Life-history variation in the short-lived herb <Emphasis Type="Italic">Rorippa palustris</Emphasis>: effect of germination date and injury timing

Life-history variation in annuals is known to be caused by size requirements for photoinduction of flowering, but the importance of germination date and injury was overlooked so far even though they may play an important role in disturbed habitats. To test the effect of germination date and timing of injury on life-history variation of an annual plant, we performed a 2-year pot experiment with the root-sprouting herb Rorippa palustris. Plants belonging to six different cohorts, and sown at monthly intervals from April to September, were injured (all stem parts removed) in three ontogenetic stages: vegetative rosettes, flowering plants and fruiting plants. Plants from the April, and partly from the May, cohort behaved as summer annuals: they started to bolt at the same time, resprouted and overwintered poorly. Plants from the June cohort flowered in the first season as well, but they entered the bolting stage a month later than the preceding cohorts, produced the least fruits, but overwintered successfully and flowered again the second year (polycarpic perennials). Cohorts germinating after the summer solstice did not flower during the first year and, with the exception of the September cohort, overwintered successfully and flowered the second year (winter annuals). After injury, the pattern of life-histories was the same as in control plants, although generative reproduction was interrupted by injury in the first year of the experiment. About one quarter of␣plants injured in the vegetative stage regenerated after injury irrespective of cohort, however, regeneration was enhanced in larger plants. Regeneration of plants in the flowering and fruiting stages depended on date of injury in relation to day length, being the most successful after the summer solstice. Life-history variation, together with the ability to resprout after severe injury in the pioneer wetland herb Rorippa palustris is caused by its ability to germinate throughout the season. Even the second tested factor, the effect of injury, is modified by germination time. The experiment points to a complex nature of factors affecting life-history variation as well as resprouting after severe injury in short lived plants. An erratum to this article can be found at     

Resprouting after disturbance: an experimental study with short-lived monocarpic herbs

We experimentally demonstrated the ability of three short-lived monocarpic species to vegetatively regenerate (resprout) from roots after severe disturbance. We assessed the relationship between resprouting ability and (1) timing of injury with respect to life-cycle stage (reproductive vs. vegetative plant), life-history mode (annual vs. winter annual) and phenological stage (flowering vs. fruiting plant), (2) nutrient availability, and (3) disturbance severity (removal of all axillary buds Yes/No). In a chamber experiment with the annual or potentially winter annualRorippa palustris, all injured plants resprouted in all nutrient levels and day-length regimes (day-length regimes simulated conditions of an annual and a winter annual cohort). The number of adventitious buds on roots was positively affected only by injury. The extent of regeneration and amount of regenerated biomass were higher at high nutrient level and long-day regime.     

Compensation of seed production after severe injury in the short-lived herb Barbarea vulgaris

A pot experiment with the common ruderal herb Barbarea vulgaris (Brassicaceae) was set up to elucidate to what extent short-lived species sprouting from roots regenerate and compensate for seed production after damage. We tested if sprouting from roots ensures survival after severe aboveground biomass damage, but the number of seeds produced declines with increasing severity of injury, decreasing nutrient availability and progress in the life cycle at the time of injury.Plants of B. vulgaris were cultivated in a 3-year garden experiment at two nutrient levels (high vs. low). During the experiment, two levels of injury severity were applied: high (removal of all aboveground biomass) and low (removal of aboveground biomass leaving basal axillary buds intact). Damage was applied at four life-cycle phases: young rosette, overwintered rosette, flowering plant and fruiting plant. All injured plants survived and resprouted irrespective of life-cycle phase, severity of injury and nutrient availability. Injury significantly affected seed production and also the plants’ life cycle. Plants injured in the second year of their life (overwintered rosette, flowering plant and fruiting plant) postponed reproduction to the third season (in the case of high injury severity) or their seed production was lower than in intact plants (in the case of low injury severity). In plants injured in the first life year, seed production and life cycle were not influenced. Nutrient level only marginally affected resprouting after injury and seed production.The experiment showed that the ability to sprout from roots enables plants to survive a 100% loss of aboveground biomass, and to keep some seed production or even compensate it. The short-lived ruderal species B. vulgaris successfully copes with severe disturbance by resprouting and does not rely only on its seed bank.     

