Silicon modifies root anatomy, and uptake and subcellular distribution of cadmium in young maize plants

Background and Aims

Silicon (Si) has been shown to ameliorate the negative influence of cadmium (Cd) on plant growth and development. However, the mechanism of this phenomenon is not fully understood. Here we describe the effect of Si on growth, and uptake and subcellular distribution of Cd in maize plants in relation to the development of root tissues.

Methods

Young maize plants (Zea mays) were cultivated for 10 d hydroponically with 5 or 50 µm Cd and/or 5 mm Si. Growth parameters and the concentrations of Cd and Si were determined in root and shoot by atomic absorption spectrometry or inductively coupled plasma mass spectroscopy. The development of apoplasmic barriers (Casparian bands and suberin lamellae) and vascular tissues in roots were analysed, and the influence of Si on apoplasmic and symplasmic distribution of ¹⁰⁹Cd applied at 34 nm was investigated between root and shoot.

Key Results

Si stimulated the growth of young maize plants exposed to Cd and influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root. Si did not affect the distribution of apoplasmic and symplasmic Cd in maize roots, but considerably decreased symplasmic and increased apoplasmic concentration of Cd in maize shoots.

Conclusions

Differences in Cd uptake of roots and shoots are probably related to the development of apoplasmic barriers and maturation of vascular tissues in roots. Alleviation of Cd toxicity by Si might be attributed to enhanced binding of Cd to the apoplasmic fraction in maize shoots.     

Hourglass cell development in the soybean seed coat

Background and Aims

Hourglass cells (HGCs) are prominent cells in the soybean seed coat, and have potential use as ‘phytofactories’ to produce specific proteins of interest. Previous studies have shown that HGCs initiate differentiation at about 9 d post-anthesis (dpa), assuming their characteristic morphology by 18 dpa. This study aims to document the structural changes in HGCs during this critical period, and to relate these changes to the concurrent development of a specific soybean peroxidase (SBP) encoded by the Ep gene.

Methods

Pods were collected from plants at specific growth stages. Fresh material was processed for analysis of Ep peroxidase activity. Tissues were processed for scanning and transmission electron microscopy, as well as extracted for western blotting. A null variety lacking expression of Ep peroxidase was grown as a control.

Key Results and Conclusions

At 9 dpa, HGCs are typical undifferentiated plant cells, but from 12–18 dpa they undergo rapid changes in their internal and external structure. By 18 dpa, they have assumed the characteristic hourglass shape with thick cell walls, intercellular air spaces and large central vacuoles. By 45 dpa, all organelles in HGCs have been degraded. Additional observations indicate that plasmodesmata connect all cell types. SBP activity and SBP protein are detectable in the HGC before they are fully differentiated (approx. 18 dpa). In very early stages, SBP activity appears localized in a vacuole as previously predicted. These results increase our understanding of the structure and development of the HGC and will be valuable for future studies aimed at protein targeting to components of the HGC endomembrane systems.     

Distinct mechanisms for aerenchyma formation in leaf sheaths of rice genotypes displaying a quiescence or escape strategy for flooding tolerance

Background and Aims

Rice is one of the few crops able to withstand periods of partial or even complete submergence. One of the adaptive traits of rice is the constitutive presence and further development of aerenchyma which enables oxygen to be transported to submerged organs. The development of lysigenous aerenchyma is promoted by ethylene accumulating within the submerged plant tissues, although other signalling mechanisms may also co-exist. In this study, aerenchyma development was analysed in two rice (Oryza sativa) varieties, ‘FR13A’ and ‘Arborio Precoce’, which show opposite traits in flooding response in terms of internode elongation and survival.

