trebouxia ) were capable of phytochelatin production and, by contrast, aposymbiotically grown mycobionts only produced glutathione. • The Cd-exposed lichens grown in Italy produced significantly larger amounts of phytochelatins than the same species grown in Poland. An opposite trend was observed in Pb-exposed lichens. • The trebouxioid lichens may gain an ecological advantage from their capability to counter heavy metal stress with prompt phytochelatin synthesis. [ABSTRACT FROM AUTHOR]

Summary • Hyperaccumulator plants in the field show significant variation in the metal concentration in their aerial parts, but little is known of the causes of this variation. This paper investigates the role of soil zinc (Zn) concentration and genetic variation in causing between and within population variation in Zn accumulation in Arabidopsis halleri . • Seed from 17 populations of A. halleri collected in central Europe were grown under standard conditions at three external Zn concentrations and tested for Zn concentration in the leaves. • Between population variation was highest at low external zinc concentrations. At 10 µm Zn some plants had very low leaf Zn concentrations, and were indistinguishable from nonaccumulators. However, at higher Zn concentrations, all plants showed hyperaccumulation. There were no differences in the accumulating abilities of populations from sites with different degrees of contamination. • Heritability of accumulation, determined for individual families from three populations, was quite high (25–50%), indicating that selection for increased accumulating ability should be possible, although selection would be easier at low external Zn concentrations. The Zn concentration of field collected plants was affected partly by plant genotype but not by the total soil Zn around their roots. [ABSTRACT FROM AUTHOR]

Summary The aim of this paper is to survey the general area of quantitative trait locus (QTL) mapping, and its specific impact on current understanding of plant domestication. Plant domestication is not only of historical interest, but is also of ongoing importance as changing human needs and availability of nonrenewable resources impel continuing (and perhaps even accelerated) investigation of prospective new crops. New genomic tools applied in conjunction with now-established approaches such as QTL mapping are opening new doors into searches for the ‘footprints’ of domestication, and promise to accelerate and streamline the identification of specific genes integral to domestication(s), building on early successes. Better understanding of plant domestication promises to enhance knowledge about the developmental basis of some of the more striking evolutionary events known, to guide efforts to catalog plant biodiversity, and to accelerate progress in improving existing and new crops to sustain humanity. [ABSTRACT FROM AUTHOR]

Summary The mitogen-activated protein kinase (MAP kinase) signal transduction cascades are routes through which eukaryotic cells deliver extracellular messages to the cytosol and nucleus. These signalling pathways direct cell division, cellular differentiation, metabolism, and both biotic and abiotic stress responses. In plants, MAP kinases and the upstream components of the cascades are represented by multigene families, organized into different pathways which are stimulated and interact in complex ways. Experimental strategies for the analysis of MAP kinase cascades include the yeast two-hybrid system; using this approach in vitro interactions between specific MAP kinase cascade components have been analysed and putative plant cascades postulated. Transient transformation of protoplasts with epitope-tagged kinases has allowed cascades to be tested in planta. There is clear evidence for the involvement of MAP kinases in plant cell division and in the regulation of auxin signalling. Biotic (pathogens and pathogen-derived elicitors from fungi, bacteria and viruses) and abiotic stresses including wounding, mechanical stimulation, cold, drought and ozone can elicit defence responses in plants through MAP kinase pathways. There are data suggesting that ABA signalling utilizes a MAP kinase pathway, and probably ethylene and perhaps cytokinins do so also. The objective of this paper is to review this rapidly advancing field. [ABSTRACT FROM AUTHOR]

