Pollen limitation of reproductive effort in willows

Floral reward, advertisement and attractiveness to honey bees in dioecious Salix caprea

In dioecious, zoophilous plants potential pollinators have to be attracted to both sexes and switch between individuals of both sexes for pollination to occur. It often has been suggested that males and females require different numbers of visits for maximum reproductive success because male fertility is more likely limited by access to mates, whereas female fertility is rather limited by resource availability. According to sexual selection theory, males therefore should invest more in pollinator attraction (advertisement, reward) than females. However, our knowledge on the sex specific investment in floral rewards and advertisement, and its effects on pollinator behaviour is limited. Here, we use an approach that includes chemical, spectrophotometric, and behavioural studies i) to elucidate differences in floral nectar reward and advertisement (visual, olfactory cues) in dioecious sallow, Salix caprea, ii) to determine the relative importance of visual and olfactory floral cues in attracting honey bee pollinators, and iii) to test for differential attractiveness of female and male inflorescence cues to honey bees. Nectar amount and sugar concentration are comparable, but sugar composition varies between the sexes. Olfactory sallow cues are more attractive to honey bees than visual cues; however, a combination of both cues elicits the strongest behavioural responses in bees. Male flowers are due to the yellow pollen more colourful and emit a higher amount of scent than females. Honey bees prefer the visual but not the olfactory display of males over those of females. In all, the data of our multifaceted study are consistent with the sexual selection theory and provide novel insights on how the model organism honey bee uses visual and olfactory floral cues for locating host plants.

The evolution of ovule number and flower size in wind-pollinated plants

In angiosperms, ovules are "packaged" within individual flowers, and an optimal strategy should occur depending on pollination and resource conditions. In animal-pollinated species, wide variation in ovule number per flower occurs, and this contrasts with wind-pollinated plants, where most species possess uniovulate flowers. This pattern is usually explained as an adaptive response to low pollen receipt in wind-pollinated species. Here, we develop a phenotypic model for the evolution of ovule number per flower that incorporates the aerodynamics of pollen capture and a fixed resource pool for provisioning of flowers, ovules, and seeds. Our results challenge the prevailing explanation for the association between uniovulate flowers and wind pollination. We demonstrate that when flowers are small and inexpensive, as they are in wind-pollinated species, ovule number should be minimized and lower than the average number of pollen tubes per style, even under stochastic pollination and fertilization regimes. The model predicts that plants benefit from producing many small inexpensive flowers, even though some flowers capture too few pollen grains to fertilize their ovules. Wind-pollinated plants with numerous flowers distributed throughout the inflorescence, each with a single ovule or a few ovules, sample more of the airstream, and this should maximize pollen capture and seed production.

Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants

BACKGROUND

The rich literature that characterizes the field of pollination biology has focused largely on animal-pollinated plants. At least 10 % of angiosperms are wind pollinated, and this mode of pollination has evolved on multiple occasions among unrelated lineages, and hence this discrepancy in research interest is surprising. Here, the evolution and functional ecology of pollination and mating in wind-pollinated plants are discussed, a theoretical framework for modelling the selection of wind pollination is outlined, and pollen capture and the occurrence of pollen limitation in diverse wind-pollinated herbs are investigated experimentally.

SCOPE AND CONCLUSIONS

Wind pollination may commonly evolve to provide reproductive assurance when pollinators are scarce. Evidence is presented that pollen limitation in wind-pollinated plants may not be as common as it is in animal-pollinated species. The studies of pollen capture in wind-pollinated herbs demonstrate that pollen transfer efficiency is not substantially lower than in animal-pollinated plants as is often assumed. These findings challenge the explanation that the evolution of few ovules in wind-pollinated flowers is associated with low pollen loads. Floral and inflorescence architecture is crucial to pollination and mating because of the aerodynamics of wind pollination. Evidence is provided for the importance of plant height, floral position, and stamen and stigma characteristics in promoting effective pollen dispersal and capture. Finally, it is proposed that geitonogamous selfing may alleviate pollen limitation in many wind-pollinated plants with unisexual flowers.

Pollen limitation meets resource allocation: towards a comprehensive methodology

The standard method of measuring pollen limitation is to add pollen to a number of flowers, preferably to a whole plant, and to compare fruit and seed set with that of naturally pollinated flowers on other plants. In 25 yr of research, this method has yielded valuable data, but it is difficult to use in large plants. This has caused a bias in the available data towards smaller, herbaceous plants with relatively few flowers. I argue that, in order to widen our knowledge of how pollen limitation affects plants, we should go beyond whole-plant pollen addition and change our concept of how a flowering plant functions. The traditional method does not take into account the variation in and dynamics of resource allocation and pollen availability. The concept of integrated physiological units (IPUs) does, but, although it has been applied to pollination biology, it has not received the attention it deserves. I use this article to present its merits again, to propose a step-by-step methodology for studying pollen limitation, and to examine factors influencing possible plant strategies.