Methods

The growth and survival of rice varieties under submergence was investigated in the leaf sheath of ‘FR13A’ and ‘Arborio Precoce’. The possible involvement of ethylene and reactive oxygen species (ROS) was evaluated in relation to aerenchyma formation. Cell viability and DNA fragmentation were determined by FDA/FM4-64 staining and TUNEL assay, respectively. Ethylene production was monitored by gas chromatography and by analysing ACO gene expression. ROS production was measured by using Amplex Red assay kit and the fluorescent dye DCFH₂-DA. The expression of APX1 was also evaluated. AVG and DPI solutions were used to test the effect of inhibiting ethylene biosynthesis and ROS production, respectively.

Key Results

Both the varieties displayed constitutive lysigenous aerenchyma formation, which was further enhanced when submerged. ‘Arborio Precoce’, which is characterized by fast elongation when submerged, showed active ethylene biosynthetic machinery associated with increased aerenchymatous areas. ‘FR13A’, which harbours the Sub1A gene that limits growth during oxygen deprivation, did not show any increase in ethylene production after submersion but still displayed increased aerenchyma. Hydrogen peroxide levels increased in ‘FR13A’ but not in ‘Arborio Precoce’.

Conclusions

While ethylene controls aerenchyma formation in the fast-elongating ‘Arborio Precoce’ variety, in ‘FR13A’ ROS accumulation plays an important role.     

Genotypic differences in Al resistance and the role of cell-wall pectin in Al exclusion from the root apex in Fagopyrum tataricum

Background and Aims

Aluminium (Al) toxicity is one of the factors limiting crop production on acid soils. However, genotypic differences exist among plant species or cultivars in response to Al toxicity. This study aims to investigate genotypic differences among eight cultivars of tatary buckwheat (Fagopyrum tataricum) for Al resistance and explore the possible mechanisms of Al resistance.

Methods

Al resistance was evaluated based on relative root elongation (root elongation with Al/root elongation without Al). Root apex Al content, pectin content and exudation of root organic acids were determined and compared.

Key Results

Genotypic differences among the eight cultivars were correlated with exclusion of Al from the root apex. However, there was a lack of correlation between Al exclusion and Al-induced oxalate secretion. Interestingly, cell-wall pectin content of the root apex was generally lower in Al-resistant cultivars than in Al-sensitive cultivars. Although we were unable to establish a significant correlation between Al exclusion and pectin content among the eight cultivars, a strong correlation could be established among six cultivars, in which the pectin content in the most Al-resistant cultivar ‘Chuan’ was significantly lower than that in the most Al-sensitive cultivar ‘Liuku2’. Furthermore, root apex cell-wall pectin methylesterase activity (PME) was similar in ‘Chuan’ and ‘Liuku2’ in the absence of Al, but Al treatment resulted in increased PME activity in ‘Liuku2’ compared with ‘Chuan’. Immunolocalization of pectins also showed that the two cultivars had similar amounts of either low-methyl-ester pectins or high-methyl-ester pectins in the absence of Al, but Al treatment resulted in a more significant increase of low-methyl-ester pectins and decrease of high-methyl-ester pectins in ‘Liuku2’.

Conclusions

Cell-wall pectin content may contribute, at least in part, to differential Al resistance among tatary buckwheat cultivars.     

Identification and characterization of the water gap in physically dormant seeds of Geraniaceae, with special reference to Geranium carolinianum

Background and Aims

Physical dormancy in seeds of species of Geraniaceae is caused by a water-impermeable palisade layer in the outer integument of the seed coat and a closed chalaza. The chalazal cleft has been reported to be the water gap (i.e. location of initial water entry) in innately permeable seeds of Geraniaceae. The primary aim of this study was to re-evaluate the location of the water gap and to characterize its morphology and anatomy in physically dormant seeds of Geraniaceae, with particular reference to G. carolinianum.

Methods

Length, width, mass, anatomy and germination of two seed types (light brown and dark brown) of G. carolinianum were compared. Location, anatomy and morphology of the water gap were characterized using free-hand and microtome tissue sectioning, light microscopy, scanning electron microscopy, dye tracking, blocking and seed-burial experiments.