Summary • Translocation of nutrients through fungal mycelia has mainly been viewed in terms of unidirectional movement from sources to sinks. This paper describes an experiment designed to test the possibility of bidirectional phosphorus transport in mycelial cords of the white rot fungus Hypholoma fasciculare growing in soil microcosms. • A novel technique is described that uses the different energy spectra of the radioactive phosphorus isotopes 32 P and 33 P, in combination with nondestructive electronic autoradiography, to monitor the distribution of the two isotopes within single mycelial cord systems. The tracer isotopes were applied to pairs of wood blocks connected by mycelial cords, and isotope translocations were monitored during a 29-d period. • Significant amounts of phosphorus were transported in both directions simultaneously between the two colonized wood blocks. • Simultaneous, bidirectional translocation of phosphorus occurs in mycelial cords of H. fasciculare . Such circulation of nutrients in fungal mycelia should equalize differences in nutrient concentrations in different parts of the mycelia, enabling net translocation of nutrients from areas of high nutrient availability to those of high nutrient demand. [ABSTRACT FROM AUTHOR]

'The view that nutrient acquisition by most plants growing in natural ecosystems is mediated by mycorrhiza-forming symbiotic fungi is now largely accepted' (Read, 2000). Is this bold claim really true for the whole suite of mineral nutrients that plants require? The case is strongest for nutrients that are not very mobile in soil, especially when present in growth-limiting amounts, and phosphate (P) is the classic example. Arbuscular mycorrhizas are by far the most widespread mycorrhizal symbioses, and the ability of arbuscular mycorrhizal (AM) fungi to take up soil nutrients such as P and transfer them to the host plant is an area of intense research. However, there is great variation in the extent to which AM plants benefit in measurable terms from the symbiosis under a given set of environmental conditions, and a paper in this issue, by Koide et al., addresses this problem (Koide et al., pp. 163-168). The variability is especially apparent in the field, thus obscuring the possible roles of mycorrhizas in community structure and succession (Fitter, 1985; McGonigle, 1988). [ABSTRACT FROM AUTHOR]

Plant species that persist during succession, from the colonization to the stabilization stages, face major environmental changes. Such changes are believed to have significant effects on species performance. In subarctic coastal dune systems, Leymus mollis colonizes the embryo dunes, on the upper limit of the beach. It reaches its maximum density on the foredune, but also grows on older, stabilized ridges. This paper reports on the phenotypic variations of some ecophysiological traits associated with the persistence of L. mollis on a dune system on the east coast of Hudson Bay (northern Quebec). Leymus mollis ramets tend to have a lower net carbon assimilation rate and water use efficiency, and a higher substomatal CO2 concentration on the stabilized dune than on the foredune. However, these physiological differences cannot be explained by differences in leaf morphology or nitrogen content. Under controlled conditions, ecophysiological differences observed in the field disappear, suggesting that these are not genetic but determined by environmental changes along the foredune-stabilized dune gradient. We propose that higher net carbon assimilation rate on the foredune might be related to higher sink strength in relation to the growth-stimulating effect of sand burial. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The objectives of this paper were to review the literature on the responses of root systems to elevated CO2 in intact, native grassland ecosystems, and to present the results from a 2-yr study of root production and mortality in an intact calcareous grassland in Switzerland. Previous work in intact native grassland systems has revealed that significant stimulation of the size of root systems (biomass, length density or root number) is not a universal response to elevated CO2. Of the 12 studies reviewed, seven showed little or no change in root-system size under elevated CO2, while five showed marked increases (average increase 38%). Insufficient data are available on the effects of elevated CO2 on root production, mortality and life span to allow generalization about effects. The diversity of experimental techniques employed in these native grassland studies also makes generalization difficult. In the present study, root production and mortality were monitored in situ in a species-rich calcareous grassland community using minirhizotrons in order to test the hypothesis that an increase in these two measures would help explain the increase in net ecosystem CO2 uptake (net ecosystem exchange) previously observed under elevated CO2 at this site (600 vs 350 μl CO2 l−1; eight 1.2-m2 experimental plots per CO2 level using the screen-aided CO2 control method). However, results from the first 2 yr showed no difference in overall root production or mortality in the top 18 cm of soil, where 80-90% of the roots occur. Elevated CO2 was associated with an upward shift in root length density: under elevated CO2 a greater proportion of roots were found in the upper 0-6-cm soil layer, and a lower proportion of roots in the lower 12-18 cm, than under ambient CO2. Elevated CO2 was also associated with an increase in root survival probability (RSP; e.g. for roots still alive 280 d after they were produced under ambient CO2, RSP = 0.30; elevated CO2, RSP = 0.56) and an increase (48%) in median root life span in the deepest (12-18 cm) soil layer. The factors driving changes in root distribution and longevity with depth under elevated CO2 were not clear, but might have been related to increases in soil moisture under elevated CO2 interacting with vertical patterns in soil temperatures. Thus extra CO2 taken up in this grassland ecosystem during the growing season under elevated CO2 could not be explained by changes in root production and mortality. However, C and nutrient cycling might be shifted closer to the soil surface, which could potentially have a substantial effect on the activities of soil heterotrophic organisms as CO2 levels rise. [ABSTRACT FROM AUTHOR]

