Timing of seed dispersal in Daucus carota L. (Apiaceae)

Abstract: This study describes the temporal pattern of seed dispersal in Daucus carota and examines the fate of seeds dispersed at different dates in SE Michigan. Plants varied greatly in both time of onset and rate of dispersal. Onset was directly related to flowering time, a phenotypically plastic character, and tended to occur earlier in newly established populations. Dispersal rate was similar for different-aged populations and for plants flowering at different times. The latter indicates that later-flowering plants dispersed a greater proportion of seeds in winter. Seed germination in outdoor plots declined when dispersal was delayed experimentally. Winter dispersal distances over snow surpassed autumn dispersal distances. However, only in some years did conditions (high winds and snow cover) required for longer distance dispersal occur while many seeds were still viable. Survival and reproduction of autumn-versus spring-germinating offspring varied greatly among years in experimental and natural populations. The fate of seeds dispersed at different times is unpredictable, which may explain the extended dispersal pattern observed in D. carota. Individual variation in dispersal rate is associated with environmental uncertainty in 1) timing of conditions suitable for dispersal over snow and 2) relative success of autumn-versus springgerminating offspring. Early onset of dispersal, more common in the youngest populations, improves chances for local population expansion; late onset of dispersal found in older populations improves chances for new site colonization

The genetic and environmental control of reproductive timing in a short-lived monocarpic species Daucus carota (Umbelliferae)

(1) Offspring of annual, biennial and triennial Daucus carota were grown under three nutrient regimes in a growth chamber to measure the effects of nutrient supply and maternal age of flowering on offspring size and growth rate and the effects of all four variables on year of flowering. (2) Offspring rosette size and recent growth rate were both good predictors of year of flowering. An increase in size but a decrease in relative growth at the end of the summer were associated with an increased probability of flowering in the next season. The results are consistent with the Wilbur—Collins model, which suggests that both size and recent growth rate influence reproductive timing and that individuals track resources by delaying the year of reproduction if resources abound or by accelerating reproduction if resources become limiting. This model, although proposed for amphibians, may describe the general relationship between growth and reproductive timing in monocarpic plants. (3) Maternal age and nutrient supply influenced rosette size, relative growth rate, and flowering time. Annual maternal plants and high nutrients produced both the largest offspring and the greatest number of annuals. Also, the maternal contribution acted directly upon the year of flowering as well as indirectly through size and recent growth. Both genetic variability and habitat heterogeneity explain the variation in year of flowering in natural populations. (4) The results provide evidence that the early-rosette growth rate and the length of the pre-reproductive period are negatively correlated, as predicted by theories about the evolution of life-history patterns. Rapid growth may help annuals to persist in habitats which generally favour biennials. (5) Response to nutrient supply did not vary among maternal age groups as might be expected in colonizing species. This suggests that phenotypic plasticity in response to nutrient supply evolves independently of year of reproduction

A Comparison of Germination and Early Growth of Four Early Successional Tree Species of the Southeastern United States in Different Soil and Water Regimes

To learn more about the basic biology of exotic relative to native tree species we conducted a greenhouse experiment comparing the germination and early seedling growth of four early successional tree species found in the southeastern United States: two exotics (Ailanthus altissima and Paulownia tomentosa) and two natives (Liquidambar styradflua and Platanus occidentalis). Five soil types and three water regimes were used for the experiment. Liquidambar and Platanus, the native species, germinated significantly more quickly and were more sensitive to soil type than were the exotics, Ailanthus and Pauloumia. Platanus grew tallest, and along with Paulownia, accumulated the greatest total biomass. Ailanthus alone exhibited a high root/shoot ratio in all soil types. In addition, species differed in their response to soil types for multiple growth traits. The results suggest that native tree species could be used to help retard the establishment of invasive tree species on bare soil

Multigenerational effects of flowering and fruiting phenology in Plantago lanceolata

Phenological patterns of flowering and fruiting can be influenced by the effects of reproductive time on seed production. We propose here that these patterns are also influenced by phenological effects on offspring quality. Furthermore, we hypothesize that there are cross-generational trade-offs between parental and offspring components of parental fitness influencing the evolution of reproductive phenology. To test our hypothesis, we examined the multigenerational effects of flowering and fruiting phenology in Plantago lanceolata. Offspring of 30 families were transplanted into field plots to measure the effects of onsets of flowering and fruiting, duration of fruiting, percentage fungal infection, and damage by grasshoppers on total seed production, our measure of the within-generational component of parental fitness. To gather information about cross-generational contributions to parental fitness, we assessed the quality of off-spring produced at different times in terms of seed mass and germination. Families significantly differed in flowering and fruiting onsets. Larger plants began flowering earlier, and earlier flowering plants matured fruits earlier and produced fruits for a longer time. Significant family-mean correlations among these traits suggest that selection on any one trait will change all three traits. A negative family-mean correlation between fruiting onset and seed production suggests that we can expect an antagonistic trade-off in response to selection on these two traits. Early fruiting significantly reduced seed predation by grasshoppers and increased seed production. In contrast, late-maturing seeds were significantly heavier and germinated more rapidly than did early-maturing seeds produced by the same plants. The directions of the multigenerational effects support the hypothesis that there are cross-generational trade-offs between parental and offspring components of parental fitness. The experiments indicate that multigenerational fitness effects should be considered in future studies addressing the evolution of flowering and fruiting phenology

Parental effects in Plantago lanceolata. L. I. A growth chamber experiment to examine pre-and post-zygotic temperature effects