Key Results

Treatment with dry heat caused a colour change in the palisade cells adjacent to the micropyle. When placed in water, the ‘hinged valve’ (blister) erupted at the site of the colour change, exposing the water gap. The morphology and anatomy in the water-gap region differs from those of the rest of the seed coat. The morphology of the seed coat of the water-gap region is similar in G. carolinianum, G. columbinum, G. molle and G. pusillum and differs from that of the closely related species Erodium cicutarium.

Conclusions

Dislodgment of swollen ‘hinged valve’ palisade cells adjacent to the micropyle caused the water gap to open in physically dormant seeds of G. carolinianum, and it was clear that initial water uptake takes place through this gap and not via the chalazal opening as previously reported. This water gap (‘hinged valve gap’) differs from water gaps previously described for other families in morphology, anatomy and location in the seed coat.     

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.     

Developmental changes in the germinability, desiccation tolerance, hardseededness, and longevity of individual seeds of Trifolium ambiguum

Background and Aims

Using two parental clones of outcrossing Trifolium ambiguum as a potential model system, we examined how during seed development the maternal parent, number of seeds per pod, seed position within the pod, and pod position within the inflorescence influenced individual seed fresh weight, dry weight, water content, germinability, desiccation tolerance, hardseededness, and subsequent longevity of individual seeds.

Methods

Near simultaneous, manual reciprocal crosses were carried out between clonal lines for two experiments. Infructescences were harvested at intervals during seed development. Each individual seed was weighed and then used to determine dry weight or one of the physiological behaviour traits.

Key Results

Whilst population mass maturity was reached at 33–36 days after pollination (DAP), seed-to-seed variation in maximum seed dry weight, when it was achieved, and when maturation drying commenced, was considerable. Individual seeds acquired germinability between 14 and 44 DAP, desiccation tolerance between 30 and 40 DAP, and the capability to become hardseeded between 30 and 47 DAP. The time for viability to fall to 50 % (p₅₀) at 60 % relative humidity and 45 °C increased between 36 and 56 DAP, when the seed coats of most individuals had become dark orange, but declined thereafter. Individual seed f. wt at harvest did not correlate with air-dry storage survival period. Analysing survival data for cohorts of seeds reduced the standard deviation of the normal distribution of seed deaths in time, but no sub-population showed complete uniformity of survival period.

Conclusions

Variation in individual seed behaviours within a developing population is inherent and inevitable. In this outbreeder, there is significant variation in seed longevity which appears dependent on embryo genotype with little effect of maternal genotype or architectural factors.     

On the mechanisms of nectar secretion: revisited

Background and Scope

Models of nectar formation and exudation in multilayered nectaries with modified stomata or permeable cuticle are evaluated. In the current symplasmic model the pre-nectar moves from terminal phloem through the symplasm into the apoplasm (cell walls and intercellular spaces) with nectar formation by either granulocrine or eccrine secretion and its diffusion outwards. It is concluded, however, that no secretory granules are actually produced by the endoplasmic reticulum, and that secretory Golgi vesicles are not involved in the transport of nectar sugar. Therefore, the concept of granulocrine secretion of nectar should be discarded. The specific function of the endomembrane system in nectary cells remains unknown. According to the apoplasmic model, the pre-nectar moves from the terminal phloem in the apoplasm and, on the way, is transformed from phloem sap into nectar. However, viewed ultrastructurally, the unloading (terminal) phloem of nectaries appears to be less active than that of the leaf minor veins, and is therefore not actively involved in the secretion of pre-nectar components into the apoplasm. This invalidates the apoplasmic model. Neither model provides an explanation for the origin of the driving force for nectar discharge.