Reproductive competence from an annual and a perennial perspective

Plants at early stages of development undergo a juvenile phase during which they are not competent to flower in response to environmental stimuli. The length of this phase varies among species and is extended in perennial plants particularly. In annuals, temporal changes in expression of microR156 (miR156), miR172, and their targets are correlated with the transition from the juvenile to the adult phase and flowering. This developmental transition in perennials is probably more complex than in other plants and the molecular mechanisms are less well understood. In addition, once perennials become adult and capable of reproduction they still keep some meristems in the vegetative state that contribute to their polycarpic growth habit. Juvenility and polycarpy, although considered as two different processes in perennials, might be related.     

Flowering requirements of Polymnia canadensis (Asteraceae) and their influence on its life history variation

The purpose of this study is to determine the flowering requirements of Polymnia canadensis and how they correspond to the occurrence of winter annuals, biennials, and short-lived monocarpic perennials in this species. Polymnia canadensis has a vernalization requirement for flowering, and even very small plants (i.e., those with one pair of leaves) can be vernalized. Vernalized plants can flower under both long- and short days. However, to flower plants must attain a minimum postvernalization size. Plants of this primarily monocarpic species that do not die after they flower once require another period of vernalization to flower a second time (i.e., to be dicarpic). Vernalized plants exposed to high temperatures can be devernalized; these must be re-vernalized in order to flower.     

Carbon source for reproduction in a spring ephemeral herb, Corydalis ambigua (Papaveraceae)

1.  The carbon source for reproduction in plants may differ between flowering and fruiting stages. To clarify how spring ephemerals use current photosynthetic products for reproduction, the allocation patterns of photosynthate at flowering and fruiting and the effects of resource limitation on reproductive performance in Corydalis ambigua were assessed.
2.  A ¹³C tracing experiment revealed that about 20% of the current photosynthetic carbon was used for reproduction at both flowering and fruiting. The proportion of ¹³C allocated to fruits was constant irrespective of the light level. In contrast, ¹³C translocation to tubers increased at fruiting, and this trend was accelerated when plants were shaded.
3.  Defoliation treatment significantly reduced nectar production and tuber mass, while seed production was not affected. Therefore, when carbon assimilation was limited, carbon was preferentially allocated to current reproduction (seeds) rather than to pollinator attraction (nectar) or storage (tuber).
4.  If seed production is partly supported by carbohydrate reserved in the old tissue of tubers, nectar and seed production may not compete strongly for carbon sources. In contrast to the ability of high seed production, the susceptibility of nectar production to current photosynthesis indicates that seed production of this species is basically limited by pollen capture.
5.  Therefore, temporal separation of resource pool for reproduction may mitigate the joint limitation of seed production between pollinator attraction and resource availability. Temporal variation of the sink–source balance of storage organ is crucial to understand the cost of reproduction in perennial plants.     

The evolution of self-fertilization in perennials

Abstract Many plants are perennials, but studies of self-fertilization do not usually include features of perennial life histories. We therefore develop models that include selfing, a simple form of perenniality, adult inbreeding depression, and an adult survivorship cost to seed production. Our analysis shows that inbreeding depression in adults diminishes the genetic transmission advantage associated with selfing, especially in long-lived perennials that experience inbreeding depression over many seasons. Perennials also pay a cost when selfing increases total seed set at the expense of future survivorship and reproduction. Such life-history considerations shed new light on the generalization that annuals self-fertilize more than perennials. Past research suggested reproductive assurance as an explanation for this association, but common modes of selfing offer equal reproductive assurance to annuals and perennials. Instead, perennials may avoid selfing because of adult inbreeding depression and the cost to future survivorship and reproduction.     

