Stamen development and winter dormancy in apricot (Prunus armeniaca)

Background and Aims

In temperate woody perennials, flower bud development is halted during the winter, when the buds enter dormancy. This dormant period is a prerequisite for adequate flowering, is genetically regulated, and plays a clear role in possibly adapting species and cultivars to climatic areas. However, information on the biological events underpinning dormancy is lacking. Stamen development, with clear differentiated stages, appears as a good framework to put dormancy in a developmental context. Here, stamen developmental changes are characterized in apricot (Prunus armeniaca) and are related to dormancy.

Methods

Stamen development was characterized cytochemically from the end of August to March, over 4 years. Developmental changes were related to dormancy, using the existing empirical information on chilling requirements.

Key Results

Stamen development continued during the autumn, and the flower buds entered dormancy with a fully developed sporogenous tissue. Although no anatomical changes were observed during dormancy, breaking of dormancy occurred following a clear sequence of events. Starch accumulated in particular places, pre-empting further development in those areas. Vascular bundles developed and pollen mother cells underwent meiosis followed by microspore development.

Conclusions

Dormancy appears to mark a boundary between the development of the sporogenous tissue and the occurrence of meiosis for further microspore development. Breaking of dormancy occurs following a clear sequence of events, providing a developmental context in which to study winter dormancy and to evaluate differences in chilling requirements among genotypes.     

Seed dormancy distribution: explanatory ecological factors

Background and Aims

Knowledge of those traits that vary with latitude should be helpful in predicting how they may evolve locally under climate change. In the sea beet Beta vulgaris ssp. maritima, seed dormancy largely controls the timing of germination, is highly heritable and varies geographically; it is therefore thought to be selected by climate. The aim here was to characterize the variation in seed dormancy among sea beet populations across the French distribution area, as well as the ecological factors in situ that are correlated with and that could therefore select for seed dormancy. The relative importance of genetic inheritance vs. non-genetic variation is also evaluated.

Methods

The proportions of dormant seeds from 85 natural populations encompassing different climates over the whole French distribution area were measured under controlled conditions. Germination phenology was observed in a common garden experiment. Dormancy variation of seeds collected in situ was compared with that of seeds collected on plants grown in the greenhouse.

Key Results

The proportions of dormant seeds in the greenhouse were highly variable, covering almost the entire range from 0 to 1, and followed a geographical pattern from lower dormancy at high latitudes to high dormancy at low latitudes. The distribution of dormancy was positively correlated with yearly temperatures, especially summer temperatures. Minimum temperatures in winter did not significantly explain the trait variation. The genetic component of the total variation was significant and is probably completed by an important adjustment to the local conditions brought about by maternal adaptive phenotypic plasticity.

Conclusions

Dormancy in sea beet could be interpreted as a way to limit summer germination and spread germination over the first autumn and spring or following autumns. This highly heritable trait has the potential to evolve in the relatively near future because of climate change.     

Evaluation of Greenbug and Yellow Sugarcane Aphid Feeding Behavior on Resistant and Susceptible Switchgrass Cultivars

Switchgrass (Panicum virgatum L.) is an emerging biofuel crop that serves as host for aphids. To discern the effects of plant age and possible resistance mechanisms, the feeding behavior of greenbugs (Schizaphis graminum Rondani.) and the yellow sugarcane aphid (Sipha flava Forbes.) was monitored on three diverse switchgrasses by the electrical penetration graph (EPG) technique. Callose deposition and genes associated with callose metabolism were also analyzed to discern their association with plant resistance. There was a strong host effect on greenbugs feeding on lowland cultivar Kanlow at the V3 stage of development, as compared to the greenbug-susceptible upland cultivar Summer and plants derived from Kanlow (♂) × Summer (♀) (K×S) crosses. These data confirmed that Kanlow at the V3 stage had antibiosis to greenbugs, which was absent in the Summer and K×S plants. In contrast, similar effects were not observed for yellow sugarcane aphids, excluding significant differences in the time to first probe on Kanlow plants at the V1 stage and reduction in time spent on pathway processes on Kanlow plants at the V3 stage. These data demonstrated that Kanlow plants may have multiple sources of resistance to the two aphids, and possibly some were phloem based. Microscopy of leaf sections stained with aniline blue for callose was suggestive of increased callose deposition in the sieve elements in Kanlow plants relative to Summer and K×S plants. RT-qPCR analysis of several genes associated with callose metabolism in infested plants was equivocal. Overall, these studies suggest the presence of multiple defense mechanisms against aphids in Kanlow plants, relative to Summer and K×S plants.     

