Mistletoes and mutant albino shoots on woody plants as mineral nutrient traps

Background and Aims

Potassium, sulphur and zinc contents of mistletoe leaves are generally higher than in their hosts. This is attributed to the fact that chemical elements which are cycled between xylem and phloem in the process of phloem loading of sugars are trapped in the mistletoe, because these parasites do not feed their hosts. Here it is hypothesized that mutant albino shoots on otherwise green plants should behave similarly, because they lack photosynthesis and thus cannot recycle elements involved in sugar loading.

Methods

The mineral nutrition of the mistletoe Scurrula elata was compared with that of albino shoots on Citrus sinensis and Nerium oleander. The potential for selective nutrient uptake by the mistletoe was studied by comparing element contents of host leaves on infected and uninfected branches and by manipulation of the haustorium–shoot ratio in mistletoes. Phloem anatomy of albino leaves was compared with that of green leaves.

Key Results

Both mistletoes and albino leaves had higher contents of potassium, sulphur and zinc than hosts or green leaves, respectively. Hypothetical discrimination of nutrient elements during the uptake by the haustorium is not supported by our data. Anatomical studies of albino leaves showed characteristics of release phloem.

Conclusions

Both albino shoots and mistletoes are traps for elements normally recycled between xylem and phloem, because retranslocation of phloem mobile elements into the mother plant or the host is low or absent. It can be assumed that the lack of photosynthetic activity in albino shoots and thus of sugars needed in phloem loading is responsible for the accumulation of elements. The absence of phloem loading is reflected in phloem anatomy of these abnormal shoots. In mistletoes the evolution of a parasitic lifestyle has obviously eliminated substantial feeding of the host with photosynthates produced by the mistletoe.     

Linking water stress effects on carbon partitioning by introducing a xylem circuit into L-PEACH

Background and Aims

Many physiological processes such as photosynthesis, respiration and transpiration can be strongly influenced by the diurnal patterns of within-tree water potential. Despite numerous experiments showing the effect of water potential on fruit-tree development and growth, there are very few models combining carbohydrate allocation with water transport. The aim of this work was to include a xylem circuit into the functional–structural L-PEACH model.

Methods

The xylem modelling was based on an electrical circuit analogy and the Hagen–Poisseuille law for hydraulic conductance. Sub-models for leaf transpiration, soil water potential and the soil–plant interface were also incorporated to provide the driving force and pathway for water flow. The model was assessed by comparing model outputs to field measurements and published knowledge.

Key Results

The model was able to simulate both the water uptake over a season and the effect of different irrigation treatments on tree development, growth and fruit yield.

Conclusions

This work opens the way to a new field of modelling where complex interactions between water transport, carbohydrate allocation and physiological functions can be simulated at the organ level and describe functioning and behaviour at the tree scale.     

Changes in vascular and transpiration flows affect the seasonal and daily growth of kiwifruit (Actinidia deliciosa) berry

Background and Aims

The kiwifruit berry is characterized by an early stage of rapid growth, followed by a relatively long stage of slow increase in size. Vascular and transpiration flows are the main processes through which water and carbon enter/exit the fruit, determining the daily and seasonal changes in fruit size. This work investigates the biophysical mechanisms underpinning the change in fruit growth rate during the season.

Methods

The daily patterns of phloem, xylem and transpiration in/outflows have been determined at several stages of kiwifruit development, during two seasons. The different flows were quantified by comparing the diurnal patterns of diameter change of fruit, which were then girdled and subsequently detached while measurements continued. The diurnal courses of leaf and stem water potential and of fruit pressure potential were also monitored at different times during the season.

Key Results

Xylem and transpiration flows were high during the first period of rapid volume growth and sharply decreased with fruit development. Specific phloem import was lower and gradually decreased during the season, whereas it remained constant at whole-fruit level, in accordance with fruit dry matter gain. On a daily basis, transpiration always responded to vapour pressure deficit and contributed to the daily reduction of fruit hydrostatic pressure. Xylem flow was positively related to stem-to-fruit pressure potential gradient during the first but not the last part of the season, when xylem conductivity appeared to be reduced.

