The parsnip webworm (Depressaria pastinacella) and the wild parsnip (Pastinaca sativa) together represent a potentially "coevolved" system in that throughout their ranges the plant has relatively few other herbivores and the insect has virtually no other hosts. Individual wild parsnip plants within a central Illinois population vary in their content and composition of furanocoumarins, secondary compounds with insecticidal properties. Half-sib and parent-offspring regression estimates of the heritability of furanocoumarins demonstrate that this variation is genetically based. Wild parsnip plants also vary in their resistance to damage by the parsnip webworm, which feeds on flowers and developing seeds. In an experimental garden, seed production in the primary umbel ranged from 0 to 1,664 seeds among individuals, and mean seed production of half-sib families ranged from 3.7 seeds to 446.0 seeds. Approximately 75% of the variation in resistance among half-sib families to D. pastinacella was attributable to four furanocoumarin characteristics-resistance is positively related to the proportion of bergapten and the amount of sphondin in seeds, and negatively related to the amount of bergapten and the proportion of sphondin in leaves. Each of the four resistance factors had significant heritability. Thus, resistance in wild parsnip to the parsnip webworm is to a large extent chemically based and genetically controlled. Genetic correlations among fitness and resistance characters, however, tend to limit coevolutionary responses between herbivore and plant. In greenhouse plants protected from herbivory, several of the resistance factors have negative genetic correlations with potential seed production. Ostensibly, highly resistant plants in the absence of herbivory would be at a competitive disadvantage in the field. The selective impact of the herbivore is also limited in this population by a negative genetic correlation among resistance factors. Selection to increase one resistance factor (e.g., the proportion of bergapten in the seed) would at the same time decrease the amount of a second resistance factor (e.g., the amount of sphondin in the seed). The wild parsnip and the parsnip webworm, then, appear to have reached an evolutionary "stalemate" in the coevolutionary arms race.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1986.tb05746.x

Constraints on chemical coevolution: wild parsnips and the parsnip webworm.

Understanding the adaptations that allow species to live in temporally variable environments is essential for predicting how they may respond to future environmental change. Variation at the intergenerational scale can allow the evolution of bet-hedging strategies: a novel genotype may be favoured over an alternative with higher arithmetic mean fitness if the new genotype experiences a sufficiently large reduction in temporal fitness variation; the successful genotype is said to have traded off its mean and variance in fitness in order to ‘hedge its evolutionary bets’. We review the evidence for bet-hedging in a range of simple plant systems that have proved particularly tractable for studying bet-hedging under natural conditions. We begin by outlining the essential theory, reiterating the important distinction between conservative and diversified bet-hedging strategies. We then examine the theory and empirical evidence for the canonical example of bet-hedging: diversification via dormant seeds in annual plants. We discuss the complications that arise when moving beyond this simple case to consider more complex life-history traits, such as flowering size in semelparous perennial plants. Finally, we outline a framework for accommodating these complications, emphasizing the central role that model-based approaches can play.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2010.0707

Evolutionary bet-hedging in the real world: empirical evidence and challenges revealed by plants

Individual plants were marked in field populations of four biennial plant species, Verbascum thapsus L., Oenothera biennis L., Daucus carota L., and Tragopogon dubius Scop., and followed for 2 or 3 years. The relationship of both rosette size and age to the probability of an individual dying, remaining vegetative, or flowering was determined for each species. In all four species, a minimum size must be reached before flowering can be induced and above that minimum the probability of flowering increases directly with rosette size. Conversely, the probability of dying decreases with increasing rosette size. In general, the fate of a rosette appears to be independent of its age although in populations where the probability of death is high for individuals above the minimum size for flowering, the age of a rosette may influence its fate to some extent.

Predictions of fate from rosette size in four "biennial" plant species: Verbascum thapsus, Oenothera biennis, Daucus carota, and Tragopogon dubius.

The article was published in American Journal of Botany, 71:6 (1984), pp. 795-802. http://www.jstor.org/stable/pdfplus/2443470.pdf Author Posting. Copyrightc Botanical Society of America 1984. This article is posted here by permission of thepublisher for personal use, not for redistribution.This paper is posted at Iowa Research Online.http://ir.uiowa.edu/biology pubs/2

https://ir.uiowa.edu/cgi/viewcontent.cgi?article=1000&context=biology_pubs

Variation in seed weight and its effects on germination in Pastinaca sativa L-(Umbelliferae)

Monocarpic plants, which flower once then die, are ideal systems for testing evolutionary ideas because the cost of reproduction is easily quantified and the timing of flowering is a key determinant of darwinian fitness. Monocarps should flower at the size that maximizes fitness, enabling models of life-history evolution to be tested. These models are becoming increasingly sophisticated and accurate, making a review of the techniques and underlying theory timely. Here, we review the long-term demography of monocarpic species, focusing on how demographic rates vary with size and age. We then examine the broad array of evolutionary models, and question what aspects of the demography are crucial for the successful prediction of the size and age at flowering, shedding light on both the important aspects of monocarp demography and current advances in life-history modelling.