Combining winter hardiness and forage yield in white clover (Trifolium repens) cultivated in northern environments

Background and Aims

White clover (Trifolium repens) is an important component of sustainable livestock systems around the world. Its exploitation for agriculture in the northern, marginal areas is, however, currently limited by the lack of cultivars that combine persistence and high production potential. The aims are to investigate whether it is feasible to create breeding material of white clover for these areas by combining winter hardiness of northerly populations with good yielding ability of more southerly cultivars.

Methods

A total of 166 crosses of 14 different parental combinations between winter-hardy, low-yielding populations of northern origin and high-yielding commercial cultivars of more southerly origin were tested under field conditions in Iceland and Norway and the parental populations were compared in Norway. Spaced plants were transplanted into a smooth meadow grass (Poa pratensis) sward. Dry matter yield was estimated for 2 years after planting in Norway and morphological characters associated with yielding capacity were measured at both sites.

Key Results

The results showed that southerly cultivars had larger leaves and higher yielding potential than northern types but suffered more winter damage. Significant variation was found between full-sib families within the different parental combinations for all morphological characteristics measured in all three trials. However, it was difficult to detect any consistent morphological patterns between progeny groups across trial sites. No significant correlations were found between leaflet area and survival.

Conclusions

The present study has confirmed that it should be possible to simultaneously select for good winter survival and larger leaves and, hence, higher yielding ability under marginal conditions.Key words: Breeding, morphology, spaced plants, Trifolium repens, winter survival, progeny testing     

Floral adaptation to local pollinator guilds in a terrestrial orchid

Background and Aims

Studies of local floral adaptation in response to geographically divergent pollinators are essential for understanding floral evolution. This study investigated local pollinator adaptation and variation in floral traits in the rewarding orchid Gymnadenia odoratissima, which spans a large altitudinal gradient and thus may depend on different pollinator guilds along this gradient.

Methods

Pollinator communities were assessed and reciprocal transfer experiments were performed between lowland and mountain populations. Differences in floral traits were characterized by measuring floral morphology traits, scent composition, colour and nectar sugar content in lowland and mountain populations.

Key Results

The composition of pollinator communities differed considerably between lowland and mountain populations; flies were only found as pollinators in mountain populations. The reciprocal transfer experiments showed that when lowland plants were transferred to mountain habitats, their reproductive success did not change significantly. However, when mountain plants were moved to the lowlands, their reproductive success decreased significantly. Transfers between populations of the same altitude did not lead to significant changes in reproductive success, disproving the potential for population-specific adaptations. Flower size of lowland plants was greater than for mountain flowers. Lowland plants also had significantly higher relative amounts of aromatic floral volatiles, while the mountain plants had higher relative amounts of other floral volatiles. The floral colour of mountain flowers was significantly lighter compared with the lowland flowers.

Conclusions

Local pollinator adaptation through pollinator attraction was shown in the mountain populations, possibly due to adaptation to pollinating flies. The mountain plants were also observed to receive pollination from a greater diversity of pollinators than the lowland plants. The different floral phenotypes of the altitudinal regions are likely to be the consequence of adaptations to local pollinator guilds.     

The effects of water regime on phosphorus responses of rainfed lowland rice cultivars

Background and Aims

Soil phosphorus (P) solubility declines sharply when a flooded soil drains, and an important component of rice (Oryza sativa) adaptation to rainfed lowland environments is the ability to absorb and utilize P under such conditions. The aim of this study was to test the hypothesis that rice cultivars differ in their P responses between water regimes because P uptake mechanisms differ.