Proposal

A new model is proposed in which nectar moves by a pressure-driven mass flow in the nectary apoplasm while pre-nectar sugars diffuse from the sieve tubes through the symplasm to the secretory cells, where nectar is formed and sugars cross the plasma membrane by active transport (‘eccrine secretion’). The pressure originates as the result of water influx in the apoplasm from the symplasm along the sugar concentration gradient. It follows from this model that there can be no combinations of apoplasmic and symplasmic pre-nectar movements. The mass-flow mechanism of nectar exudation appears to be universal and applicable to all nectaries irrespective of their type, morphology and anatomy, presence or absence of modified stomata, and their own vascular system.     

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.     

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.     

Characterization of a rice variety with high hydraulic conductance and identification of the chromosome region responsible using chromosome segment substitution lines

Background and Aims

The rate of photosynthesis in paddy rice often decreases at noon on sunny days because of water stress, even under submerged conditions. Maintenance of higher rates of photosynthesis during the day might improve both yield and dry matter production in paddy rice. A high-yielding indica variety, ‘Habataki’, maintains a high rate of leaf photosynthesis during the daytime because of the higher hydraulic conductance from roots to leaves than in the standard japonica variety ‘Sasanishiki’. This research was conducted to characterize the trait responsible for the higher hydraulic conductance in ‘Habataki’ and identified a chromosome region for the high hydraulic conductance.

Methods

Hydraulic conductance to passive water transport and to osmotic water transport was determined for plants under intense transpiration and for plants without transpiration, respectively. The varietal difference in hydraulic conductance was examined with respect to root surface area and hydraulic conductivity (hydraulic conductance per root surface area, L_p). To identify the chromosome region responsible for higher hydraulic conductance, chromosome segment substitution lines (CSSLs) derived from a cross between ‘Sasanishiki’ and ‘Habataki’ were used.

Key Results

The significantly higher hydraulic conductance resulted from the larger root surface area not from L_p in ‘Habataki’. A chromosome region associated with the elevated hydraulic conductance was detected between RM3916 and RM2431 on the long arm of chromosome 4. The CSSL, in which this region was substituted with the ‘Habataki’ chromosome segment in the ‘Sasanishiki’ background, had a larger root mass than ‘Sasanishiki’.

Conclusions

The trait for increasing plant hydraulic conductance and, therefore, maintaining the higher rate of leaf photosynthesis under the conditions of intense transpiration in ‘Habataki’ was identified, and it was estimated that there is at least one chromosome region for the trait located on chromosome 4.     

A new type of specialized morphophysiological dormancy and seed storage behaviour in Hydatellaceae, an early-divergent angiosperm family

Background and Aims

Recent phylogenetic analysis has placed the aquatic family Hydatellaceae as an early-divergent angiosperm. Understanding seed dormancy, germination and desiccation tolerance of Hydatellaceae will facilitate ex situ conservation and advance hypotheses regarding angiosperm evolution.

Methods

Seed germination experiments were completed on three species of south-west Australian Hydatellaceae, Trithuria austinensis, T. bibracteata and T. submersa, to test the effects of temperature, light, germination stimulant and storage. Seeds were sectioned to examine embryo growth during germination in T. austinensis and T. submersa.

Key Results

Some embryo growth and cell division in T. austinensis and T. submersa occurred prior to the emergence of an undifferentiated embryo from the seed coat (‘germination’). Embryo differentiation occurred later, following further growth and a 3- to 4-fold increase in the number of cells. The time taken to achieve 50 % of maximum germination for seeds on water agar was 50, 35 and 37 d for T. austinensis, T bibracteata and T. submersa, respectively.

Conclusions

Seeds of Hydatellaceae have a new kind of specialized morphophysiological dormancy in which neither root nor shoot differentiates until after the embryo emerges from the seed coat. Seed biology is discussed in relation to early angiosperm evolution, together with ex situ conservation of this phylogenetically significant group.     

Asynchronous flowering and within-plant flowering diversity in wheat and the implications for crop resilience to heat

Background and Aims

Self-pollination dominates in wheat, with a small level of out-crossing due to flowering asynchrony and male sterility. However, the timing and synchrony of male and female flowering in wheat is a crucial determinant of seed set and may be an important factor affecting gene flow and resilience to climate change. Here, a methodology is presented for assessing the timing and synchrony of flowering in wheat, Triticum aestivum.