Conclusions

The fruit growth model adopted by this species changes during the season due to anatomical modifications in the fruit features.     

Plant traits and ecosystem effects of clonality: a new research agenda

Background

Clonal plants spread laterally by spacers between their ramets (shoot–root units); these spacers can transport and store resources. While much is known about how clonality promotes plant fitness, we know little about how different clonal plants influence ecosystem functions related to carbon, nutrient and water cycling.

Approach

The response–effect trait framework is used to formulate hypotheses about the impact of clonality on ecosystems. Central to this framework is the degree of correspondence between interspecific variation in clonal ‘response traits’ that promote plant fitness and interspecific variation in ‘effect traits’, which define a plant''s potential effect on ecosystem functions. The main example presented to illustrate this concept concerns clonal traits of vascular plant species that determine their lateral extension patterns. In combination with the different degrees of decomposability of litter derived from their spacers, leaves, roots and stems, these clonal traits should determine associated spatial and temporal patterns in soil organic matter accumulation, nutrient availability and water retention.

Conclusions

This review gives some concrete pointers as to how to implement this new research agenda through a combination of (1) standardized screening of predominant species in ecosystems for clonal response traits and for effect traits related to carbon, nutrient and water cycling; (2) analysing the overlap between variation in these response traits and effect traits across species; (3) linking spatial and temporal patterns of clonal species in the field to those for soil properties related to carbon, nutrient and water stocks and dynamics; and (4) studying the effects of biotic interactions and feedbacks between resource heterogeneity and clonality. Linking these to environmental changes may help us to better understand and predict the role of clonal plants in modulating impacts of climate change and human activities on ecosystem functions.     

Changes in the localization and levels of starch and lipids in cambium and phloem during cambial reactivation by artificial heating of main stems of Cryptomeria japonica trees

Background and Aims

Cambial reactivation in trees occurs from late winter to early spring when photosynthesis is minimal or almost non-existent. Reserve materials might be important for wood formation in trees. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules in cambium and phloem were examined from cambial dormancy to the start of xylem differentiation in locally heated stems of Cryptomeria japonica trees in winter.

Methods

Electric heating tape was wrapped on one side of the stem of Cryptomeria japonica trees at breast height in winter. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules were determined by image analysis of optical digital images obtained by confocal laser scanning microscopy.

Key Results

Localized heating induced earlier cambial reactivation and xylem differentiation in stems of Cryptomeria japonica, as compared with non-heated stems. There were clear changes in the respective localizations and levels of starch and lipids (as droplets) determined in terms of relative areas on images, from cambial dormancy to the start of xylem differentiation in heated stems. In heated stems, the levels and number of starch granules fell from cambial reactivation to the start of xylem differentiation. There was a significant decrease in the relative area occupied by lipid droplets in the cambium from cambial reactivation to the start of xylem differentiation in heated stems.

Conclusions

The results showed clearly that the levels and number of storage starch granules in cambium and phloem cells and levels of lipids (as droplets) in the cambium decreased from cambial reactivation to the start of xylem differentiation in heated stems during the winter. The observations suggest that starch and lipid droplets might be needed as sources of energy for the initiation of cambial cell division and the differentiation of xylem in Cryptomeria japonica.     

Morphology of nectaries and biology of nectar production in the distylous species Fagopyrum esculentum

Background and Aims

The mechanisms of floral nectar production in buckwheat (Fagopyrum esculentum, Polygonaceae), a distylous pseudo-cereal, have received relatively little attention, prompting an investigation of the factors that regulate this process. The aim was to perform a refined study of the structures that secrete nectar and of the internal and external parameters influencing nectar volumes and sugar concentrations.

Methods

In order to control environmental parameters, plants were cultivated in growth rooms under controlled conditions. The structure of nectaries was studied based on histological sections from flowers and flower buds. Nectar was extracted using glass micropipettes and the sugar concentration was measured with a hand refractometer. Sugar concentration in the phloem sap was measured using the anthrone method. To test the influence of photosynthesis on nectar production, different light and defoliation treatments were applied.

