Recovery dynamics of growth, photosynthesis and carbohydrate accumulation after de-submergence: a comparison between two wetland plants showing escape and quiescence strategies

Background and Aims

The capacity for fast-growth recovery after de-submergence is important for establishment of riparian species in a water-level-fluctuation zone. Recovery patterns of two wetland plants, Alternanthera philoxeroides and Hemarthria altissima, showing ‘escape’ and ‘quiescence’ responses, respectively, during submergence were investigated.

Methods

Leaf and root growth and photosynthesis were monitored continuously during 10 d of recovery following 20 d of complete submergence. Above- and below-ground dry weights, as well as carbohydrate concentrations, were measured several times during the experiment.

Key Results

Both species remobilized stored carbohydrate during submergence. Although enhanced internode elongation depleted the carbohydrate storage in A. philoxeroides during submergence, this species resumed leaf growth 3 d after de-submergence concomitant with restoration of the maximal photosynthetic capacity. In contrast, some sucrose was conserved in shoots of H. altissima during submergence, which promoted rapid re-growth of leaves 2 d after de-submergence and earlier than the full recovery of photosynthesis. The recovery of root growth was delayed by 1–2 d compared with leaves in both species.

Conclusions

Submergence tolerance of the escape and quiescence strategies entails not only the corresponding regulation of growth, carbohydrate catabolism and energy metabolism during submergence but also co-ordinated recovery of photosynthesis, growth and carbohydrate partitioning following de-submergence.     

Endodermal cell-cell contact is required for the spatial control of Casparian band development in Arabidopsis thaliana

Background and Aims

Apoplasmic barriers in plants fulfil important roles such as the control of apoplasmic movement of substances and the protection against invasion of pathogens. The aim of this study was to describe the development of apoplasmic barriers (Casparian bands and suberin lamellae) in endodermal cells of Arabidopsis thaliana primary root and during lateral root initiation.

Methods

Modifications of the endodermal cell walls in roots of wild-type Landsberg erecta (Ler) and mutants with defective endodermal development – scarecrow-3 (scr-3) and shortroot (shr) – of A. thaliana plants were characterized by light, fluorescent, confocal laser scanning, transmission and cryo-scanning electron microscopy.

Key Results

In wild-type plant roots Casparian bands initiate at approx. 1600 µm from the root cap junction and suberin lamellae first appear on the inner primary cell walls at approx. 7000–8000 µm from the root apex in the region of developing lateral root primordia. When a single cell replaces a pair of endodermal and cortical cells in the scr-3 mutant, Casparian band-like material is deposited ectopically at the junction between this ‘cortical’ cell and adjacent pericycle cells. Shr mutant roots with an undeveloped endodermis deposit Casparian band-like material in patches in the middle lamellae of cells of the vascular cylinder. Endodermal cells in the vicinity of developing lateral root primordia develop suberin lamellae earlier, and these are thicker, compared wih the neighbouring endodermal cells. Protruding primordia are protected by an endodermal pocket covered by suberin lamellae.

Conclusions

The data suggest that endodermal cell–cell contact is required for the spatial control of Casparian band development. Additionally, the endodermal cells form a collet (collar) of short cells covered by a thick suberin layer at the base of lateral root, which may serve as a barrier constituting a ‘safety zone’ protecting the vascular cylinder against uncontrolled movement of water, solutes or various pathogens.     

Genotypic differences in pod wall and seed growth relate to invertase activities and assimilate transport pathways in asparagus bean

Background and Aims

Coordination of sugar transport and metabolism between developing seeds and their enclosing fruit tissues is little understood. In this study the physiological mechanism is examined using two genotypes of asparagus bean (Vigna unguiculata ssp. sesquipedialis) differing in pod wall and seed growth rates. Pod growth dominates over seed growth in genotype ‘Zhijiang 121’ but not in ‘Zhijiang 282’ in which a ‘bulging pod’ phenotype is apparent from 8 d post-anthesis (dpa) onward.

Methods

Seed and pod wall growth rates and degree of pod-bulging were measured in the two genotypes together with assays of activities of sucrose-degrading enzymes and sugar content in pod wall and seed and evaluation of cellular pathways of phloem unloading in seed coat using a symplasmic fluorescent dye, 5(6)-carboxyfluorescein (CF).