Methods

From the onset of flowering until the end of anthesis, the anther and stigma activity of each floret was assessed on the first five developing ears in potted plants grown under ambient conditions and originating from ‘Paragon’ or ‘Spark-Rialto’ backgrounds. At harvest maturity, seed presence, size and weight was recorded for each floret scored.

Key Results and Conclusions

The synchrony between pollen dehiscence and stigma collapse within a flower was dependent on its relative position in a spike and within a floret. Determined on the basis of synchrony within each flower, the level of pollination by pollen originating from other flowers reached approx. 30 % and did not change throughout the duration of flowering. A modelling exercise parameterized by flowering observations indicated that the temporal and spatial variability of anther activity within and between spikes may influence the relative resilience of wheat to sudden, extreme climatic events which has direct relevance to predicted future climate scenarios in the UK.     

Evolution and history of grapevine (Vitis vinifera) under domestication: new morphometric perspectives to understand seed domestication syndrome and reveal origins of ancient European cultivars

Background and Aims

In spite of the abundance of archaeological, bio-archaeological, historical and genetic data, the origins, historical biogeography, identity of ancient grapevine cultivars and mechanisms of domestication are still largely unknown. Here, analysis of variation in seed morphology aims to provide accurate criteria for the discrimination between wild grapes and modern cultivars and to understand changes in functional traits in relation to the domestication process. This approach is also used to quantify the phenotypic diversity in the wild and cultivated compartments and to provide a starting point for comparing well-preserved archaeological material, in order to elucidate the history of grapevine varieties.

Methods

Geometrical analysis (elliptic Fourier transform method) was applied to grapevine seed outlines from modern wild individuals, cultivars and well-preserved archaeological material from southern France, dating back to the first to second centuries.

Key Results and Conclusions

Significant relationships between seed shape and taxonomic status, geographical origin (country or region) of accessions and parentage of varieties are highlighted, as previously noted based on genetic approaches. The combination of the analysis of modern reference material and well-preserved archaeological seeds provides original data about the history of ancient cultivated forms, some of them morphologically close to the current ‘Clairette’ and ‘Mondeuse blanche’ cultivars. Archaeobiological records seem to confirm the complexity of human contact, exchanges and migrations which spread grapevine cultivation in Europe and in Mediterranean areas, and argue in favour of the existence of local domestication in the Languedoc (southern France) region during Antiquity.     

Among-individual variation in pollen limitation and inbreeding depression in a mixed-mating shrub

Background and Aims

Variation in inbreeding depression (δ) among individual plants is considered to play a central role in mating system evolution and population genetics. Moreover, such variation could be linked to individual susceptibility to pollen limitation (PL) because those individuals strongly affected by δ for seed production will require more outcross pollen for setting a given number of fruits or seeds. However, no study has tested explicitly for associations between PL and δ at the individual plant level. This study assesses the extent of among-individual variation in PL and δ, the consistency of δ across life stages, and the relationships between individual PL and δ in the mixed-mating shrub Myrtus communis.

Methods

Controlled hand-pollinations were performed in a natural M. communis population. Marked flowers were monitored until fruit production and a greenhouse experiment was conducted with the seeds produced.

Key Results

Compared with selfing, outcross-pollination enhanced seed number per fruit, germination rate and seedling growth, but did not enhance fruit-set. Only seed number per fruit was pollen limited and, thus, cumulative pollen limitation depended more on pollen quality (outcross pollen) than on quantity. The effects of δ varied considerably across life stages and individual plants. Cumulative δ was high across individuals (mean δ = 0·65), although there were no positive correlations between δ values at different life stages. Interestingly, maternal plants showing stronger δ for seed production were more pollen limited, but they were also less affected by δ for seedling growth because of a seed size/number trade-off.