Key Results

Unicellular trichomes were located in the epidermis at the ventral part of eight nectary glands situated on the flower receptacle alternately with stamens. Vascular bundles consisting of both phloem and xylem were identified at the boundary between a multilayered nectary parenchyma and a sub-nectary parenchyma with chloroplasts. A higher volume of nectar in thrum morphs was observed. No other difference was found in morphology or in sugar supply to inflorescences between morphs. Nectar secretion was strongly influenced by plant age and inflorescence position. Nectar volumes were higher in the upper inflorescences and during the flowering peak. Light had a dual role, (1) acting directly on reproductive structures to trigger flower opening, which conditions nectar secretion, and (2) stimulating photosynthetic activity, which regulates nectar accumulation in open flowers.

Conclusions

In buckwheat, nectar is secreted by trichomes and probably proceeds, at least in part, from phloem sap. Nectar secretion is strongly influenced by floral morph type, plant age, inflorescence position and light.Key words: Buckwheat, distyly, Fagopyrum esculentum, inflorescence position, morph comparisons, nectary histology, nectar sugar concentration, nectar volume, light intensity, organ biomass, phloem sap, plant age     

A systems biology approach to dissect the contribution of brassinosteroid and auxin hormones to vascular patterning in the shoot of Arabidopsis thaliana

Systems biology can foster our understanding of hormonal regulation of plant vasculature. One such example is our recent study on the role of plant hormones brassinosteroids (BRs) and auxin in vascular patterning of Arabidopsis thaliana (Arabidopsis) shoots. By using a combined approach of mathematical modelling and molecular genetics, we have reported that auxin and BRs have complementary effects in the formation of the shoot vascular pattern. We proposed that auxin maxima, driven by auxin polar transport, position vascular bundles in the stem. BRs in turn modulate the number of vascular bundles, potentially by controlling cell division dynamics that enhance the number of provascular cells. Future interdisciplinary studies connecting vascular initiation at the shoot apex with the established vascular pattern in the basal part of the plant stem are now required to understand how and when the shoot vascular pattern emerges in the plant.Key words: Arabidopsis, vascular, auxin, brassinosteroids, mathematical model, computer simulationsThe plant vascular system is responsible for the long-distance transport of water, solutes and molecules throughout the plant, being essential for plant growth and development. It is formed by two different functional tissues: the xylem, which transports water from roots to aerial organs, and the phloem, through which nutrients and photosynthetic products and signaling molecules are transported.During embryogenesis, the vasculature is characterized as an undifferentiated procambial tissue in the innermost part of the plant embryo.¹ Later in development, the procambium (i.e., a group of pluripotent stem cells²) begins to divide and differentiate into xylem and phloem tissues through oriented cell divisions. In the shoot, procambium generates xylem tissue centripetally and phloem tissue centrifugally, driving the formation of collateral vascular bundles around it.^3,4 In the inflorescence stem of the model plant Arabidopsis, the radial pattern of the vasculature exhibits a periodic organization made by the alternation of vascular bundles and interfascicular fibers, which altogether form the vascular ring (Fig. 1A).Open in a separate windowFigure 1Vascular patterning in Arabidopsis shoot inflorescence stem. (A) Radial section of DR5::GUS expression at the base of the inflorescence stem in Arabidopsis Col-0 plants. (B) Computer simulation result for auxin concentration ([Auxin]) in arbitrary units (a.u.) along a ring of cells; x and y stand for spatial coordinates. Auxin is distributed in maxima which, according to the model hypothesis, position vascular bundles. (C) Longitudinal section of Arabidopsis Col-0 wild-type plant at the most apical zone, immediately below the shoot apical meristem. Arrows point to xylem strains coming from the lateral organs.Previous studies have documented the importance of plant hormones such as auxin and BRs in vascular cell differentiation and patterning.⁵ Defective polar auxin transport distorts shoot vascular patterning^6,7 and BR loss-of-function mutants exhibit few vascular bundles.^8,9 But how do these hormones control shoot vascular patterning? In order to answer this question, we used both quantitative measurements of vascular phenotypes and computational modeling.¹⁰     

The three-dimensional structure of vascular tissues in Agrobacterium tumefaciens-induced crown galls and in the host stems of Ricinus communis L.