Key Results

Activities of cell wall, cytoplasmic and vacuolar invertases (CWIN, CIN and VIN) were significantly smaller in pod walls of ‘282’ than in ‘121’ at 10 dpa onwards. Low INV activities were associated with weak pod wall growth of ‘282’. In seed coats, CF was confined within the vasculature in ‘282’ but moved beyond the vasculature in ‘121’, indicating apoplasmic and symplasmic phloem unloading, respectively. Higher CWIN activity in ‘282’ seed coats at 6–8 dpa correlated with high hexose concentration in embryos and enhanced early seed growth. However, CWIN activity in ‘282’ decreased significantly compared with ‘121’ from 10 dpa onwards, coinciding with earlier commencement of nuclei endoreduplication in their embryos.

Conclusions

The study shows genotypic differences between ‘bulging pod’ and ‘non-bulging’ phenotypes of asparagus bean in sucrose metabolism in relation to the pathway of phloem unloading in developing seed coats, and to pod and seed growth. Low INV activity in pod wall corresponds to its shortened and weak growth period; by contrast, the apoplasmic path in the seed coat is associated with high CWIN activity and strong early seed growth.     

Juvenility and flowering of Brunonia australis (Goodeniaceae) and Calandrinia sp. (Portulacaceae) in relation to vernalization and daylength

Background and Aims

The time at which plants are transferred to floral inductive conditions affects the onset of flowering and plant morphology, due to juvenility. Plants of Brunonia australis and Calandrinia sp. were used to investigate whether Australian native ephemeral species show a distinct juvenile phase that can be extended to increase vegetative growth and flowering.

Methods

The juvenile phase was quantified by transferring seedlings from less inductive (short day and 30/20°C) to inductive (vernalization or long day) conditions at six different plant ages ranging from 4 to 35 d after seed germination. An increase in days to first visible floral bud and leaf number were used to signify the end of juvenility.

Key Results

Brunonia australis was receptive to floral inductive long day conditions about 18–22 d after seed germination, whereas plants aged 4–35 d appeared vernalization sensitive. Overall, transferring plants of B. australis from short to long day conditions reduced the time to anthesis compared with vernalization or constant short day conditions. Calandrinia sp. showed a facultative requirement for vernalization and an insensitive phase was not detected. Floral bud and branch production increased favourably as plant age at time of transfer to inductive conditions increased. Younger plants showed the shortest crop production time.

Conclusions

Both species can perceive the vernalization floral stimulus from a very young age, whereas the photoperiodic stimulus is perceived by B. australis after a period of vegetative growth. However, extending the juvenile phase can promote foliage development and enhance flower production of both species.     

Historical and contemporary gene dispersal in wild carrot (Daucus carota ssp. carota) populations

Background and Aims

Wild carrot is the ancestor of cultivated carrot and is the most important gene pool for carrot breeding. Transgenic carrot may be released into the environment in the future. The aim of the present study was to determine how far a gene can disperse in wild carrot populations, facilitating risk assessment and management of transgene introgression from cultivated to wild carrots and helping to design sampling strategies for germplasm collections.

Methods

Wild carrots were sampled from Meijendel and Alkmaar in The Netherlands and genotyped with 12 microsatellite markers. Spatial autocorrelation analyses were used to detect spatial genetic structures (SGSs). Historical gene dispersal estimates were based on an isolation by distance model. Mating system and contemporary pollen dispersal were estimated using 437 offspring of 20 mothers with different spatial distances and a correlated paternity analysis in the Meijendel population.

Key Results

Significant SGSs are found in both populations and they are not significantly different from each other. Combined SGS analysis indicated significant positive genetic correlations up to 27 m. Historical gene dispersal σ_g and neighbourhood size N_b were estimated to be 4–12 m [95 % confidence interval (CI): 3–25] and 42–73 plants (95 % CI: 28–322) in Meijendel and 10–31 m (95 % CI: 7–∞) and 57–198 plants (95 % CI: 28–∞) in Alkmaar with longer gene dispersal in lower density populations. Contemporary pollen dispersal follows a fat-tailed exponential-power distribution, implying pollen of wild carrots could be dispersed by insects over long distance. The estimated outcrossing rate was 96 %.