Adaptive genetic differentiation in life-history traits between populations of <Emphasis Type="Italic">Mimulus guttatus</Emphasis> with annual and perennial life-cycles

The optimal allocation to sexual and vegetative reproduction as well as the optimal values of other life-history characteristics such as phenology, growth and mating system are likely to depend on the life-cycle of the organism. I tested whether plants of Mimulus guttatus originating from temporarily wet populations where the species has an enforced annual life-cycle have higher allocation to sexual reproduction, lower allocation to vegetative reproduction, more rapid phenology, faster growth, and floral traits associated with a self-fertilizing mating system than plants from permanently wet populations where the species has a perennial life-cycle. I grew a total of 1377 plants originating from three populations with an annual life-cycle and 11 populations with a perennial life-cycle in a greenhouse under permanently and temporarily wet conditions. Plants of M. guttatus in permanently wet conditions had significantly more vegetative reproduction and tended to have a faster growth than plants in the temporarily wet conditions, indicating plasticity in these life-history traits. Plants from populations with an annual life-cycle invested significantly more in sexual reproduction and significantly less in vegetative reproduction than the ones from populations with a perennial life-cycle. Moreover, this study showed that plants originating from populations with an annual life-cycle have a significantly faster development and floral traits associated with autonomous self-fertilization. In conclusion, this study suggests that there has been adaptive evolution of life history traits of M. guttatus in response to natural watering conditions that determine the life span of the species.     

Herbivores and the evolution of the semelparous perennial life-history of plants

The relationship between a plant and its potential enemies changes drastically after reproduction has started. Using a dynamic modelling approach we study the effects of herbivores and pathogens, that are attracted by reproducing plants, on optimal allocation of resources, and life-history strategies. We assume that the level of attack increases with the investment in reproduction, which may lead to a reduction of current years reproductive success, a reduction of storage efficiency or an increase of plant mortality. If herbivores or pathogens attracted by flowering plants mainly reduce current years reproductive success, the optimal life-history is annual or iteroparous perennial if the attack is an all or nothing event. If the level of consumption increases with the number of herbivores attracted, the optimal life-history is most likely iteroparity with or without mast years. Only under very restricted conditions this may lead to semelparity. If herbivores mainly reduce the efficiency of the resources stored for next year, the optimal life-history is iteroparity. If herbivores mainly reduce survival, the optimal solution is likely to be mast years or semelparity. For parameter values that are realistic for Cynoglossum officinale, a semelparous perennial from calcereous soils, the model predicts that reproduction should start in the third year and that 99% of the available resources at the end of season should be invested in reproduction and only 1% saved for growth next year. With such an investment only c. 1% of the plants would survive after reproduction, so the optimal life-history is close to semelparity. For the small fraction of plants that reproduce more than once, years of vegetative growth only and years with reproduction should alternate. Multiple reproduction is rare in C. officinale. However, such a life history is very common for plants known as semelparous perennial. Although the available empirical evidence is, as yet, circumstantial rather than conclus ive we propose that reproduction related mortality mediated through herbivores or pathogens may play a role in the evolution of the semelparous perennial life-history.     

Seasonal patterns of carbon assimilation and allocation of a summer-green forest herb, <Emphasis Type="Italic">Parasenecio</Emphasis> <Emphasis Type="Italic">auriculata</Emphasis> (Senecioneae; Asteraceae)

Summer-green herbs inhabiting deciduous forests often put out aerial shoots under bright conditions before tree-canopy closure and grow until late summer under the closed canopy. Some of them produce leaves continuously even after the initiation of canopy closure, indicating an exploitation of the low light period. The manner of carbon assimilation during bright and shade periods within a growth season should reflect the seasonal patterns of vegetative growth and reproductive allocation of individual species. We examined the seasonal patterns of assimilation, partitioning of photosynthate between reproduction and storage, and the budget of reproduction of a perennial understory herb, Parasenecio auriculata. Although photosynthetic rates per unit leaf area decreased with the seasonal reduction in light level, net assimilation at the whole-plant level was maintained at a high level even after canopy closure owing to the increase in the total leaf area. Stored resource in tubers contributed to the rapid development of aerial shoots in the early season, and annual tuber growth was completed before flowering. Instant photosynthetic products considerably contributed to the maintenance of flowers but not to fruit development because of low assimilation rate during fruiting. These findings indicated that carbon assimilation during flowering contributes to sexual reproduction without influencing the development of storage organs. Stable carbon assimilation over summer by shade-acclimatized leaves enabled the maintenance of high productivity associated with high sexual reproduction.     