In spite of the potential evolutionary importance of parental effects, many aspects of these effects remain inadequately explained. This paper explores both their causes and potential consequences for the evolution of life-history traits in plants. In a growth chamber experiment, I manipulated the pre- and postzygotic temperatures of both parents of controlled crosses of Plantago lanceolata. All offspring traits were affected by parental temperature. On average, low parental temperature increased seed weight, reduced germination and offspring growth rate, and accelerated onset of reproduction by 7%, 50%, 5%, and 47%, respectively, when compared to the effects of high parental temperature. Both pre- and postzygotic parental temperatures (i.e., prior to fertilization vs. during fertilization and seed set, respectively) influenced offspring traits but not always in the same direction. In all cases, however, the postzygotic effect was stronger. The prezygotic effects were more often transmitted paternally than maternally. Growth and onset of reproduction were influenced both directly by parental temperature as well as indirectly via the effects of parental temperature on seed weight and germination. Significant interactions between parental genotypes and prezygotic temperature treatment (G × E interactions) show that genotypes differ in their intergenerational responses to temperature with respect to germination and growth. The data suggest that temperature is involved in both genetically based and environmentally induced parental effects and that parental temperature may accelerate the rate of evolutionary change in flowering time in natural populations of P. lanceolata. The environmentally induced temperature effects, as mediated through G × (prezygotic) E interactions are not likely to affect the rate or direction of evolutionary change in the traits examined because postzygotic temperature effects greatly exceed prezygotic effects

Seed Mortality in Daucus Carota Populations: Latitudinal Effects

Daucus carota, a common herbaceous weed, grows over a wide latitudinal range in eastern North America. Viability and germination tests of mature seeds collected from 36° to 45°N were conducted to measure predispersal seed mortality. Viability and germination declined as latitude of the seed source decreased. Only 30-50% of the seeds from southern populations germinated owing to high embryo inviability and absence of embryos. Sixty to ninety percent of the seeds from northern populations germinated. Reciprocal planting of seeds in outdoor experimental plots at three latitudes and testing of seeds over two generations together showed that the environment in which seeds mature, rather than environmental preconditioning over generations or genetically-based differences among populations, explain this variation in germination ability. Within-latitude germination declined in experimental plots as population age of the seed source within latitudes increased. The data indicate that predispersal seed mortality can influence local population persistence and that seed mortality is an increasingly important factor in population regulation at the southern limit of the species' range

Timing of seed dispersal in Daucus carota L. (Apiaceae)

This study describes the temporal pattern of seed dispersal in Daucus carota and examines the fate of seeds dispersed at different dates in SE Michigan. Plants varied greatly in both time of onset and rate of dispersal. Onset was directly related to flowering time, a phenotypically plastic character, and tended to occur earlier in newly established populations. Dispersal rate was similar for different-aged populations and for plants flowering at different times. The latter indicates that later-flowering plants dispersed a greater proportion of seeds in winter. Seed germination in outdoor plots declined when dispersal was delayed experimentally. Winter dispersal distances over snow surpassed autumn dispersal distances. However, only in some years did conditions (high winds and snow cover) required for longer distance dispersal occur while many seeds were still viable. Survival and reproduction of autumn-versus spring-germinating offspring varied greatly among years in experimental and natural populations. The fate of seeds dispersed at different times is unpredictable, which may explain the extended dispersal pattern observed in D. carota. Individual variation in dispersal rate is associated with environmental uncertainty in 1) timing of conditions suitable for dispersal over snow and 2) relative success of autumn- versus spring-germinating offspring. Early onset of dispersal, more common in the youngest populations, improves chances for local population expansion; late onset of dispersal found in older populations improves chances for new site colonization

Sexual Dimorphism in a Temperate Dioecious Tree, Ilex montana (Aquifoliaceae)

A study of a population of Ilex montana mountain winterberry showed a strongly skewed age distribution with few individuals reaching reproductive maturity. Sexual differences of the reproductive adults were manifested in several ways. The tertiary sex ratio of 436 trees was significantly male-biased; males produced more flowers per tree than did females, and there was a slight spatial segregation between males and females. However, males and females did not differ in estimated age and size distributions, nor did the proportion of males vary with density. The data suggest that females flower less often than do males and that microenvironmental variation influences the spatial patterning of sexes within populations of this temperate dioe- cious tree

Parental Effects on Seed Mass: Seed Coat but Not Embryo/Endosperm Effects

Many biologists studying environmentally induced parental effects have indirectly suggested that the parental environment alters seed mass by altering the amount of endosperm or embryo tissue in the seed. We tested this hypothesis by measuring the effects of parental temperature on total seed mass, seed coat mass, and embryo/endosperm mass in offspring of Plantago lanceolata. Parental temperature significantly affected total seed and coat mass but not endosperm/embryo mass. Thus, larger seeds do not contain more resources in the embryo or endosperm than do small seeds. Rather they have more coat mass, which probably strongly influences germination. These results suggest caution when making assumptions about the pathways by which environmentally induced parental effects are transmitted in plant species. We also observed that controlled crosses differed significantly in their response to parental temperature, which provides evidence for genetic variation in environmentally induced parental effects, i.e., intergenerational phenotypic plasticity, in natural populations of P. lanceolata

Phenological patterns of terrestrial plants

The term phenology is derived from the Greek word phaino meaning to show or to appear. Hence, phenology is defined as the study of the seasonal timing of life cycle events. For plants the seasonal timing of such events can be critical to survival and reproduction. In agriculture the most common failure of introduced crops is the inability to adjust to the seasons imposed by the new, environment (68). In the past few years, interest in the ecology and evolution of timing of life cycle events has grown. Here we review the literature on phenological patterns of germination, flowering, and fruiting (including dispersal)