Conclusions

SGSs in wild carrots may be the result of high outcrossing, restricted seed dispersal and long-distance pollen dispersal. High outcrossing and long-distance pollen dispersal suggest high frequency of transgene flow might occur from cultivated to wild carrots and that they could easily spread within and between populations.     

Dichogamy correlates with outcrossing rate and defines the selfing syndrome in the mixed-mating genus Collinsia

Background and Aims

How and why plants evolve to become selfing is a long-standing evolutionary puzzle. The transition from outcrossing to highly selfing is less well understood in self-compatible (SC) mixed-mating (MM) species where potentially subtle interactions between floral phenotypes and the environment are at play. We examined floral morphological and developmental traits across an entire SC MM genus, Collinsia, to determine which, if any, predict potential autonomous selfing ability when pollinators are absent (AS) and actual selfing rates in the wild, s_m, and to best define the selfing syndrome for this clade.

Methods

Using polymorphic microsatellite markers, we obtained 30 population-level estimates of s_m across 19 Collinsia taxa. Species grand means for the timing of herkogamy (stigma–anther contact) and dichogamy (stigmatic receptivity, SR), AS, floral size, longevity and their genetic correlations were quantified for 22 taxa.

Key Results

Species fell into discrete selfing and outcrossing groups based on floral traits. Loss of dichogamy defines Collinsia''s selfing syndrome. Floral size, longevity and herkogamy also differ significantly between these groups. Most taxa have high AS rates (>80 %), but AS is uncorrelated with any measured trait. In contrast, s_m is significantly correlated only with SR. High variance in s_m was observed in the two groups.

Conclusions

Collinsia species exhibit clear morphological and developmental traits diagnostic of ‘selfing’ or ‘outcrossing’ groups. However, many species in both the ‘selfing’ and the ‘outcrossing’ groups were MM, pointing to the critical influence of the pollination environment, the timing of AS and outcross pollen prepotency on s_m. Flower size is a poor predictor of Collinsia species'' field selfing rates and this result may apply to many SC species. Assessment of the variation in the pollination environment, which can increase selfing rates in more ‘outcrossing’ species but can also decrease selfing rates in more ‘selfing’ species, is critical to understanding mating system evolution of SC MM taxa.     

An efficient method for transient gene expression in monocots applied to modify the Brachypodium distachyon cell wall

Background

Agrobacterium-mediated transformation is widely used to produce insertions into plant genomes. There are a number of well-developed Agrobacterium-mediated transformation methods for dicotyledonous plants, but there are few for monocotyledonous plants.

Methods

Three hydrolase genes were transiently expressed in Brachypodium distachyon plants using specially designed vectors that express the gene product of interest and target it to the plant cell wall. Expression of functional hydrolases in genotyped plants was confirmed using western blotting, activity assays, cell wall compositional analysis and digestibility tests.

Key Results

An efficient, new, Agrobacterium-mediated approach was developed for transient gene expression in the grass B. distachyon, using co-cultivation of mature seeds with bacterial cells. This method allows transformed tissues to be obtained rapidly, within 3–4 weeks after co-cultivation. Also, the plants carried transgenic tissue and maintained transgenic protein expression throughout plant maturation. The efficiency of transformation was estimated at around 5 % of initially co-cultivated seeds. Application of this approach to express three Aspergillus nidulans hydrolases in the Brachypodium cell wall successfully confirmed its utility and resulted in the expected expression of active microbial proteins and alterations of cell wall composition. Cell wall modifications caused by expression of A. nidulans α-arabinofuranosidase and α-galactosidase increased the biodegradability of plant biomass.

Conclusions

This newly developed approach is a quick and efficient technique for expressing genes of interest in Brachypodium plants, which express the gene product throughout development. In the future, this could be used for broad functional genomics studies of monocots and for biotechnological applications, such as plant biomass modification for biofuel production.     

Aquatic adventitious root development in partially and completely submerged wetland plants Cotula coronopifolia and Meionectes brownii

Background and Aims

A common response of wetland plants to flooding is the formation of aquatic adventitious roots. Observations of aquatic root growth are widespread; however, controlled studies of aquatic roots of terrestrial herbaceous species are scarce. Submergence tolerance and aquatic root growth and physiology were evaluated in two herbaceous, perennial wetland species Cotula coronopifolia and Meionectes brownii.