Methods

Six lowland rice cultivars (three considered tolerant of low P soils, three sensitive) were grown in a factorial experiment with three water regimes (flooded, moist and flooded-then-moist) and four soil P levels, and growth and P uptake were measured. Small volumes of soil were used to maximize inter-root competition and uptake per unit root surface. The results were compared with the predictions of a model allowing for the effects of water regime on P solubility and diffusion.

Key Results

The plants were P stressed but not water stressed in all the water regimes at all P levels except the higher P additions in the flooded soil. The cultivar rankings scarcely differed between the water regimes and P additions. In all the treatments, the soil P concentrations required to explain the measured uptake were several times the concentration of freely available P in the soil.

Conclusions

The cultivar rankings were driven more by differences in growth habit than specific P uptake mechanisms, so the hypothesis cannot be corroborated with these data. Evidently all the plants could tap sparingly soluble forms of P by releasing a solubilizing agent or producing a greater root length than measured, or both. However, any cultivar differences in this were not apparent in greater net P uptake, possibly because the restricted rooting volume meant that additional P uptake could not be converted into new root growth to explore new soil volumes.Key words: Oryza sativa, rainfed lowland, phosphorus efficiency, root morphology, solubilization, rice cultivar     

Morphological and physiological responses of rice seedlings to complete submergence (flash flooding)

Background and Aims

Reducing damage to rice seedlings caused by flash flooding will improve the productivity of rainfed lowland rice in West Africa. Accordingly, the morphological and physiological responses of different forms of rice to complete submergence were examined in field and pot experiments to identify primary causes of damage.

Methods

To characterize the physiological responses, seedlings from a wide genetic base including Oryza sativa, O. glaberrima and interspecific hybrids were compared using principle component analysis.

Key Results

Important factors linked to flash-flood tolerance included minimal shoot elongation underwater, increase in dry matter weight during submergence and post-submergence resistance to lodging. In particular, fast shoot elongation during submergence negatively affected plant growth after de-submergence. Also shoot-elongating cultivars showed a strong negative correlation between dry matter weight of the leaves that developed before submergence and leaves developing during submergence.

Conclusions

Enhancement of shoot elongation during submergence in water that is too deep to permit re-emergence by small seedlings represents a futile escape strategy that takes place at the expense of existing dry matter in circumstances where underwater photosynthetic carbon fixation is negligible. Consequently, it compromises survival or recovery growth once flood water levels recede and plants are re-exposed to the aerial environment. Tolerance is greater in cultivars where acceleration of elongation caused by submergence is minimal.Key words: Africa, flash floods, Oryza glaberrima, rainfed lowland, rice, shoot elongation, stress tolerance, submergence     

Ecotypic variation of summer dormancy relaxation associated with rainfall gradient in the geophytic grass Poa bulbosa

Background and Aims

Summer dormancy is an adaptive trait in geophytes inhabiting regions with a Mediterranean climate, allowing their survival through the hot and dry summers. Summer dormancy in Poa bulbosa is induced by increasing day-length and temperature and decreasing water availability during spring. Populations from arid habitats became dormant earlier than those from mesic habitats. Relaxation of dormancy was promoted by the hot, dry summer conditions. Here we test the hypothesis that dormancy relaxation is also delayed in ecotypes of P. bulbosa inhabiting arid regions, as a cautious strategy related to the greater unpredictability of autumn rains associated with decreasing precipitation.

Methods

Ecotypes collected across a precipitation gradient (100–1200 mm year⁻¹) in the Mediterranean climate region were grown under similar conditions in a net-house in Israel. Differences among ecotypes in dormancy induction and dormancy relaxation were determined by measuring time to dormancy onset in spring, and time to sprouting after the first effective rain in autumn. Seasonal and ecotype variation in dormancy relaxation were assessed by measuring time to sprouting initiation, rate of sprouting and maximal sprouting of resting dry bulbs sampled in the net-house during late spring, and mid- and late summer, and planted in a wet substrate at temperatures promoting (10 °C) or limiting (20 °C) sprouting.