Evolutionary demography of monocarpic perennials.

(1) Wild parsnip (Pastinaca sativa L.) is a monocarpic perennial herb which grows as a weed in North America in grassy habitats that are usually mown in summer. Flowering occurs in May and June, and seeds are mature by early July. Seeds remain attached to the dead erect shoots until autumn, unless the plants are disturbed, as by mowing. (2) At maturity small numbers of seeds can germinate at summer temperatures. As the seeds after-ripen during summer their ability to germinate at high temperatures increases, but there is little increase in ability to germinate at autumn temperatures. Stratification (moist chilling) over winter lowers the temperature requirement for germination, and therefore the majority of seeds in the field germinate in early spring. (3) Seedling mortality is high; of 3980 seedlings marked in a natural population in Kentucky, only thirty-three (0.83%4) flowered before dying. Cohorts of seedlings marked in August, November, December, January and April had concave survivorship curves; cohorts marked in February and March initially had concave survivorship curves, but the curves subsequently became linear. (4) Plants produce rosettes and grow until the second, third, fourth or fifth growing season before producing flowers. Vernalization is required for flowering, and plants reach the flowering stage quicker under long than under short days. (5) An old-field population showed a wide range of individual plant sizes in late May: flowering plants had root-crown diameters (RCDs) of 5-39 mm, and non-flowering plants had RCDs of < 1 (first-year seedlings) to 10 mm. It is suggested that plants must reach a critical size before they can respond to the environmental stimuli of vernalization and photoperiod.

Studies on the autecology and population biology of the weedy monocarpic perennial, Pastinaca sativa

Increasing deposition of atmospheric nitrogen (N) due to accelerated human activities is a threat to various ecosystems. However, there is a lack of long‐term experimental evidence demonstrating the seasonal dynamics of plant species turnover that ultimately determines species richness in natural ecosystems under N enrichment. Moreover, the frequency of N addition also may affect species turnover in a community, but it is rarely studied. To assess the responses of a plant community to N addition, we manipulated the amounts (0–50 g N m−2 year−1) and frequency (2 vs. 12 times year−1) of N addition in an Inner Mongolian typical steppe ecosystem in northern China for 12 consecutive years (2008–2020). We measured species richness and density of plant in the growing seasons (May–September) from 2018 to 2020, starting 10 years after the initial N addition treatment. Both species gain and species loss decreased with increasing amounts of N addition, resulting in a lower plant species turnover rate and greater similarity in the community between two adjacent months throughout the growing season. Species loss and species gain increased modestly under high N addition frequency. Species gain was more important than species loss in determining species richness after a decade of N application. In addition, plant density increased at high N amounts, mainly driven by enhanced clonal growth of the dominant species, Leymus chinensis. Synthesis. Together, these results suggest that high levels of N deposition may suppress species richness due to aggravated soil chemical properties and may favour growth of a limited number of N‐tolerant species compared to systems that experience low levels of N deposition. To conserve biodiversity and to facilitate restoration of degraded grassland ecosystems exposed to long‐term N deposition, amelioration of the acidified soils induced by N deposition may be an important strategy to use.

Intra‐annual species gain overrides species loss in determining species richness in a typical steppe ecosystem after a decade of nitrogen enrichment

The herbaceous perennial legume Mimosa pudica is an invasive weed in many tropical and subtropical regions and a serious problem for farmers since it is difficult to eliminate from crop field by hand. Moreover, it has water impermeable seeds, i.e., physical dormancy (PY), which could persist in the soil seed bank for a long period of time, thus making it a big challenge to control. The aims of this study were to test the effect of various laboratory methods on breaking PY in seeds of M. pudica, to identify the site(s) of water entry into seeds of M. pudica and compare results of dormancy-breaking methods for seeds of M. pudica with those of 36 other species of Mimosa reported in the literature. Mechanical scarification, wet heat and cycles of wet heat and ice water effectively broke PY in seeds of M. pudica. Following wet heat at 80 °C for 10 min, water uptake was via the hilar region but not the pleurogram; small cracks made in the pleurogram by this treatment were not deep enough for water to enter the seed. Neither tolerance to summer temperatures nor PY is likely to be the cause of invasiveness of this species, since seeds of rare and endemic species of Mimosa also tolerate summer temperatures and have PY.

Carol M. Baskin

Papers

Studies on the autecology and population biology of the weedy monocarpic perennial, Pastinaca sativa

Intra‐annual species gain overrides species loss in determining species richness in a typical steppe ecosystem after a decade of nitrogen enrichment

Methods of breaking physical dormancy in seeds of the invasive weed Mimosa pudica (Fabaceae) and a comparison with 36 other species in the genus