Pollination system and the effect of inflorescence size on fruit set in the deceptive orchid Cephalanthera falcata.

Larger inflorescences in reward-producing plants can benefit plants by increasing both pollinator attraction and the duration of visits by individual pollinators. However, ultimately, inflorescence size is determined by the balance between the benefits of large inflorescences and the increased cost of geitonogamy. At present, little is known about the relationship between inflorescence size and fecundity in deceptive plants. Given that pollinators are likely to leave inflorescences lacking rewards quickly, it seems unlikely that longer pollinator visits and the risk of geitonogamy would be strong selective pressures in these species, which indicates that pollinator attraction might be the most important factor influencing their inflorescence size. Here we examined the pollination ecology of the deceptive orchid Cephalanthera falcata in order to clarify the effects of inflorescence size on the fruit set of this non-rewarding species. Field observations of the floral visitors showed that C. falcata is pollinated by the andrenid bee Andrena aburana, whilst pollination experiments demonstrated that this orchid species is neither autogamous nor apogamous, but is strongly pollinator dependent. Three consecutive years of field observations revealed that fruit set was positively correlated with the number of flowers per inflorescence. These results provide strong evidence that the nectarless orchid C. falcata benefits from producing larger inflorescences that attract a greater number of innate pollinators. Large inflorescences may have a greater positive effect on fruit set in deceptive plants because a growing number of studies suggest that fruit set in reward-producing plants is usually unaffected by display size.

Radiocesium leaching from contaminated litter in forest streams.

In Japanese forests suffering from the Fukushima Daiichi Nuclear Power Plant accident, litter fall provides a large amount of radiocesium from forests to streams. Submerged litter is processed to become a vital food resource for various stream organisms through initial leaching and subsequent decomposition. Although leaching from litter can detach radiocesium similarly to potassium, radiocesium leaching and its migration are poorly understood. We examined both radiocesium and potassium leaching to the water column and radiocesium allocation to minerals (glass beads, silica sand, and vermiculite) in the laboratory using soaked litter with and without minerals on a water column. The mineral types did not affect radiocesium leaching from litter, but soaking in water for 1, 7, and 30 days decreased the radiocesium concentration in litter by ×0.71, ×0.66, and ×0.56, respectively. Meanwhile, the 1-, 7-, and 30-day experiments decreased potassium concentration in litter by ×0.17, ×0.11, and ×0.09, respectively. Leached radiocesium remained in a dissolved form when there was no mineral phases present in the water, whereas there was sorption onto the minerals when they were present. In particular, vermiculite adsorbed radiocesium by two to three orders of magnitude more effectively than the other minerals. Because radiocesium forms (such as that dissolved or adsorbed to organic matter or minerals) can further mobilize to ecosystems, our findings will increase our understanding regarding the dynamics of radiocesium in stream ecosystems.