Key Results

Earlier dormancy in the spring and delayed sprouting in autumn were correlated with decreasing mean annual rainfall at the site of ecotype origin. Seasonal and ecotype differences in dormancy relaxation were expressed in bulbs planted at 20 °C. During the summer, time to sprouting decreased while rate of sprouting and maximal sprouting increased, indicating dormancy relaxation. Ecotypes from more arid sites across the rainfall gradient showed delayed onset of sprouting and lower maximal sprouting, but did not differ in rate of sprouting. Planting at 10 °C promoted sprouting and cancelled differences among ecotypes in dormancy relaxation.

Conclusions

Both the induction and the relaxation of summer dormancy in P. bulbosa are correlated with mean annual precipitation at the site of population origin. Ecotypes from arid habitats have earlier dormancy induction and delayed dormancy relaxation, compared with those from mesic habitats.     

Adaptation to flooding in upland and lowland ecotypes of Cyperus rotundus, a troublesome sedge weed of rice: tuber morphology and carbohydrate metabolism

Background and aims

In recent years, Cyperus rotundus has become a problem weed in lowland rice (Oryza sativa) grown in rotation with vegetables in the Philippines. As the growth of C. rotundus is commonly suppressed by prolonged flooding, the ability of the weed to grow vigorously in flooded as well as upland conditions suggests that adapted ecotypes occur in these rotations. Studies were conducted to elucidate the mechanisms that permit C. rotundus to tolerate flooded soil conditions.

Methods

Upland and lowland ecotypes of C. rotundus were compared in terms of growth habit, carbohydrate reserves and metabolism, and activities of enzymes involved in alcoholic fermentation – alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC).

Key Results

The lowland ecotype has much larger tubers than the upland ecotype. Prior to germination, the amylase activity and total non-structural carbohydrate content in the form of soluble sugars were greater in the tubers of lowland plants than in those of upland C. rotundus. At 24 h after germination in hypoxic conditions, PDC and ADH activities in the lowland plants increased, before decreasing at 48 h following germination. In contrast, ADH and PDC activities in the upland plants increased from 24 to 48 h after germination.

Conclusions

Tolerance of lowland C. rotundus of flooding may be attributed to large carbohydrate content and amylase activity, and the ability to maintain high levels of soluble sugars in the tubers during germination and early growth. This is coupled with the modulation of ADH and PDC activities during germination, possibly to control the use of carbohydrate reserves and sustain substrate supply in order to avoid starvation and death of seedlings with prolonged flooding.Key words: Anoxia, ethanol fermentation, flooding tolerance, nutsedge, Cyperus rotundus, Pasteur effect, weed ecology     

Water deficit and induction of summer dormancy in perennial Mediterranean grasses

Background and Aims

Summer dormancy is a trait conferring superior drought survival in Mediterranean perennial grasses. As the respective roles of environmental factors and water deficit on induction of summer dormancy are unclear, the effect of intense drought were tested under contrasting day lengths in a range of forage and native grasses.

Methods

Plants of Poa bulbosa, Dactylis glomerata ‘Kasbah’ and Lolium arundinaceum ‘Flecha’ were grown in pots (a) from winter to summer in a glasshouse and subjected to either an early or a late-spring drought period followed by a summer water deficit and (b) in controlled conditions, with long days (LD, 16 h) or short days (SD, 9 h) and either full irrigation or water deficit followed by rehydration. Leaf elongation, senescence of aerial tissues and dehydration of basal tissues were measured to assess dormancy. Endogenous abscisic acid (ABA) in basal tissues was determined by monoclonal immunoassay analysis.

Key Results

Even under irrigation, cessation of leaf elongation, senescence of lamina and relative dehydration of basal tissues were triggered only by a day length longer than 13 h 30 min (late spring and LD) in plants of Poa bulbosa and Dactylis glomerata ‘Kasbah’ which exhibit complete dormancy. Plants of Lolium arundinaceum ‘Flecha’ maintained leaf growth under irrigation irrespective of the day length since its dormancy is incomplete. ABA concentrations were not higher during late-spring drought than early, and could not be associated with spring dormancy induction. In summer, ABA concentration in bulbs of the desiccation-tolerant Poa were greater than in basal tissues of other species.

