Effects Of Airborne Gymnosperms Pollen On Pollination Of The Co-flowering Insect-pollinated Plants

In natural communities,plants often co-exist with other plant species.For example,in North Temperate Zone or at high altitude,conifers(gymnosperms)usually release pollen in spring while some flowering plants(angiosperms)begin to flower.If different species share the same pollen vector,heterospecific pollen transfer(HPT)may occur.These pollen grains are most from pine trees.Although it is known that HPT is widespread among co-flowering angiosperms which share pollinators,it remains unclear whether airborne pollen from conifers which is wind dispersed may interfere pollen receipt of animal-pollinated flowers.The effects of airborne pollen on reproductive success of animal-pollinated plants have been rarely studied.First,to estimate the degree of HPT from airborne pollen on stigmas of insect-pollinated plants,we examined whether the wind pollination of gymnosperms,especially pine,has deleterious effects on the reproduction process of insect-pollinated plants.We studied 8 flowering species with overlapping flowering stages of Pinus densala and Piceae purpurea for 2 consecutive years in a field station,Shangri-La Alpine Botanical Garden,southwest China.To investigate whether the stigma of the insect-pollinated plants can specifically identify or avoid the deposition of pine pollen grains,heterospecific pollen deposited on stigmas of insect-pollinated plants under natural populations and artificial heterospecific pollen release experiments were surveyed.Our results showed that the proportion of pine pollen on stigmas of insect-pollinated plants was closely related to the quantity of pine pollen in air.An increase of pine pollen floated in the air,a higher proportion of airborne pollen would be deposited on the stigmas of insect-pollinated plants.Floral morphologies,such as the position of the stigma,corolla openness and floral opening orientation may also affect the deposition of airborne pollen.Species with exposed stigmas in the air were likely to receive high loads of airborne pollen.The open shaped,upward flower was more likely to receive airborne pollen than the tubular,downward flower.We did not find evidence for stigmas of insect-pollinated plants which could specifically prevent airborne pollen from receipt of wind-pollinated gymnosperms.To further explore the effects of airborne gymnosperm pollen on sexual reproduction in insect-pollinated plants,we conducted seven kinds of artificial pollination experiments.The results showed that pollen grains from Pinus densata and Picea purpurea did not result in a significant decrease in seed set of two spring flowers(Lonicera webbiana and Malus rockii).Our analyses suggest that pollen grains from pine trees are light,each pollen with two air sacs.These pollen property makes it easily blown away by the wind and difficult to retain on stigmas for a long time.The airborne pollen did not affect pollen germination and pollen tube growth of insect-pollinated plants.Even if some pine pollen grains were deposited on stigmas,as long as there were some conspecific pollen grains,these insect-pollinated angiosperms could achieve reproductive success given that both Lonicera webbiana and Malus rockii have a small number of ovules(no more than 10).