Conclusions

Results show a general inconsistency in δ across life stages and individuals, suggesting that different deleterious loci are acting at different stages. The association between δ and PL at the individual level corroborates the idea that pollen limitation may be ‘genotype-dependent’ regardless of other factors.     

Peperomia leaf cell wall interface between the multiple hypodermis and crystal-containing photosynthetic layer displays unusual pit fields

Background and Aims

Leaves of succulent Peperomia obtusifolia (Piperaceae), and its related species, contain a large multilayered hypodermis (epidermis) subtended by a very small single-layered photosynthetic palisade parenchyma, the latter containing spherical aggregates of crystals called druses. Each druse is in a central vacuole surrounded by chloroplasts. All hypodermal cell walls are thin, except for thick lowermost periclinal walls associated with the upper periclinal walls of the subtending palisade cells. These thick walls display ‘quilted’ impressions (mounds) formed by many subtending palisade cells. Conspicuous depressions occur in most mounds, and each depression contains what appear to be many plasmodesmata. These depressions are opposite similar regions in adjacent thin palisade periclinal walls, and they can be considered special pit fields that represent thin translucent regions (‘windows’ or ‘skylights’). Druses in the vacuoles of palisade cells occur below these pit field regions and are surrounded by conspicuous cytoplasmic chloroplasts with massive grana oriented perpendicular to the crystals, probably providing for an efficient photosynthetic system under low-intensity light.

Methods

Leaf clearings and fractures, light microscopy and crossed polarizers, general and histochemical staining, and transmission and scanning electron microscopy were used to examine these structures.

Key Results

Druses in the vacuoles of palisade cells occur below the thin pit field regions in the wall interface, suggesting an interesting physical relationship that could provide a pathway for light waves, filtered through the multiple hypodermis. The light waves pass into the palisade cells and are collected and dispersed by the druses to surrounding chloroplasts with large grana.

Conclusions

These results imply an intriguing possible efficient photosynthetic adaptation for species growing in low-light environments, and provide an opportunity for future research on how evolution through environmental adaptation aids plants containing crystals associated with photosynthetic tissues to exist under low-light intensity and with other stresses.     

Host responses and metabolic profiles of wood components in Dutch elm hybrids with a contrasting tolerance to Dutch elm disease

Background and Aims

Changes occurring in the macromolecular traits of cell wall components in elm wood following attack by Ophiostoma novo-ulmi, the causative agent of Dutch elm disease (DED), are poorly understood. The purpose of this study was to compare host responses and the metabolic profiles of wood components for two Dutch elm (Ulmus) hybrids, ‘Groeneveld’ (a susceptible clone) and ‘Dodoens’ (a tolerant clone), that have contrasting survival strategies upon infection with the current prevalent strain of DED.

Methods

Ten-year-old plants of the hybrid elms were inoculated with O. novo-ulmi ssp. americana × novo-ulmi. Measurements were made of the content of main cell wall components and extractives, lignin monomer composition, macromolecular traits of cellulose and neutral saccharide composition.

Key Results

Upon infection, medium molecular weight macromolecules of cellulose were degraded in both the susceptible and tolerant elm hybrids, resulting in the occurrence of secondary cell wall ruptures and cracks in the vessels, but rarely in the fibres. The ¹³C nuclear magnetic resonance spectra revealed that loss of crystalline and non-crystalline cellulose regions occurred in parallel. The rate of cellulose degradation was influenced by the syringyl:guaiacyl ratio in lignin. Both hybrids commonly responded to the medium molecular weight cellulose degradation with the biosynthesis of high molecular weight macromolecules of cellulose, resulting in a significant increase in values for the degree of polymerization and polydispersity. Other responses of the hybrids included an increase in lignin content, a decrease in relative proportions of d-glucose, and an increase in proportions of d-xylose. Differential responses between the hybrids were found in the syringyl:guaiacyl ratio in lignin.