Results presented in this paper demonstrate the relationship between two data sets from Mubwindi Swamp, south-west Uganda. The first data set is form a survey of dryland montane forest within the immediate catchment and the second is in the form of pollen spectra recovered from surface samples of sediment. Numerical analysis grouped pollen types encountered into five categories, according to their level of representivity when compared with abundances of the same taxa in the sampled vegetation; under representative, moderately under representative, representative, moderately over representative, and over representative. The northern part of Mubwindi Swamp also yielded a 5-m-long sequence of organic-rich sediments (core MB6) from which sub-fossil pollen and radiocarbon data have been previously obtained. This provides an opportunity to apply calibration factors derived from modern pollen-vegetation relationships to sub-fossil pollen data that were obtained from the same site. These calibration factors effectively down-weighted the influence of over representative pollen types and permitted the discrimination of a phase of forest disturbance that had not previously been so clearly visible in sub-fossil pollen data. [ABSTRACT FROM AUTHOR]

In this paper we distinguish between invasive and noninvasive (finite) saprotrophic spread of the soil-borne fungal plant pathogen, Rhizoctonia soloni amongst discrete sites of nutrient resource. Using simple concepts of percolation theory, we predict the critical threshold distance, associated with a threshold probability, between donor (colonized) and recipient (uncolonized) nutrient sites at which R. solani can spread invasively by mycelial growth through a population of nutrient sites on a lattice. The critical distance for invasive spread is estimated from colonization profiles derived from placement experiments that summarize the probability of colonization with instance between replicated pairs of colonized and uncolonized sites. Colonization profiles were highly nonlinear, decaying sigmoidally with distance. Thresholds for invasive spread were predicted at inter-site distances of 8.1 mm and 11.8 mm for sites of low and high nutrient agar, respectively. In population experiments with inter-site distances below the predicted thresholds, the spread of the fungus was invasive in all replicates. At large distances (> 10mm for low, and > 14 mm for high nutrient sites) the spread of the fungus was always finite with the proportion of finite replicates decreasing sharply closet the percolation threshold. Invasive spread did not depend on the furthest extent of growth of the fungus but on distances predicted by the percolation thresholds. Invasive spread of the fungus is also examined in a more natural and variable, nonsterile system involving the growth and colonization of a lattice of poppy seeds over sand. The system is characterized by a decay in the probability of colonization between older poppy seeds, which effectively 'quenches' saprotrophic spread. Hence in the population experiments with poppy seeds all growth was ultimately finite. The threshold distance, corresponding to the critical percolation probability for invasive growth changed from 18 mm to 4 mm over 21 leading to a switch from invasive to finite growth. We conclude that percolation theory can be used to link the growth of individual mycelial colonies to the formation of patches that result from the colonization of particulate organic mater. The nonlinearity of the colonization profiles combined with the presence of a percolation threshold means that small changes in the distance between nutrient sites can results in large differences in final patch size. The rapid decay of particulate organic matter in a more natural system can have a profound effect on the dynamics of colonization, restricting saprotrophic invasion of the soil. The consequences of invasion thresholds for colony growth of saprotrophic and parasitic fungi in dynamical systems, are briefly discussed. [ABSTRACT FROM AUTHOR]