The three-dimensional pattern of phloem and xylem in 10-d-to two-month-old tumors induced by Agrobacterium tumefaciens (C58) and in adjacent Ricinus communis L. stem tissues was studied in thick sections by clearing with lactic acid and by staining with lacmoid. The crown galls contained two types of vascular strands: treelike branched bundles, which developed towards the tumor surface in fast-growing regions, and globular bundles in the slowly developing parts. Both types of vascular bundles contained xylem and phloem and were continuous with the vascular system of the host plant. The tumor bundles were interconnected by a dense net of phloem anastomoses, consisting of sieve tubes but no vessels. These vascular patterns reflect the apparent synthesis sites, concentration gradients and flow pathways of the plant hormones additionally produced in the tumors upon expression of the T-DNA-encoded genes. The A. tumefaciens-induced crown gall affected vascular differentiation in the host stem. In the basipetal direction, the tumor induced more xylem differentiation directly below it, where the crown-gall bundles joined the vascular system of the host. In the centripetal direction, the crown gall caused the development of pathologic xylem characterized by narrow vessels, giant rays and absence of fibers. On the other hand, most probably as a consequence of its gibberellic acid content, the host plant stimulated a local differentiation of regenerative phloem and xylem fibers with unique ramifications, only at the base of the tumor. However, fibers were absent from the main body of the crown gall. The study shows that A. tumefaciens-induced crown galls are characterized by a sophisticated network of vascular tissues in the tumor and are accompanied by a perturbated vessel system in the host. The hormonal mechanisms controlling vascular differentiation in the tumor and neighboring host tissues are discussed. In addition, the gall constriction hypothesis is proposed for explaining the mechanism which gives priority in water supply to the growing gall over the host shoot.We thank Dr. Zs. Koncz (Max-Planck-Institut für Züchtungsforschung, Köln, Germany) for Agrobacterium strains and the Deutsche Forschungsgemeinschaft (SFB 199) for financial support to C.I.U.     

Xylem structure of four grape varieties and 12 alternative hosts to the xylem-limited bacterium Xylella fastidious

Background and Aims

The bacterium Xylella fastidiosa (Xf), responsible for Pierce''s disease (PD) of grapevine, colonizes the xylem conduits of vines, ultimately killing the plant. However, Vitis vinifera grapevine varieties differ in their susceptibility to Xf and numerous other plant species tolerate Xf populations without showing symptoms. The aim of this study was to examine the xylem structure of grapevines with different susceptibilities to Xf infection, as well as the xylem structure of non-grape plant species that support or limit movement of Xf to determine if anatomical differences might explain some of the differences in susceptibility to Xf.

Methods

Air and paint were introduced into leaves and stems to examine the connectivity between stem and leaves and the length distribution of their vessels. Leaf petiole and stem anatomies were studied to determine the basis for the free or restricted movement of Xf into the plant.

Key Results

There were no obvious differences in stem or petiole vascular anatomy among the grape varieties examined, nor among the other plant species that would explain differences in resistance to Xf. Among grape varieties, the more tolerant ‘Sylvaner’ had smaller stem vessel diameters and 20 % more parenchyma rays than the other three varieties. Alternative hosts supporting Xf movement had slightly longer open xylem conduits within leaves, and more connection between stem and leaves, when compared with alternative hosts that limit Xf movement.

Conclusions

Stem–leaf connectivity via open xylem conduits and vessel length is not responsible for differences in PD tolerance among grape varieties, or for limiting bacterial movement in the tolerant plant species. However, it was found that tolerant host plants had narrower vessels and more parenchyma rays, possibly restricting bacterial movement at the level of the vessels. The implications of xylem structure and connectivity for the means and regulation of bacterial movement are discussed.     