Conclusions

In susceptible ‘Groeneveld’ plants, syringyl-rich lignin provided a far greater degree of protection from cellulose degradation than in ‘Dodoens’, but only guaiacyl-rich lignin in ‘Dodoens’ plants was involved in successful defence against the fungus. This finding was confirmed by the associations of vanillin and vanillic acid with the DED-tolerant ‘Dodoens’ plants in a multivariate analysis of wood traits.     

False paracarpy in Seemannaralia (Araliaceae): from bilocular ovary to unilocular fruit

Background and Aims

Seemannaralia appears to be fundamentally different from all other Araliaceae in the presence of a well-developed symplicate zone in its gynoecium, as well as in the ovule insertion in the symplicate zone (rather than in the cross-zone). The present investigation re-examined the floral structure of Seemannaralia with emphasis on the morphology and evolution of its gynoecium.

Methods

Flowers and fruits of Seemannaralia gerrardii at various developmental stages were examined using light microscopy and scanning electron microscopy.

Key Results

Ovaries in the flowers of Seemannaralia are bilocular. Each ovary locule corresponds to a carpel whose ascidiate part is distinctly longer than the plicate part. Each carpel contains one fertile ovule attached to the cross-zone, and one sterile ovule as well. The fruit is unilocular: its central cavity is occupied by a single large seed. In the course of fruit development, the growth of one ovule stops while another ovule develops into the mature seed. When this ovule outgrows the available space in the locule, the septum is ruptured, forming a united cavity of two carpels.

Conclusions

Despite literature data, the synascidiate zone is well developed in the gynoecium of Seemannaralia, and the ovules are attached to the cross-zone. Its preanthetic and anthetic gynoecium has nearly the same structure as gynoecia of most other Araliaceae. The Seemannaralia fruit resembles the paracarpous gynoecium but its ground plan is very different because the central cavity is formed by mechanical rupture of the septum. The term ‘pseudoparacarpy’ (‘false paracarpy’) is proposed to describe this condition, which has not been reported to date for indehiscent fruits in any taxa other than Seemannaralia. In this genus, the pseudoparacarpy has probably resulted from a decrease in seed number in the course of the transition from zoochory to anemochory.     

Quantitative trait loci for seed yield and yield-related traits, and their responses to reduced phosphorus supply in Brassica napus

Background and Aims

One of the key targets of breeding programmes in rapeseed (Brassica napus) is to develop high-yield varieties. However, the lack of available phosphorus (P) in soils seriously limits rapeseed production. The aim of this study was to dissect the genetic control of seed yield and yield-related traits in B. napus grown with contrasting P supplies.

Methods

Two-year field trials were conducted at one site with normal and low P treatments using a population of 124 recombinant inbred lines derived from a cross between ‘B104-2’ and ‘Eyou Changjia’. Seed yield, seed weight, seed number, pod number, plant height, branch number and P efficiency coefficient (PEC) were investigated. Quantitative trait locus (QTL) analysis was performed by composite interval mapping.

Key Results

The phenotypic values of most of the tested traits were reduced under the low P conditions. In total, 74 putative QTLs were identified, contributing 7·3–25·4 % of the phenotypic variation. Of these QTLs, 16 (21·6 %) were detected in two seasons and in the mean value of two seasons, and eight QTLs for two traits were conserved across P levels. Low-P-specific QTLs were clustered on chromosomes A1, A6 and A8. By comparative mapping between Arabidopsis and B. napus, 161 orthologues of 146 genes involved in Arabidopsis P homeostasis and/or yield-related trait control were associated with 45 QTLs corresponding to 23 chromosomal regions. Four gene-based markers developed from genes involved in Arabidopsis P homeostasis were mapped to QTL intervals.

Conclusions

Different genetic determinants were involved in controlling seed yield and yield-related traits in B. napus under normal and low P conditions. The QTLs detected under reduced P supply may provide useful information for improving the seed yield of B. napus in soils with low P availability in marker-assisted selection.