In previously reported laboratory experiments, infection of Rumex obtusifolius by the rust fungus Uromyces rumicis was decreased on leaves which had prior herbivory by the beetle Gastrophysa viridula. In this paper we investigate whether this interaction is robust for natural infection by a variety of fungi in field experiments carried out in spring and autumn with plants given different levels of nitrogen fertilization. Grazing by G. viridula led to a decrease in lesion density of Ramularia rubella and Venturia rumicis in the spring and V. rumicis and U. rumicis in the autumn experiment. For V. rumicis and U. rumicis significant reductions in lesion density occurred on the undamaged leaves of damaged plants, compared with similar leaves on undamaged plants, suggesting systemic induced resistance. This induced resistance was usually independent of the amount of nitrogen fertilization, although the inhibitory effect of grazing on R. rubella in the spring and V. rumicis in the autumn experiment was enhanced by increasing nitrogen fertilization and was inhibited by increasing nitrogen fertilization for V. rumicis in the spring. In both experiments, the lesion density of V. rumicis was greater on leaves on which R. rubella was also present, and the presence of U. Rumicis in the autumn experiment was linked to a similar but greater effect on V. rumicis lesion density. We found no evidence of induced resistance by fungi against fungi in these experiments. We highlight the complex interactions between inhibitory and facilitatory processes acting on leaf fungal infection. These results are compared with the proposed molecular mechanism of induced resistance(s) and we consider the benefits of closer integration between molecular and ecological investigations of induced resistances that occur in the field. [ABSTRACT FROM AUTHOR]

This paper reports the effects of nutrient magnesium (Mg) concentrations on the growth and photosynthetic physiology of clonal Pinus radiata from four female parents (families) known to differ in their tolerance to Mg deficiency and in their needle Mg concentrations. Plants were grown in flowing nutrient solutions with 2 mg l-1 (control) and 0.8 mg l-1 (low) Mg. Plant growth, needle Mg concentration, photosynthesis, chlorophyll fluorescence and carotenoid pigment content were measured. At low Mg, needle Mg concentration was about half that of control plants, height growth was reduced 15-25%, and the needles showed strong visual characteristics of Mg deficiency. Photosynthesis was also halved, and was associated with closure of the stomata under low Mg and with reductions in the residual conductance. In needles from plants grown at low Mg, photochemical yield was reduced both in the light and in the dark, and was strongly dependent on needle Mg concentrations below a threshold concentration of 0.02-0.025% (d. wt basis). The electron transport rate (ETR) at saturating photon flux density in low-Mg-grown needles was reduced to about half that of their Mg controls, but the photon efficiency of ETR was unaffected by the Mg concentration the plants were grown in. Photosynthetic quenching was markedly reduced and non-photosynthetic quenching was increased following growth in low Mg. Growth under low Mg also increased levels of zeaxanthin. Although family differences in growth and photosynthetic physiology were present, few family × Mg interactions were significant. We conclude that Mg deficiency probably affects growth through severe reductions in photosynthesis. [ABSTRACT FROM AUTHOR]

During the initial phases of elongation of pea internodes, oat and rice coleoptiles, oat mesocotyls, soybean hypocotyls and dandelion peduncles, net transverse orientation of cellulose wall microfibrils (Mfs) was found in the outer epidermal wall. This paper demonstrates that in all these axes, with the exception of rice coleoptile, net longitudinal orientation of microfibrils occurs in the outer epidermal wall in portions of the axes that were still elongating at the time of sampling. The timing of the transition to net longitudinal orientation and whether the transition proceeded acropetally or basipetally varied with the type of axis under study. The variability of the relationship between extension and the transition from net transverse to net longitudinal orientation suggests that factors other than extension are important in determining the transition. Layers of longitudinal wall microfibrils may be added to the extending epidermal wall to bolster its tensile strength commensurate with its function during and after extension. Attention is drawn to the parallels between the concept of tissue tension in growing axes and the concept that the epidermis functions as a stressed skin in the support of mature plant parts in primary growth. [ABSTRACT FROM AUTHOR]