Conclusions

The results of both experiments tend to invalidate the hypothesis that water deficit has a role in early summer-dormancy induction in the range of tested grasses. However, a late-spring drought tends to increase plant senescence and ABA accumulation in basal tissues of forage grasses which could enhance summer drought survival.Key words: Poa bulbosa, Dactylis glomerata, Lolium arundinaceum, abscisic acid, water deficit, photoperiod, summer dormancy, perennial grass     

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.     

Characterization of the Rust Fungus, Puccinia emaculata, and Evaluation of Genetic Variability for Rust Resistance in Switchgrass Populations

Several fungal pathogens have been identified on ornamental and native stands of switchgrass (Panicum virgatum L.). Diseases of switchgrass, particularly rust, have been largely neglected and are likely to become the major limiting factor to biomass yield and quality, especially when monocultured over a large acreage. Based on teliospore morphology and internal transcribed spacer-based diagnostic primers, the rust pathogen collected from switchgrass research fields in Oklahoma was identified as Puccinia emaculata. Furthermore, to identify genetically diverse source(s) of rust resistance, several switchgrass genotypes from both upland (cv. ‘Summer’ and ‘Cave-in-Rock’) and lowland (cv. ‘Alamo’ and ‘Kanlow’) ecotypes were evaluated in Ardmore, Oklahoma during 2008 and 2009 and in growth chamber assays. Field and growth chamber evaluations revealed a high degree of genetic variation within and among switchgrass cultivars. In general, Alamo and Kanlow showed moderate resistance to P. emaculata, while Summer was highly susceptible. Distinct ecotypic variations for reactions to rust were also prevalent with the lowlands maintaining a high level of resistance. These results suggest the potential for improvement of rust resistance via the selection of resistant individuals from currently available cultivars. Further, the selection pressure on the pathogen would also be reduced by employing several rust resistant cultivars in production-scale situations.     

Adaptation of rhizome connections in drylands: increasing tolerance of clones to wind erosion

Background and Aims

Wind erosion is a severe stress for plants in drylands, but the mechanisms by which plants withstand erosion remain largely unknown. Here, the hypothesis is tested that maintaining rhizome connections helps plants to tolerate erosion.

Methods

Five transects were established across an inland dune in Inner Mongolia, China, and measurements were made of leaf number, biomass per ramet and rhizome depth of Psammochloa villosa in 45 plots. In 40 × 40 cm plots of P. villosa on another dune, the top 15 or 30 cm of sand was removed for 1·5 or 3 months to simulate short- and long-term moderate and severe erosion, respectively, with untreated plots as controls, and the rhizomes at the edges of half of the plots were severed to mimic loss of rhizome connections.

Key Results

Leaf number and biomass per ramet showed quadric relationships with rhizome depth; when rhizomes were exposed to the air, the associated ramets either died or became very weak. Ramet number, leaf number and biomass per plot decreased with increasing erosion severity. Rhizome connections did not affect these traits under control or short-term erosion, but increased them under long-term erosion.

Conclusions

Rhizome connections alleviated the negative effects of erosion on P. villosa, very likely because the erosion-stressed ramets received water and/or photosynthates translocated from those connected ramets that were not subject to erosion. This study provides the first evidence that maintaining rhizome connections helps plants to tolerate erosion in drylands.Key words: Clonal integration, inland-dune grass, Psammochloa villosa, resource sharing, rhizome severing, wind erosion     

Quantitative characterization of clumping in Scots pine crowns

Background and Aims

Proper characterization of the clumped structure of forests is needed for calculation of the absorbed radiation and photosynthetic production by a canopy. This study examined the dependency of crown-level clumping on tree size and growth conditions in Scots pine (Pinus sylvestris), and determined the ability of statistical canopy radiation models to quantify the degree of self-shading within crowns as a result of the clumping effect.