Methods

Plants were raised in large pots with ‘sediment’ roots in nutrient solution and then placed into individual tanks and shoots were left in air or submerged (completely or partially). The effects on growth of aquatic root removal, and of light availability to submerged plant organs, were evaluated. Responses of aquatic root porosity, chlorophyll and underwater photosynthesis, were studied.

Key Results

Both species tolerated 4 weeks of complete or partial submergence. Extensive, photosynthetically active, aquatic adventitious roots grew from submerged stems and contributed up to 90 % of the total root dry mass. When aquatic roots were pruned, completely submerged plants grew less and had lower stem and leaf chlorophyll a, as compared with controls with intact roots. Roots exposed to the lowest PAR (daily mean 4·7 ± 2·4 µmol m⁻² s⁻¹) under water contained less chlorophyll, but there was no difference in aquatic root biomass after 4 weeks, regardless of light availability in the water column (high PAR was available to all emergent shoots).

Conclusions

Both M. brownii and C. coronopifolia responded to submergence with growth of aquatic adventitious roots, which essentially replaced the existing sediment root system. These aquatic roots contained chlorophyll and were photosynthetically active. Removal of aquatic roots had negative effects on plant growth during partial and complete submergence.     

Phenology and growth adjustments of oil palm (Elaeis guineensis) to photoperiod and climate variability

Background and Aims

Oil palm flowering and fruit production show seasonal maxima whose causes are unknown. Drought periods confound these rhythms, making it difficult to analyse or predict dynamics of production. The present work aims to analyse phenological and growth responses of adult oil palms to seasonal and inter-annual climatic variability.

Methods

Two oil palm genotypes planted in a replicated design at two sites in Indonesia underwent monthly observations during 22 months in 2006–2008. Measurements included growth of vegetative and reproductive organs, morphology and phenology. Drought was estimated from climatic water balance (rainfall – potential evapotranspiration) and simulated fraction of transpirable soil water. Production history of the same plants for 2001–2005 was used for inter-annual analyses.

Key Results

Drought was absent at the equatorial Kandista site (0°55′N) but the Batu Mulia site (3°12′S) had a dry season with variable severity. Vegetative growth and leaf appearance rate fluctuated with drought level. Yield of fruit, a function of the number of female inflorescences produced, was negatively correlated with photoperiod at Kandista. Dual annual maxima were observed supporting a recent theory of circadian control. The photoperiod-sensitive phases were estimated at 9 (or 9 + 12 × n) months before bunch maturity for a given phytomer. The main sensitive phase for drought effects was estimated at 29 months before bunch maturity, presumably associated with inflorescence sex determination.

Conclusion

It is assumed that seasonal peaks of flowering in oil palm are controlled even near the equator by photoperiod response within a phytomer. These patterns are confounded with drought effects that affect flowering (yield) with long time-lag. Resulting dynamics are complex, but if the present results are confirmed it will be possible to predict them with models.     

Root morphology, hydraulic conductivity and plant water relations of high-yielding rice grown under aerobic conditions

Background and Aims

Increasing physical water scarcity is a major constraint for irrigated rice (Oryza sativa) production. ‘Aerobic rice culture’ aims to maximize yield per unit water input by growing plants in aerobic soil without flooding or puddling. The objective was to determine (a) the effect of water management on root morphology and hydraulic conductance, and (b) their roles in plant–water relationships and stomatal conductance in aerobic culture.

Methods

Root system development, stomatal conductance (g_s) and leaf water potential (Ψ_leaf) were monitored in a high-yielding rice cultivar (‘Takanari’) under flooded and aerobic conditions at two soil moisture levels [nearly saturated (> –10 kPa) and mildly dry (> –30 kPa)] over 2 years. In an ancillary pot experiment, whole-plant hydraulic conductivity (soil-leaf hydraulic conductance; K_pa) was measured under flooded and aerobic conditions.

Key Results

Adventitious root emergence and lateral root proliferation were restricted even under nearly saturated conditions, resulting in a 72–85 % reduction in total root length under aerobic culture conditions. Because of their reduced rooting size, plants grown under aerobic conditions tended to have lower K_pa than plants grown under flooded conditions. Ψ_leaf was always significantly lower in aerobic culture than in flooded culture, while g_s was unchanged when the soil moisture was at around field capacity. g_s was inevitably reduced when the soil water potential at 20-cm depth reached –20 kPa.