Life history traits of the pseudometallophyte Thlaspi caerulescens in natural populations from Northern Europe

We examined recruitment, survival, life cycle and fecundity of two metallicolous (M, on metalliferous calamine soils) and two non-metallicolous (NM, on normal soils) populations of Thlaspi caerulescens in Belgium and Luxemburg. In each population, permanent plots were monitored over two reproductive seasons. In M populations, plots were located in two contrasting environments (grass versus grove) in order to test the influence of vegetation cover on life strategy. Our results show that the monocarpic life cycle is dominant in all populations of T. caerulescens. However the length of the pre-reproductive period varies from several months (winter annuals) to 1 year or more (perennials), and is partly related to plant origin (M versus NM). Most plants growing in metalliferous environments were annuals, whereas NM plants were mostly perennials. These differences in life cycle were related to differences in survival during summer, which was better in NM than in M populations. Within each M population, different survival conditions and life cycles were observed according to vegetation cover. Plants growing in grass areas were mostly annuals and had a low survival rate in summer whereas grove plants were mostly perennials and survived better in summer. Our results suggest the selection of stress avoiders (shortening of life cycle) in M populations of T. caerulescens but only for individuals growing in grass areas. Summer survival seems to play a key role in selection of life strategy in T. caerulescens.     

Vegetation development on deposit soils starting at different seasons

Permanent plots were created in different seasons (autumn and spring) and filled with two substrates: nutrient-rich topsoil and nutrient-poor ruderal soil (n = 5 for each treatment). My objectives were to assess the influence of starting season on initial species composition, whether differences at the start cause divergent or convergent pathways of succession and which mechanisms are operating during vegetation development. Mean species richness (number of species per plot) and mean total cover of herb layer differed significantly between substrates and changed significantly during 10 year succession, but there were no significant differences with respect to starting season. However, seasonal as well as substrate effects were evident for particular dominant species and for the pattern of successional sequences. When succession on topsoil plots started in spring, first summer annuals dominated, then monocarpic and polycarpic perennial herbs, then herbaceous perennials together with woody perennials, and at the end of the decade woody perennials. When succession started in autumn, polycarpic perennial herbs dominated from the beginning, and then were replaced by woody perennials in the second half of the decade. On ruderal soil, there was a less rapid but continuous increase of polycarpic perennial herbs and woody species, both on spring and on autumn plots, whereas short-lived plants were more abundant in the first years and then decreased. Species turnover was very high from the first to the second year for all treatments (except topsoil plots starting in autumn), but slowed down during succession. Priority effects due to starting season caused high dissimilarity at the start on the nutrient-rich substrate, but convergent succession towards the end of the first decade. The main mechanisms during early succession on the nutrient-rich topsoil were tolerance based on different life-history traits and inhibition due to reduced light availability. There was no evidence for obligate facilitation. However, an indirect facilitative effect by annuals, which slowed the development of herbaceous perennials down, and thus facilitated growth of woody species, could be seen on topsoil when succession started in spring.     

Interrelationships among life-history traits in three California oaks

Life-history traits interact in important ways. Relatively few studies, however, have explored the relationships between life-history traits in long-lived taxa such as trees. We examined patterns of energy allocation to components of reproduction and growth in three species of California oaks (Quercus spp.) using a combination of annual acorn censuses, dendrometer bands to measure radial increment, and litterfall traps. Our results are generally consistent with the hypothesis that energy invested in reproduction detracts from the amount of energy available for growth in these long-lived taxa; i.e., there are trade-offs between these traits. The relationships between reproduction and growth varied substantially among specific trait combinations and tree species, however, and in some cases were in the direction opposite that expected based on the assumption of trade-offs between them. This latter finding appears to be a consequence of the pattern of resource use across years in these long-lived trees contrasting with the expected partitioning of resource use within years in short-lived taxa. Thus, the existence and magnitude of putative trade-offs varied depending on whether the time scale considered was within or across years. Collectively, our results indicate that negative relationships between fundamental life-history traits can be important at multiple levels of modular organization and that energy invested in reproduction can have measurable consequences in terms of the amount of energy available for future reproduction and both current and future growth.     