Methods

Twelve 3-D Scots pine trees were generated using an application of the LIGNUM model, and the crown-level clumping as quantified by the crown silhouette to total needle area ratio (STAR_crown) was calculated. The results were compared with those produced by the stochastic approach of modelling tree crowns as geometric shapes filled with a random medium.

Key Results

Crown clumping was independent of tree height, needle area and growth conditions. The results supported the capability of the stochastic approach in characterizing clumping in crowns given that the outer shell of the tree crown is well represented.

Conclusions

Variation in the whole-stand clumping index is induced by differences in the spatial pattern of trees as a function of, for example, stand age rather than by changes in the degree of self-shading within individual crowns as they grow bigger.     

Pre- and post-harvest influences on seed dormancy status of an Australian Goodeniaceae species, Goodenia fascicularis

Background and Aims

The period during which seeds develop on the parent plant has been found to affect many seed characteristics, including dormancy, through interactions with the environment. Goodenia fascicularis (Goodeniaceae) seeds were used to investigate whether seeds of an Australian native forb, harvested from different environments and produced at different stages of the reproductive period, differ in dormancy status.

Methods

During the reproductive phase, plants were grown ex situ in warm (39/21 °C) or cool (26/13 °C) conditions, with adequate or limited water availability. The physiological dormancy of resulting seeds was measured in terms of the germination response to warm stratification (34/20 °C, 100 % RH, darkness).

Key Results

Plants in the cool environment were tall and had high above-ground biomass, yet yielded fewer seeds over a shorter, later harvest period when compared with plants in the warm environment. Seeds from the cool environment also had higher viability and greater mass, despite a significant proportion (7 % from the cool-wet environment) containing no obvious embryo. In the warm environment, the reproductive phase was accelerated and plants produced more seeds despite being shorter and having lower above-ground biomass than those in the cool environment. Ten weeks of warm stratification alleviated physiological dormancy in seeds from all treatments resulting in 80–100 % germination. Seeds that developed at warm temperatures were less dormant (i.e. germination percentages were higher) than seeds from the cool environment. Water availability had less effect on plant and seed traits than air temperature, although plants with reduced soil moisture were shorter, had lower biomass and produced fewer, less dormant seeds than plants watered regularly.

Conclusions

Goodenia fascicularis seeds are likely to exhibit physiological dormancy regardless of the maternal environment. However, seeds collected from warm, dry environments are likely to be more responsive to warm stratification than seeds from cooler, wetter environments.Key words: Goodenia fascicularis, Goodeniaceae, Australia, physiological dormancy, seeds, temperature, soil moisture, maternal influence, climate     

Understanding the germination of bulbils from an ecological perspective: a case study on Chinese yam (Dioscorea polystachya)

Background and Aims

Bulbils serve as a means of vegetative reproduction and of dispersal for many plants; this latter aspect making them analogous to seeds. However, germination of bulbils may differ considerably from seeds due to dissimilar anatomical structures and perhaps environmental cue perception. The few laboratory studies done on bulbils suggest that their germination is similar to that of seeds in the same habitats and to vegetative buds of winter-dormant plants. The present study is the first to examine how bulbil germination is controlled in nature in relation to dispersal (before vs. after winter of the same cohort) and to ambient temperatures.

Methods

Under laboratory conditions, temperature and light requirements for root and shoot emergences from bulbils of Dioscorea polystachya collected in September, 2005, February, 2006 (produced in 2005) and July, 2006 were determined. Effects of cold stratification and dry storage for releasing dormancy were tested on September and July bulbils. The phenology of dormancy release and of root and shoot emergences and the persistence of bulbils in soil were followed over time under field conditions.

Key Results

Although a low percentage of bulbils collected in July or in September produced roots, but no shoots, in the laboratory and field, these roots died within approx. 1 month. Regardless of collection date, cold stratification markedly increased root and shoot emergences. Bulbils sown outdoors in October produced roots and shoots the following March and April, respectively. The soil bulbil bank is short lived.