Conclusions

Unstable performance of rice in water-saving cultivations is often associated with reduction in Ψ_leaf. Ψ_leaf may reduce even if K_pa is not significantly changed, but the lower Ψ_leaf would certainly occur in case K_pa reduces as a result of lower water-uptake capacity under aerobic conditions. Rice performance in aerobic culture might be improved through genetic manipulation that promotes lateral root branching and rhizogenesis as well as deep rooting.     

Tests for inbreeding and outbreeding depression and estimation of population differentiation in the bird-pollinated shrub Grevillea mucronulata

Background and Aims

Plants show patterns of spatial genetic differentiation reflecting gene flow mediated by pollen and seed dispersal and genotype × environment interactions. If patterns of genetic structure are determined largely by gene flow then they may be useful in predicting the likelihood of inbreeding or outbreeding depression but should be less useful if there is strong site-specific selection. For many Australian plants little is known about either their population genetics or the effects on mating systems of variation in pollen transfer distances. Experimental pollinations were used to compare the reproductive success of bird-adapted Grevillea mucronulata plants mated with individuals from a range of spatial scales. A hierarchical survey of microsatellite DNA variation was also conducted to describe the scale of population differentiation for neutral markers.

Methods

The effects of four pollen treatments on reproductive performance were compared. These treatments were characterized by transfer of pollen from (a) neighbouring adults; (b) an adjacent cluster of adults (30–50 m distant); (c) a distant cluster (>5 km distant); and (d) open pollination. Sets of 17·9 ± 3·3 leaves from each of 15 clusters of plants were genotyped and spatial autocorrelation and F statistics were used to describe patterns of genetic structure.

Key Results

Grevillea mucronulata displayed evidence of both inbreeding and outbreeding depression, with ‘intermediate’ pollen producing consistently superior outcomes for most aspects of fitness including seed set, seed size, germination and seedling growth. Significant genotypic structuring was detected within clusters (spatial autocorrelation) and among adjacent clusters and clusters separated by >5 km distance (F_ST = 0·07 and 0·10).

Conclusions

The superior outcome of intermediate pollen transfer and genetic differentiation of adjacent clusters suggests that G. mucronulata selection disfavours matings among closely and distantly related neighbours. Moreover, the performance of open-pollinated seedlings was poor, implying that current mating patterns are suboptimal.     

Nitric oxide is the shared signalling molecule in phosphorus- and iron-deficiency-induced formation of cluster roots in white lupin (Lupinus albus)

Background and Aims

Formation of cluster roots is one of the most specific root adaptations to nutrient deficiency. In white lupin (Lupinus albus), cluster roots can be induced by phosphorus (P) or iron (Fe) deficiency. The aim of the present work was to investigate the potential shared signalling pathway in P- and Fe-deficiency-induced cluster root formation.

Methods

Measurements were made of the internal concentration of nutrients, levels of nitric oxide (NO), citrate exudation and expression of some specific genes under four P × Fe combinations, namely (1) 50 µm P and 10 µm Fe (+P + Fe); (2) 0 P and 10 µm Fe (–P + Fe); (3) 50 µm P and 0 Fe (+P–Fe); and (4) 0 P and 0 Fe (–P–Fe), and these were examined in relation to the formation of cluster roots.

Key Results

The deficiency of P, Fe or both increased the cluster root number and cluster zones. It also enhanced NO accumulation in pericycle cells and rootlet primordia at various stages of cluster root development. The formation of cluster roots and rootlet primordia, together with the expression of LaSCR1 and LaSCR2 which is crucial in cluster root formation, were induced by the exogenous NO donor S-nitrosoglutathione (GSNO) under the +P + Fe condition, but were inhibited by the NO-specific endogenous scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl- 3-oxide (cPTIO) under –P + Fe, +P–Fe and –P–Fe conditions. However, cluster roots induced by an exogenous supply of the NO donor did not secrete citrate, unlike those formed under –P or –Fe conditions.

Conclusions

NO plays an important role in the shared signalling pathway of the P- and Fe-deficiency-induced formation of cluster roots in white lupin.     