Size-dependent gender modification in a hermaphroditic perennial herb

The size-advantage model predicts that hermaphroditic organisms adjust sex allocation depending on their resource status. We investigated the relationship between size and sex allocation in the co-sexual perennial herbs Trillium erectum and Trillium grandiflorum at two sites in southern Ontario, Canada by measuring pollen and ovule production and biomass allocation at flowering and fruiting. In both species, there was a strong relationship between size and gender; larger plants allocated proportionately more biomass to female reproduction and produced fewer pollen grains relative to ovules than smaller plants. Variation in gender was better explained by size than age, although age and size were correlated. While the relationship between size and gender was similar between species, T. erectum allocated proportionately more to female reproduction than T. grandiflorum, independent of size. In the absence of pollen limitation, there was no evidence of secondary adjustment of gender at fruiting. The results are discussed in the context of models predicting size-dependent gender modification in animal-pollinated plants. Evidence about the pollination and seed dispersal biology of Trillium spp. suggests that the relative effects of local mate and resource competition may be important in driving size-dependent sex allocation in these species.     

Reproductive phenology and biology in annual and perennial Alismataceae

Two annual (Echinodorus rostratus (Nutt.) Engelm., Sagittaria calycina Engelm.) and 3 perennial (Alisma subcordatum Rat., S. brevirostra Mack & Bush, S. latifolia Wild.) species of Alismataceae were compared to determine differences in reproductive effort and timing. The annuals produce flowers and fruits earlier and later than the perennials and, under optimal conditions, the annuals also produce more (but smaller) seeds annually per plant. Populations of the annuals occur on exposed mud and are erratically abundant from year to year, while populations of the perennials are more stable. Seed viability of the annuals decreases more slowly over time. There is general congruence of these life-history attributes with the concepts of “r” and “K” strategies.     

Reproductive strategies may explain plant tolerance to inundation: A mesocosm experiment using six marsh species

We investigated the impact of inundation duration on sexual and asexual reproduction strategies in six hygrophilic angiosperm species (2 annuals - Ranunculus sardous and Ranunculus ophioglossifolius, 2 rhizomatous - Juncus articulatus and Eleocharis palustris, and 2 stoloniferous species - Glyceria fluitans and Agrostis stolonifera). Plant growth during three inundation durations (natural, +3 weeks, +6 weeks) was assessed in 20 mesocosms. Biomass and reproductive traits were measured at the onset of flowering and at seed set.The six species maintained or increased their total mass in response to increased inundation period, suggesting a good tolerance to inundation stress. No changes were recorded in flowering time for the five species that flowered. The two annual species increased the number of flowers and mass allocated to sexual reproduction with increased duration of inundation, promoting extensive seed production. The response was species-specific in perennial species with an either a positive or a negative Gaussian curve of both traits in response to the increased duration of inundation. The duration of inundation affected ramet production in all perennial species except A. stolonifera, promoting ramet production in two out of the three species. In all species except E. palustris, the length and number of stolons or rhizomes were maintained, suggesting the importance of these connections in the resistance of perennial plants to flooding stress. These species occurring in repeatedly inundated habitats thus appeared to be tolerant to long inundations thanks to both sexual reproduction and vegetative propagation.     

Node-specific branching and heterochronic changes underlie population-level differences in Mimulus guttatus (Phrymaceae) shoot architecture

? Premise of the study: Shoot architecture is a fundamentally developmental aspect of plant biology with implications for plant form, function, reproduction, and life history evolution. Mimulus guttatus is morphologically diverse and becoming a model for evolutionary biology. Shoot architecture, however, has never been studied from a developmental perspective in M. guttatus. ? Methods: We examined the development of branches and flowers in plants from two locally adapted populations of M. guttatus with contrasting flowering times, life histories, and branch numbers. We planted second-generation seed in growth chambers to control for maternal and environmental effects. ? Key results: Most branches occurred at nodes one and two of the main axis. Onset of branching occurred earlier and at a greater frequency in perennials than in annuals. In perennials, almost all flowers occurred at the fifth or more distal nodes. In annuals, most flowers occurred at the third and more distal nodes. Accessory axillary meristems and higher-order branching did not influence shoot architecture. ? Conclusions: We found no evidence for trade-offs between flowers and branches because axillary meristem number was not limiting: a large number of meristems remained quiescent. If, however, quiescence is a component of meristem allocation strategy, then meristems may be limited despite presence of quiescent meristems. At the two basalmost nodes, branch number was determined by mechanisms governing either meristem initiation or outgrowth, rather than flowering vs. branching. At the third and more distal nodes, heterochronic processes contributed to flowering time and branch number differences between populations.     

