Folivory has long-term effects on sexual but not on asexual reproduction in woodland strawberry

Plant fitness is often a result of both sexual and asexual reproductive success and, in perennial plants, over several years. Folivory can affect both modes of reproduction. However, little is known about the effects of folivory on resource allocation to the two modes of reproduction simultaneously and across years. In a 2-year common garden experiment, we examined the effects of different levels of folivory by the strawberry leaf beetle, Galerucella tenella, on current growth, as well as current and future sexual and asexual reproduction (runners) of perennial woodland strawberry, Fragaria vesca. In addition, we measured the chlorophyll content in leaves in the year of experimental damage to determine whether there was increased photosynthetic activity, and, thus, a compensatory response to herbivory. Finally, we tested whether the previous year's folivory, as a result of its effect on plant fitness, affected the level of natural herbivory the plant experienced during the subsequent year. In the year of experimental damage, plants that were exposed to moderate and high levels of folivory (25% and 50% leaf area consumed, respectively) increased their photosynthetic activity compared to control plants. However, only plants exposed to high folivory exhibited negative effects, with a lower probability of flowering compared to control plants, indicating that plants exposed to low or moderate folivory were able to compensate for the damage. Negative effects of folivory were carried over to the subsequent year. Plants that were exposed to moderate folivory (25% leaf area consumed) during first year produced fewer flowers and fruits in the subsequent year. Runner production was consistently unaffected by folivory. The effects of experimental folivory on the level of natural herbivory were mediated via its effects on plant fitness. Our results show that the negative effects of folivory only influence sexual reproduction in woodland strawberry. Furthermore, even though woodland strawberry can tolerate moderate amounts of folivory in the short term, the negative effects on fitness appear later; this highlights the importance of studying the effects of herbivory over consecutive years in perennial plants.

Sexual stability in the nearly dioecious Pinus johannis (Pinaceae)

PREMISE OF THE STUDY

Even though dioecy is a dominant sexual system among gymnosperms, little is known about its evolutionary history. Pinus johannis may represent a model system because unisexual and monoecious individuals compose its populations. The presence of unisexual individuals in other Pinus species is a consequence of sexual lability. Here we determined whether P. johannis represents the first example of a dioecious or nearly dioecious reproductive system in conifers by evaluating its sexual stability. •

METHODS

To assess the stability of sexual expression, we quantified the proportion of male vs. female reproductive structures produced by trees over multiple years and tested for the presence of sexual dimorphism. Sexual lability hypotheses were also examined by looking at the relationship between environmental factors and sexual expression and by comparing the reproductive behavior of P. johannis with its closest labile relative, P. edulis. •

KEY RESULTS

Pinus johannis is nearly dioecious: ~99% of individuals are unisexual or express a low proportion of the opposite gender with few changes in sexual expression through time. We found sexual dimorphism consistent with sexual stability. Sexual expression did not vary with tree size/age, abiotic environment, or herbivore removal, providing evidence against sexual lability. Individuals of P. johannis tended to produce only male or female strobili, whereas those of P. edulis were mainly monoecious with a gradient in the female to male strobili ratio. •

CONCLUSIONS

This study represents the first report of a nearly stable dioecious Pinus species. The variety of sexual morphs coexisting in the same population makes P. johannis a model for studying the evolution of dioecy in gymnosperms.

Additive and non-additive effects of simulated leaf and inflorescence damage on survival, growth and reproduction of the perennial herb Arabidopsis lyrata

Herbivores may damage both leaves and reproductive structures, and although such combined damage may affect plant fitness non-additively, this has received little attention. We conducted a 2-year field experiment with a factorial design to examine the effects of simulated leaf (0, 12.5, 25, or 50% of leaf area removed) and inflorescence damage (0 vs. 50% of inflorescences removed) on survival, growth and reproduction in the perennial herb Arabidopsis lyrata. Leaf and inflorescence damage negatively and independently reduced flower, fruit and seed production in the year of damage; leaf damage also reduced rosette size by the end of the first season and flower production in the second year. Leaf damage alone reduced the proportion of flowers forming a fruit and fruit production per plant the second year, but when combined with inflorescence damage no such effect was observed (significant leaf × inflorescence damage interaction). Damage to leaves (sources) caused a greater reduction in future reproduction than did simultaneous damage to leaves and inflorescences (sinks). This demonstrates that a full understanding of the effects of herbivore damage on plant fitness requires that consequences of damage to vegetative and reproductive structures are evaluated over more than 1 year and that non-additive effects are considered.

The impact of plant and flower age on mating patterns

BACKGROUND

Over a season, plant condition, amount of ongoing reproduction and biotic and abiotic environmental factors vary. As flowers age, flower condition and amount of pollen donated and received also vary. These internal and external changes are significant for fitness if they result in changes in reproduction and mating.

SCOPE

Literature from several fields was reviewed to provide a picture of the changes that occur in plants and flowers that can affect mating over a season. As flowers age, both the entire flower and individual floral whorls show changes in appearance and function. Over a season, changes in mating often appear as alteration in seed production vs. pollen donation. In several species, older, unpollinated flowers are more likely to self. If flowers are receiving pollen, staying open longer may increase the number of mates. In wild radish, for which there is considerable information on seed paternity, older flowers produce fewer seeds and appear to discriminate less among pollen donors. Pollen donor performance can also be linked to maternal plant age. Different pollinators and mates are available across the season. Also in wild radish, maternal plants appear to exert the most control over paternity when they are of intermediate age.

CONCLUSIONS

Although much is known about the characters of plants and flowers that can change over a season, there is less information on the effects of age on mating. Several studies document changes in self-pollination over time, but very few, other than those on wild radish, consider more subtle aspects of differential success of pollen donors over time.

Herbivory differentially affects male and female reproductive traits of Cucumis sativus

Herbivory is an important selection pressure in the life history of plants. Most studies use seed or fruit production as an indication of plant fitness, but the impact of herbivory on male reproductive success is usually ignored. It is possible that plants compensate for resources lost to herbivory by shifting the allocation from seed production to pollen production and export, or vice versa. This study examined the impact of herbivory by Helix aspersa on both male and female reproductive traits of a monoecious plant, Cucumis sativus. The effects of herbivory on the relative allocation to male and female flowers were assessed through measurements of the number and size of flowers of both sexes, and the amount of pollinator visitation. We performed two glasshouse experiments; the first looked at the impact of three levels of pre-flowering herbivory, and the second looked at four levels of herbivory after the plants had started to flower. We found that herbivory during the flowering phase led to a significant increase in the number of plants without male flowers. As a consequence there was significantly less pollen export from this population, as estimated by movement of a pollen analog. The size of female flowers was reduced by severe herbivory, but there was no affect on pollen receipt by the female flowers of damaged plants. The decrease in allocation to male function after severe herbivory may be adaptive when male reproductive success is very unpredictable.