Flavonols: old compounds for old roles

Background

New roles for flavonoids, as developmental regulators and/or signalling molecules, have recently been proposed in eukaryotic cells exposed to a wide range of environmental stimuli. In plants, these functions are actually restricted to flavonols, the ancient and widespread class of flavonoids. In mosses and liverworts, the whole set of genes for flavonol biosynthesis – CHS, CHI, F3H, FLS and F3′H – has been detected. The flavonol branch pathway has remained intact for millions of years, and is almost exclusively involved in the responses of plants to a wide array of stressful agents, despite the fact that evolution of flavonoid metabolism has produced >10 000 structures.

Scope

Here the emerging functional roles of flavonoids in the responses of present-day plants to different stresses are discussed based on early, authoritative views of their primary functions during the colonization of land by plants. Flavonols are not as efficient as other secondary metabolites in absorbing wavelengths in the 290–320 nm spectral region, but display the greatest potential to keep stress-induced changes in cellular reactive oxygen species homeostasis under control, and to regulate the development of individual organs and the whole plant. Very low flavonol concentrations, as probably occurred in early terrestrial plants, may fully accomplish these regulatory functions.

Conclusions

During the last two decades the routine use of genomic, chromatography/mass spectrometry and fluorescence microimaging techniques has provided new insights into the regulation of flavonol metabolism as well as on the inter- and intracellular distribution of stress-responsive flavonols. These findings offer new evidence on how flavonols may have performed a wide array of functional roles during the colonization of land by plants. In our opinion this ancient flavonoid class is still playing the same old and robust roles in present-day plants.     

Experimental growing of wild pea in Israel and its bearing on Near Eastern plant domestication

Background and Aims

The wild progenitors of the Near Eastern legumes have low germination rates mediated by hardseededness. Hence it was argued that cultivation of these wild legumes would probably result in no yield gain. Based on the meagre natural yield of wild lentil and its poor germination, it was suggested that wild Near Eastern grain legumes were unlikely to have been adopted for cultivation unless freely germinating types were available for the incipient farmers. Unlike wild cereals, data from experimental cultivation of wild legumes are lacking.

Methods

Replicated nurseries of wild pea (Pisum elatius, P. humile and P. fulvum) were sown during 2007–2010 in the Mediterranean district of Israel. To assess the effect of hardseededness on the yield potential, seeds of the wild species were either subjected to scarification (to ensure germination) or left intact, and compared with domesticated controls.

Key Results

Sowing intact wild pea seeds mostly resulted in net yield loss due to poor establishment caused by wild-type low germination rates, while ensuring crop establishment by scarification resulted in net, although modest, yield gain, despite considerable losses due to pod dehiscence. Harvest efficiency of the wild pea plots was significantly higher (2–5 kg seeds h⁻¹) compared with foraging efficiency in wild pea populations (ranging from a few grams to 0·6 kg h⁻¹).

Conclusions

Germination and yield data from ‘cultivation’ of wild pea suggest that Near Eastern legumes are unlikely to have been domesticated via a protracted process. Put differently, the agronomic implications of the hardseededness of wild legumes are incompatible with a millennia-long scenario of unconscious selection processes leading to ‘full’ domestication. This is because net yield loss in cultivation attempts is most likely to have resulted in abandonment of the respective species within a short time frame, rather than perpetual unprofitable cultivation for several centuries or millennia.     

Trampling, defoliation and physiological integration affect growth, morphological and mechanical properties of a root-suckering clonal tree

Background and Aims

Grazing is a complex process involving the simultaneous occurrence of both trampling and defoliation. Clonal plants are a common feature of heavily grazed ecosystems where large herbivores inflict the simultaneous pressures of trampling and defoliation on the vegetation. We test the hypothesis that physiological integration (resource sharing between interconnected ramets) may help plants to deal with the interactive effects of trampling and defoliation.

Methods

In a field study, small and large ramets of the root-suckering clonal tree Populus simonii were subjected to two levels of trampling and defoliation, while connected or disconnected to other ramets. Plant responses were quantified via survival, growth, morphological and stem mechanical traits.