An experimental study on the growth and flowering of riparian pioneer plants under long- and short-day conditions

Plants of 19 annual taxa from the hygro-nitrophilous vegetation of Central European river banks and dried ponds were exposed as seedlings for 42 days to either natural long days (LD=ca. 15.5 h) or artificial short days (SD=11 h). Phenological and morphological growth parameters such as first flowering, main stem length, leaf size and plant dry weight were measured and compared. Based on their day-length responses, the plants were classified as 11 short-day plants (SDP), 6 long-day plants (LDP) and 2±day neutral plants (DNP). Differences of morphological plant responses under SD (compared with those under LD) reached from none to almost negligible ones in Ranunculus sceleratus, Bidens cernua, Chenopodium ficifolium and Rumex palustris to an extreme reduction in all parameters, as in Chenopodium glaucum×rubrum, Ch. rubrum, Ch. glaucum, Ch. polyspermum, Bidens radiata, Persicaria lapathifolia subsp. brittingeri and subsp. lapathifolia. Growth inhibition was usually combined with neotenic flowering in SDP. Some LDP exhibited clear dormancy responses, like the formation of smaller (winter) rosettes. Erosulate therophytes were mostly SDP while (hemi)rosulate to caespitose winter annuals or short-lived hemicryptophytes were LDP. Plants of seven species that were exposed to natural SD in spring (increasing day length) showed rather different responses. While Bidens radiata flowered in late spring soon after floral induction, other species showed increasing degrees of flowering delay (Chenopodium polyspermum→Xanthium saccharatum→Chenopodium rubrum). Only Atriplex prostrata did not exhibit any sign of floral induction until (decreasing) SD in late summer arrived. In the case of Chenopodium rubrum details of the divergent morphogenesis under SD and LD were studied. In this species SD lead to strong reduction of stem length, leaf number and leaf size, simplification of leaf outline and margins as well as an enhanced ramification in the axils of primary leaves (and cotyledons) instead of metaphylls. The study reveals that for the phytosociological vegetation group Chenopodion rubri, whose populations are mainly confined to river banks with moist but well-aerated sediments, erosulate, therophytic and often neotenic SDP are typical. For the species of the Bidention – that thrive preferentially at the margins of stagnant water on rather wet soils – (hemi)rosulate or caespitose winter annuals are frequent. They are often LDP and survive the cold season in a vegetative state, even under a shallow level of water. The SD response of therophytic pioneers together with an often strong neotenic plasticity is interpreted here as a reassurance for the plants to flower and set seed in riparian habitats that are available for colonization only for a short and insecure period of the year, which due to the asymmetric position of annual low water levels in larger rivers is often not before mid-summer.     

Do annual and perennial populations of an insect-pollinated plant species differ in mating system?

Background and AimsTheory predicts that outcrossing should be more prevalent among perennials than annuals, a pattern confirmed by comparative evidence from diverse angiosperm families. However, intraspecific comparisons between annual and perennial populations are few because such variation is uncommon among flowering plants. Here, we test the hypothesis that perennial populations outcross more than annual populations by investigating Incarvillea sinensis, a wide-ranging insect-pollinated herb native to China. The occurrence of both allopatric and sympatric populations allows us to examine the stability of mating system differences between life histories under varying ecological conditions.MethodsWe estimated outcrossing rates and biparental inbreeding in 16 allopatric and five sympatric populations in which both life histories coexisted using 20 microsatellite loci. In each population we measured height, branch number, corolla size, tube length and herkogamy for ~30 individuals. In a sympatric population, we recorded daily flower number, pollinator visitation and the fruit and seed set of annual and perennial plants.Key ResultsAs predicted, outcrossing rates (t) were considerably higher in perennial (mean = 0.76) than annual (mean = 0.09) populations. This difference in mating system was also maintained at sympatric sites where plants grew intermixed. In both allopatric and sympatric populations the degree of herkogamy was consistently larger in outcrossing than selfing plants. Perennials were more branched, with more and larger flowers than in annuals. In a sympatric population, annuals had a significantly higher fruit and seed set than perennials.ConclusionsGenetically based differences in herkogamy between annuals and perennials appear to play a key role in governing outcrossing rates in populations, regardless of variation in local ecological conditions. The maintenance of mating system and life history trait differentiation between perennial and annual populations of I. sinensis probably results from correlated evolution in response to local environmental conditions.