Conclusions

Bulbils of D. polystachya are similar to seeds of many temperate plants being mostly dormant when dispersed in summer or autumn and overcoming dormancy with cold stratification during winter. Adaptively, bulbil germination primarily occurs in spring at the beginning of a favourable period for survivorship and growth.     

Co-adaptation of seed dormancy and flowering time in the arable weed Capsella bursa-pastoris (shepherd's purse)

Background and Aims

The duration of the plant life cycle is an important attribute that determines fitness and coexistence of weeds in arable fields. It depends on the timing of two key life-history traits: time from seed dispersal to germination and time from germination to flowering. These traits are components of the time to reproduction. Dormancy results in reduced and delayed germination, thus increasing time to reproduction. Genotypes in the arable seedbank predominantly have short time to flowering. Synergy between reduced seed dormancy and reduced flowering time would create stronger contrasts between genotypes, offering greater adaptation in-field. Therefore, we studied differences in seed dormancy between in-field flowering time genotypes of shepherd''s purse.

Methods

Genotypes with early, intermediate or late flowering time were grown in a glasshouse to provide seed stock for germination tests. Secondary dormancy was assessed by comparing germination before and after dark-incubation. Dormancy was characterized separately for seed myxospermy heteromorphs, observed in each genotype. Seed carbon and nitrogen content and seed mass were determined as indicators of seed filling and resource partitioning associated with dormancy.

Key Results

Although no differences were observed in primary dormancy, secondary dormancy was weaker among the seeds of early-flowering genotypes. On average, myxospermous seeds showed stronger secondary dormancy than non-myxospermous seeds in all genotypes. Seed filling was similar between the genotypes, but nitrogen partitioning was higher in early-flowering genotypes and in non-myxospermous seeds.

Conclusions

In shepherd''s purse, early flowering and reduced seed dormancy coincide and appear to be linked. The seed heteromorphism contributes to variation in dormancy. Three functional groups of seed dormancy were identified, varying in dormancy depth and nitrate response. One of these groups (FG-III) was distinct for early-flowering genotypes. The weaker secondary dormancy of early-flowering genotypes confers a selective advantage in arable fields.     

The effect of temperature on reproduction in the summer and winter annual Arabidopsis thaliana ecotypes Bur and Cvi

Background and Aims

Seed yield and dormancy status are key components of species fitness that are influenced by the maternal environment, in particular temperature. Responses to environmental conditions can differ between ecotypes of the same species. Therefore, to investigate the effect of maternal environment on seed production, this study compared two contrasting Arabidopsis thaliana ecotypes, Cape Verdi Isle (Cvi) and Burren (Bur). Cvi is adapted to a hot dry climate and Bur to a cool damp climate, and they exhibit winter and summer annual phenotypes, respectively.

Methods

Bur and Cvi plants were grown in reciprocal controlled environments that simulated their native environments. Reproductive development, seed production and subsequent germination behaviour were investigated. Measurements included: pollen viability, the development of floral structure, and germination at 10 and 25 °C in the light to determine dormancy status. Floral development was further investigated by applying gibberellins (GAs) to alter the pistil:stamen ratio.

Key Results

Temperature during seed development determined seed dormancy status. In addition, seed yield was greatly reduced by higher temperature, especially in Bur (>90 %) compared with Cvi (approx. 50 %). The reproductive organs (i.e. stamens) of Bur plants were very sensitive to high temperature during early flowering. Viability of pollen was unaffected, but limited filament extension relative to that of the pistils resulted in failure to pollinate. Thus GA applied to flowers to enhance filament extension largely overcame the effect of high temperature on yield.

Conclusions

High temperature in the maternal environment reduced dormancy and negatively affected the final seed yield of both ecotypes; however, the extent of these responses differed, demonstrating natural variation. Reduced seed yield in Bur resulted from altered floral development not reduced pollen viability. Future higher temperatures will impact on seed performance, but the consequences may differ significantly between ecotypes of the same species.