Key Results

Disconnection and trampling increased mortality, especially in small ramets. Trampling increased stem length, basal diameter, fibrous root mass, stem stiffness and resistance to deflection in connected ramets, but decreased them in disconnected ones. Trampling decreased vertical height more in disconnected than in connected ramets, and reduced stem mass in disconnected ramets but not in connected ramets. Defoliation reduced basal diameter, leaf mass, stem mass and leaf area ratio, but did not interact with trampling or disconnection.

Conclusions

Although clonal integration did not influence defoliation response, it did alleviate the effects of trampling. We suggest that by facilitating resource transport between ramets, clonal integration compensates for trampling-induced damage to fine roots.     

Ontogeny and structure of the acervulate partial inflorescence in Hyophorbe lagenicaulis (Arecaceae; Arecoideae)

Background and Aims

The palm tribe Chamaedoreeae displays flowers arranged in a complex partial inflorescence called an acervulus. This type of partial inflorescence has so far not been reported elsewhere in the largest palm subfamily Arecoideae, which is traditionally characterized by flowers predominantly arranged in triads of one central female and two lateral male flowers. The ontogenetic basis of the acervulus is as yet unknown and its structural diversity throughout the genera of the Chamaedoreeae poorly recorded. This study aims to provide critical information on these aspects.

Methods

Developmental series and mature inflorescences were sampled from plants cultivated in international botanical gardens and wild populations. The main techniques employed included scanning electronic microscopy and serial anatomical sectioning of resin-embedded fragments of rachillae.

Key Results

Inflorescence ontogeny in Hyophorbe lagenicaulis demonstrates that the acervulus and the inflorescence rachilla form a condensed and cymose branching system resembling a coenosome. Syndesmy results from a combined process of rapid development and adnation, without or with reduced axis elongation. Acervulus diversity in the ten taxa of the Chamaedoreeae studied is displayed at the level of their positioning within the inflorescence, their arrangement, the number of floral buds and their sexual expression.

Conclusions

The results show that a more general definition of the type of partial inflorescence observed within the large subfamily Arecoideae would correspond to a cyme rather than to a floral triad. In spite of their common cymose architecture, the floral triad and the acervulus present differences with respect to the number and arrangement of floral buds, the superficial pattern of development and sexual expression.     

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.     

Light and desiccation responses of some Hymenophyllaceae (filmy ferns) from Trinidad, Venezuela and New Zealand: poikilohydry in a light-limited but low evaporation ecological niche

Background and Aims

Hymenophyllaceae (filmy ferns) are typically plants of shady, constantly moist habitats. They attain greatest species diversity and biomass in humid tropical montane forests and temperate hyperoceanic climates. This paper presents ecophysiological data bearing on their worldwide ecological niche space and its limits.

Methods

Chlorophyll fluorescence was used to monitor recovery in desiccation experiments, and for measurements of 95 % saturating irradiance [photosynthetic photon flux density (PPFD_{95 %})] of photosynthetic electron flow and other parameters, in the New Zealand Hymenophyllum sanguinolentum, and three species each of Hymenophyllum and Trichomanes from forests in Trinidad and Venezuela.

Key Results

Hymenophyllum sanguinolentum was comparable in desiccation tolerance and light responses with the European species. The more common species in the two tropical forests showed PPFD_{95 %} >100 µmol m⁻² s⁻¹, and withstood moderate desiccation (–40 MPa) for several days. The four most shade-adapted species had PPFD_{95 %} ≤51 µmol m⁻² s⁻¹, and were sensitive to even mild and brief desiccation (–22 MPa for 3 d).

Conclusions

Light and desiccation responses of filmy ferns can be seen as an integrated package. At low light and windspeed in humid forests, net radiation and saturation deficit are low, and diffusion resistance high. Water loss is slow and can be supported by modest conduction from the sub-stratum. With higher irradiance, selection pressure for desiccation tolerance increases progressively. With low light and high humidity, the filmy fern pattern of adaptation is probably optimal, and the vascular plant leaf with mesophyll and stomata offers no advantage in light capture, water economy or CO₂ uptake. Trade-offs between light adaptation and desiccation tolerance, and between stem conduction and water absorption through the leaf surface, underlie adaptive radiation and niche differentiation of species within the family. Hymenophyllaceae are a rare example of an evolutionary shift of adaptive strategy from typical vascular plant adaptation to the poikilohydry most typical of bryophytes.