Pollinator visitation, pollen limitation, and selection on flower size through female function in contrasting habitats within a population of Campanula persicifolia

We looked at whether flower visitation rates, pollen limitation on seed production, and phenotypic selection on flower size through female function varied across a sharp gradient in light intensity (open meadow vs. forest) within a population of the perennial plant Campanula persicifolia L. (Campanulaceae). Flower visitation rates of putative pollinators were similar in both habitats. Seed number per fruit was strongly pollen limited, with no difference in the magnitude of pollen limitation between the two habitats. This strong pollen limitation set the basis for significant phenotypic selection, through female function, on a trait that probably is important for pollinator attraction: flower size. This was revealed by path analysis and structural equation modelling. The lack of difference in pollen limitation on seed production in the two habitats may be explained by the similarity in flower visitation rates in the two habitats. Moreover, the similarity in pollen limitation in the two habitats probably resulted in a similar magnitude and direction of selection on flower size through female function. Our results suggest that pollen limitation and selection through female function may vary little across space within a popualtion despite large variation in the environmental conditions experienced by plants.

Wind to insect pollination ratios and floral traits in five alpine Salix species

This study examines the ratio of wind to insect pollination of five alpine-arctic Salix species and possible correlations between plant traits and this ratio. The field work was done in an alpine area in southwest Norway, at an elevation of 1250 m. Female catkins were enclosed within mesh cloth to prevent insect visitation, and their fruit set was determined and compared with that of open-pollinated control catkins. Simultaneously, pollen production per anther, pollen dispersal, standing crop of nectar, flower length, number of flowers per catkin, and catkin length were measured. The amount of wind pollination of the species varied from 2 to 52%. All species spread pollen in the air. Nectar content, catkin length, flower number, and flower length showed a negative relationship with the percentage of wind pollination. Significant negative linear correlations between female catkin length and flower length and amount of wind pollination were detected. Pollen production per anther did not show any relationship to the pollination system. We suggest that the evolution to insect pollination in alpine Salix species may have resulted in a lengthening of anthers and pistils, and an increase in nectar production.Key words: alpine, anemophily, entomophily, floral traits, pollination, Salix.

Pollination, capsule damage, and the production of seeds in Salix setchelliana (Salicaceae), an Alaskan glacial river gravel bar willow

Salix setchelliana, a clonal prostrate willow, is a dominant species of glacial river gravel bars in Denali National Park, Alaska. This habitat, which is located between migrating river channels, is unpredictable in time, patchy in space, and essentially alpine in nature. The species takes advantage of both entomophily and anemophily, thus perhaps improving its success under these conditions. It is dependent primarily on insect pollination, carried out by a variety of species, but wind pollination accounted for 7–25% of seed production. The number of seeds per capsule was much below the potential maximum. Seed prédation, and probably insufficient pollination, influenced seed production. Prédation accounted for a 70% reduction in seed production per capsule in an open pollination treatment (P = 0.005). There were significant differences in number of seeds per capsule between collection sites (P = 0.0022). An inverse relationship was observed between seed predation levels and seed numbers in both insect-damaged and undamaged capsules. Key words: alpine, clonal, pollination, seed prédation.

Limitations on reproduction in alpine Ranunculus acris

This study examines experimentally the influence of limiting factors on seed to ovule ratio and seed weight in three alpine populations of the perennial herb Ranunculus acris L. at Finse, southwest Norway. To test for pollen and resource limitation, seed production of supplementally pollinated or completely defoliated plants was compared to that of untreated plants. In one population the experiment was repeated twice during the season. In mid-season, supplemental hand-pollination had no significant effect on seed to ovule ratio, but significantly increased the seed weight, whereas complete defoliation resulted in a significant decrease in seed to ovule ratio and a marginally significant decrease in seed weight. These results indicate that pollen limitation occurred on seed weight and resource limitation mainly on seed to ovule ratio. Seed production in late-flowering control and defoliated plants in one population was nearly completely absent, whereas supplemental pollination increased seed production substantially. Most seeds were produced at the lowest elevation, and frost damage on developing seeds was most pronounced at the highest elevation population, suggesting that severe weather conditions at the highest altitudes had a negative influence on reproductive processes. Significant positive relationships between seed number and seed weight were found, possibly reflecting a large among-plant variation in total resource availability. Key words: altitude gradient, climate, defoliation, pollen and resource availability, seasonal variation, seed size.