| Pollen,as the male gametophyte in seed plants,in most species relies on animals delivering to the surface of female reproductive structures(i.e.,stigmas in flowering plants).When floral visitors are foraging or feeding nectar and/or pollen or other rewards from flowers,they help plants of transporting pollen grains.Bees are considered to be one of the most important pollinator groups in both natural and agricultural ecosystems.As pollen is the only protein source for bees,they may consume large amounts of pollen grains during visiting flowers.Therefore,there is a conflicting interest for pollen source between plants and pollen feeding animals.How do animal-pollinated plants reduce pollen loss from pollen consumers during pollen delivery?Previous studies suggested that pollen size,pollen wall thickness and spines on pollen surface could affect bee's pollen-collecting behavior and collecting intensity.And there are two hypotheses that indicate the adaptive significance of spines:(1)spines on pollen surface can interfere with pollen-collecting by bees;(2)spines on pollen surface could promote adherence of pollen grains to the surface of pollinators body.However,empirical evidences were absent to support these viewpoints.In this study,we measured pollen morphology in 106 species of 85 genera in 35 angiosperm families,including polar diameter,equatorial diameter,wall thickness,spine length and spine quantity of pollen grains.These parameters were comprehensively analyzed in combination with the situation of anther visibility and pollen-collecting behavior of wild bees(mainly bumblebees),to systematically investigate the role of pollen size and wall thickness in pollination.To investigate the role of spines on pollen surface in pollination,pollinator behavior and visit frequency were observed in 12 species in Asteraceae with echinate pollen.To investigate the packability of echinate pollen in Asteraceae,we added pollen from Cirsium shansiense(Asteraceae)onto the anthers of Rosa soulieana flowers,and examined whether echinate pollen could be collected into bumblebees'corbiculae.To investigate the effect of spine length on pollen adhesion,we compared proportion of pollen grains in two species of Asteraceae with different spine length placed on the bumblebee bodies in the field.Our results showed a significant positive correlation between pollen volume and wall thickness.In addition,we observed that anther-concealed plants usually had pollen grains with relatively smaller volume and thinner pollen wall,which were often collected by bumblebees.By contrast,anther-exposed plants usually had pollen grains with relatively larger volume and thicker pollen wall,rarely collected by bumblebees.Therefore,we hypothesized that an increase of pollen wall thickness might reduce pollen attraction to bumblebees for anther-exposed plants.Our results showed that flies and bumblebees were the main flower visitors in the 12 Asteraceae species.When visiting flowers,flies would forage nectar and pollen,while bumblebees only foraged nectar but not collect pollen.Our comparison showed that six species mainly visited by flies produced pollen grains with smaller size,thinner pollen wall and longer spines with higher density,while five species mainly visited by bumblebees produced pollen grains with larger size,thicker pollen wall,and shorter spines with lower density.In the experiment of packability of echinate pollen in Asteraceae,we observed agglomerate pollen grains of Cirsium shansiense in all 14 pollen corbiculae on the bumblebees,which didn't support hypothesis(1).The experiment of the effect of spine length on pollen adhesion showed that the proportion of pollen grains carried on bumblebee bodies was significantly higher in Cirsium shansiense with long-spine pollen than that in Saussurea costus with short-spine pollen.This result indicated that long spines on pollen surface could improve pollen adhered capability to the bumblebee bodies and promote pollen transfer,which supported hypothesis(2). |
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