Defence on demand: mechanisms behind optimal defence patterns

Background

The optimal defence hypothesis (ODH) predicts that tissues that contribute most to a plant''s fitness and have the highest probability of being attacked will be the parts best defended against biotic threats, including herbivores. In general, young sink tissues and reproductive structures show stronger induced defence responses after attack from pathogens and herbivores and contain higher basal levels of specialized defensive metabolites than other plant parts. However, the underlying physiological mechanisms responsible for these developmentally regulated defence patterns remain unknown.

Scope

This review summarizes current knowledge about optimal defence patterns in above- and below-ground plant tissues, including information on basal and induced defence metabolite accumulation, defensive structures and their regulation by jasmonic acid (JA). Physiological regulations underlying developmental differences of tissues with contrasting defence patterns are highlighted, with a special focus on the role of classical plant growth hormones, including auxins, cytokinins, gibberellins and brassinosteroids, and their interactions with the JA pathway. By synthesizing recent findings about the dual roles of these growth hormones in plant development and defence responses, this review aims to provide a framework for new discoveries on the molecular basis of patterns predicted by the ODH.

Conclusions

Almost four decades after its formulation, we are just beginning to understand the underlying molecular mechanisms responsible for the patterns of defence allocation predicted by the ODH. A requirement for future advances will be to understand how developmental and defence processes are integrated.     

Ontogenetic tissue modification in Malus fruit peduncles: the role of sclereids

Background and Aims

Apple (Malus) fruit peduncles are highly modified stems with limited secondary growth because fruit ripening lasts only one season. They must reliably connect rather heavy fruits to the branch and cope with increasing fruit weight, which induces dynamic stresses under oscillating wind loads. This study focuses on tissue modification of these small, exposed structures during fruit development.

Methods

A combination of microscopic, static and dynamic mechanical tests, as well as Raman spectroscopy, was used to study structure–function relationships in peduncles of one cultivar and 12 wild species, representatively chosen from all sections of the genus Malus. Tissue differentiation and ontogenetic changes in mechanical properties of Malus peduncles were observed throughout one growing season and after successive removal of tissues.

Key Results

Unlike in regular stems, the vascular cambium produces mainly phloem during secondary growth. Hence, in addition to a reduced xylem, all species developed a centrally arranged sclerenchyma ring composed of fibres and brachysclereids. Based on differences in cell-wall thickness, and proportions and arrangement of sclereids, two types of peduncle construction could be distinguished. Fibres provide an increased maximum tensile strength and contribute most to the overall axial rigidity of the peduncles. Sclereids contribute insignificantly to peduncle strength; however, despite being shown to have a lower elastic modulus than fibres, they are the most effective tissue in stiffening peduncles against bending.

Conclusions

The experimental data revealed that sclereids originating from cortical parenchyma act as ‘accessory’ cells to enhance proportions of sclerenchyma during secondary growth in peduncles. The mechanism can be interpreted as an adaptation to continuously increasing fruit loads. Under oscillating longitudinal stresses, sclereids may be regarded as regulating elements between maintenance of stiffness and viscous damping, the latter property being attributed to the cortical parenchyma.     

Tree girdling responses simulated by a water and carbon transport model

Background and Aims

Girdling, or the removal of a strip of bark around a tree''s outer circumference, is often used to study carbon relationships, as it triggers several carbon responses which seem to be interrelated.

Methods

An existing plant model describing water and carbon transport in a tree was used to evaluate the mechanisms behind the girdling responses. Therefore, the (un)loading functions of the original model were adapted and became a function of the phloem turgor pressure.

Key Results

The adapted model successfully simulated the measured changes in stem growth induced by girdling. The model indicated that the key driving variables for the girdling responses were changes in turgor pressure due to local changes in sugar concentrations. Information about the local damage to the phloem system was transferred to the other plant parts (crown and roots) by a change in phloem pressure. After girdling, the loading rate was affected and corresponded to the experimentally observed feedback inhibition. In addition, the unloading rate decreased after girdling and even reversed in some instances. The model enabled continuous simulation of changes in starch content, although a slight underestimation was observed compared with measured values.

Conclusions

For the first time a mechanistic plant model enabled simulation of tree girdling responses, which have thus far only been experimentally observed and fragmentally reported in literature. The close agreement between measured and simulated data confirms the underlying mechanisms introduced in the model.     

Orientation of vascular cell divisions in Arabidopsis

Orientation of cell division is essential for plant development as the direction of growth is determined by the direction of cell expansion and orientation of cell division. We have demonstrated that cell division orientation in vascular tissue is regulated by the interactions between a receptor kinase (PXY) expressed in dividing cells and its peptide ligand (CLE41) that is localized to adjacent phloem cells. Given that other receptor kinases have been identified as orienting the cell division plane in several developmental processes, we suggest that localized signaling from adjacent cells may be a general mechanism for defining the plane of cell division.Key words: xylem, phloem, cell division orientation, procambium, cambiumThroughout the life of plants, new organs are generated from meristems which contain stem cells at their center. Meristematic cells divide in regulated processes resulting in displacement of daughter cells to the periphery of the meristem where they differentiate, taking on new cell identities.¹ Vascular meristems (cambium and procambium) are responsible for radial growth and are the main source of plant biomass.² Their regulation has a come under increasing scrutiny as biomass is likely to play an increasing role in generation of renewable energy.³Arabidopsis vascular tissue is organized into discrete collateral bundles in stems,⁴ whereas in hypocotyls, vasculature forms in a continuous ring, much like that of trees.⁵ In both cases spatially separated xylem and phloem are formed along the stem mediolateral axis and are populated with cells derived from the procambium or cambium (Fig. 1). Vascular initials displaced from the meristematic zone towards the center of the stem differentiate into xylem whereas those displaced towards the outside of the stem differentiate into phloem. This organization occurs because vascular meristematic cells are long and thin and divide periclinally down their long axis, perpendicular to the mediolateral axis. Because these are highly ordered divisions, vascular tissue is characterized by long files of cells. Until recently regulatory factors which influence the highly ordered nature of these divisions—and therefore plant vascular tissue organization were entirely unknown.Open in a separate windowFigure 1Arabidopsis vascular tissue at the base of inflorescence stems (A) and hypocotyls (B). The mediolateral axes are marked with arrows, x is xylem, ph is phloem, pc is procambium, c is cambium. Scale bars are 50 µm.     

Differences in membrane selectivity drive phloem transport to the apoplast from which maize florets develop

Background and Aims

Floral development depends on photosynthetic products delivered by the phloem. Previous work suggested the path to the flower involved either the apoplast or the symplast. The objective of the present work was to determine the path and its mechanism of operation.

Methods

Maize (Zea mays) plants were grown until pollination. For simplicity, florets were harvested before fertilization to ensure that all tissues were of maternal origin. Because sucrose from phloem is hydrolysed to glucose on its way to the floret, the tissues were imaged and analysed for glucose using an enzyme-based assay. Also, carboxyfluorescein diacetate was fed to the stems and similarly imaged and analysed.

Key Results

The images of live sections revealed that phloem contents were released to the pedicel apoplast below the nucellus of the florets. Glucose or carboxyfluorescein were detected and could be washed out. For carboxyfluorescein, the plasma membranes of the phloem parenchyma appeared to control the release. After release, the nucellus absorbed apoplast glucose selectively, rejecting carboxyfluorescein.

Conclusions

Despite the absence of an embryo, the apoplast below the nucellus was a depot for phloem contents, and the strictly symplast path is rejected. Because glucose and carboxyfluorescein were released non-selectively, the path to the floret resembled the one later when an embryo is present. The non-selective release indicates that turgor at phloem termini cannot balance the full osmotic potential of the phloem contents and would create a downward pressure gradient driving bulk flow toward the sink. Such a gradient was previously measured by Fisher and Cash-Clark in wheat. At the same time, selective absorption from the apoplast by the nucellar membranes would support full turgor in this tissue, isolating the embryo sac from the maternal plant. The isolation should continue later when an embryo develops.     

Using virtual 3-D plant architecture to assess fungal pathogen splash dispersal in heterogeneous canopies: a case study with cultivar mixtures and a non-specialized disease causal agent

Background and Aims

Recent developments in plant disease management have led to a growing interest in alternative strategies, such as increasing host diversity and decreasing the use of pesticides. Use of cultivar mixtures is one option, allowing the spread of plant epidemics to be slowed down. As dispersal of fungal foliar pathogens over short distances by rain-splash droplets is a major contibutor to the spread of disease, this study focused on modelling the physical mechanisms involved in dispersal of a non-specialized pathogen within heterogeneous canopies of cultivar mixtures, with the aim of optimizing host diversification at the intra-field level.

Methods

Virtual 3-D wheat-like plants (Triticum aestivum) were used to consider interactions between plant architecture and disease progression in heterogeneous canopies. A combined mechanistic and stochastic model, taking into account splash droplet dispersal and host quantitative resistance within a 3-D heterogeneous canopy, was developed. It consists of four sub-models that describe the spatial patterns of two cultivars within a complex canopy, the pathway of rain-splash droplets within this canopy, the proportion of leaf surface area impacted by dispersal via the droplets and the progression of disease severity after each dispersal event.

Key Results

Different spatial organization, proportions and resistance levels of the cultivars of two-component mixtures were investigated. For the eight spatial patterns tested, the protective effect against disease was found to vary by almost 2-fold, with the greatest effect being obtained with the smallest genotype unit area, i.e. the ground area occupied by an independent unit of the host population that is genetically homogeneous. Increasing both the difference between resistance levels and the proportion of the most resistant cultivar often resulted in a greater protective effect; however, this was not observed for situations in which the most resistant of the two cultivars in the mixture had a relatively low level of resistance.

Conclusions

The results show agreement with previous data obtained using experimental approaches. They demonstrate that in order to maximize the potential mixture efficiency against a splash-dispersed pathogen, optimal susceptible/resistant cultivar proportions (ranging from 1/9 to 5/5) have to be established based on host resistance levels. The results also show that taking into account dispersal processes in explicit 3-D plant canopies can be a key tool for investigating disease progression in heterogeneous canopies such as cultivar mixtures.     

Water stress-induced xylem hydraulic failure is a causal factor of tree mortality in beech and poplar

Background and Aims

Extreme water stress episodes induce tree mortality, but the physiological mechanisms causing tree death are still poorly understood. This study tests the hypothesis that a potted tree''s ability to survive extreme monotonic water stress is determined by the cavitation resistance of its xylem tissue.

Methods

Two species were selected with contrasting cavitation resistance (beech and poplar), and potted juvenile trees were exposed to a range of water stresses, causing up to 100 % plant death.

Key Results

The lethal dose of water stress, defined as the xylem pressure inducing 50 % mortality, differed sharply across species (1·75 and 4·5 MPa in poplar and beech, respectively). However, the relationships between tree mortality and the degree of cavitation in the stems were similar, with mortality occurring suddenly when >90 % cavitation had occurred.

Conclusions

Overall, the results suggest that cavitation resistance is a causal factor of tree mortality under extreme drought conditions.     

A local dormancy cline is related to the seed maturation environment,population genetic composition and climate

Background and Aims

Seed dormancy varies within species in response to climate, both in the long term (through ecotypes or clines) and in the short term (through the influence of the seed maturation environment). Disentangling both processes is crucial to understand plant adaptation to environmental changes. In this study, the local patterns of seed dormancy were investigated in a narrow endemic species, Centaurium somedanum, in order to determine the influence of the seed maturation environment, population genetic composition and climate.

Methods

Laboratory germination experiments were performed to measure dormancy in (1) seeds collected from different wild populations along a local altitudinal gradient and (2) seeds of a subsequent generation produced in a common garden. The genetic composition of the original populations was characterized using intersimple sequence repeat (ISSR) PCR and principal co-ordinate analysis (PCoA), and its correlation with the dormancy patterns of both generations was analysed. The effect of the local climate on dormancy was also modelled.

Key Results

An altitudinal dormancy cline was found in the wild populations, which was maintained by the plants grown in the common garden. However, seeds from the common garden responded better to stratification, and their release from dormancy was more intense. The patterns of dormancy variation were correlated with genetic composition, whereas lower temperature and summer precipitation at the population sites predicted higher dormancy in the seeds of both generations.

Conclusions

The dormancy cline in C. somedanum is related to a local climatic gradient and also corresponds to genetic differentiation among populations. This cline is further affected by the weather conditions during seed maturation, which influence the receptiveness to dormancy-breaking factors. These results show that dormancy is influenced by both long-and short-term climatic variation. Such processes at such a reduced spatial scale highlight the potential of plants to adapt to fast environmental changes.     

Comparative glycan profiling of Ceratopteris richardii ‘C-Fern’ gametophytes and sporophytes links cell-wall composition to functional specialization

Background and Aims

Innovations in vegetative and reproductive characters were key factors in the evolutionary history of land plants and most of these transformations, including dramatic changes in life cycle structure and strategy, necessarily involved cell-wall modifications. To provide more insight into the role of cell walls in effecting changes in plant structure and function, and in particular their role in the generation of vascularization, an antibody-based approach was implemented to compare the presence and distribution of cell-wall glycan epitopes between (free-living) gametophytes and sporophytes of Ceratopteris richardii ‘C-Fern’, a widely used model system for ferns.

Methods

Microarrays of sequential diamino-cyclohexane-tetraacetic acid (CDTA) and NaOH extractions of gametophytes, spores and different organs of ‘C-Fern’ sporophytes were probed with glycan-directed monoclonal antibodies. The same probes were employed to investigate the tissue- and cell-specific distribution of glycan epitopes.

Key Results

While monoclonal antibodies against pectic homogalacturonan, mannan and xyloglucan widely labelled gametophytic and sporophytic tissues, xylans were only detected in secondary cell walls of the sporophyte. The LM5 pectic galactan epitope was restricted to sporophytic phloem tissue. Rhizoids and root hairs showed similarities in arabinogalactan protein (AGP) and xyloglucan epitope distribution patterns.

Conclusions

The differences and similarities in glycan cell-wall composition between ‘C-Fern’ gametophytes and sporophytes indicate that the molecular design of cell walls reflects functional specialization rather than genetic origin. Glycan epitopes that were not detected in gametophytes were associated with cell walls of specialized tissues in the sporophyte.     

A novel insight into the structure of amphivasal secondary bundles on the example of Dracaena draco L. stem

Key message

Pattern of tracheids found along the bundles extends understanding of their cross - sectional anatomy and sheds a new light on the issue of radial transport in monocotyledons with secondary growth.

Abstract

Secondary growth of Dracaena draco L. stem is connected with the formation of amphivasal vascular bundles in which a centrally located phloem is surrounded by a ring of xylem cells (tracheids). However, as visible in a single transverse section, there is a tendency towards variation among the secondary bundles from such with a xylem ring to ones in which the tracheids do not completely surround the phloem, i.e., are separated by vascular parenchyma cells. We aimed to elucidate the cross-sectional anatomy of amphivasal secondary bundles using the method of serial sectioning (with sections 3 μm thick), which allowed us to follow very precisely the bundle structure along its length. The analysis revealed that the xylem arrangement in these bundles depends on the position of a section in the bundle path. Each amphivasal bundle is composed of sectors where tracheids form a ring, as well as of such where tracheids are separated by vascular parenchyma cells. We hypothesize that this structure of amphivasal vascular bundles facilitates radial transport of assimilates to the sink tissues. The result of the anatomical analysis is discussed in a physiological context.