Wind Pollination In A Gymnosperm Gnetum Montanum (Gnetales)

Gymnosperms are generally considered to be pollinated by wind,yet insect pollination is prevalent in the phylogeny-specific genus Gnetum(Gnetales),whose distribution is restricted to tropical rainforests where anemophily is detrimental.However,Gnetum exhibits various anemophilous syndromes,including dioecy,degeneration of perianth,exposure of anthers and stigmas.Further,pollen limitation may occurs more frequently in rainforests because the diverse plant species in this area may compete for pollinators,which may leads to the transition of pollination modes from entomophily to ambophily(wind and insect pollination),or even to anemophily for reproductive assurance.Fossil evidences indicate that early gymnosperms and early angiosperms coexisted in the mesic floodplain habits in the Cretaceous and shared the similar floral traits and pollination modes.Thus,pollination study on Gnetum will provide key evidences to the origin and diversification of Gnetales,or even the angiosperms.Based on the previous survey on Gnetum parvifolium in our laboratory,field ecological surveys were carried out to reveal the potential pollination mode of anemophily in Gnetum montanum.The main results are as follows.First,to clarify the existence of airborne pollen grains of G.montanum,vaseline slides were suspended in two natural populations of Hainan and Shenzhen for two continuous years(2015-2016).We found that airborne pollen could be found on both slides around male and female plants in a radius of 2m,indicating that pollen of G.montanum could be dispersed by wind.Specifically,the number of airborne pollen grains trapped at night was significant higher than that in the daytime,suggesting that nighttime is more suitable for the occurrence of wind pollination.Second,in order to calculate the contribution of anemophily on reproductive success,we carried out four pollination treatments and calculated the swollen rates of ovules to illustrate pollination success in Hainan and Shenzhen G.montanum populations.The results were similar between the two populations:there were no swollen ovules produced in the bagged treatment,suggesting that G.montanum is incapable of apomixis.The swollen rates of supplemental pollination were significantly higher than that of open treatment,indicating the presence of pollen limitation.The swollen rates of netted treatment were significantly lower than that with open pollination,but higher than that of bagged treatment,further confirming the existence of anemophily in G.montanum.Besides,the rates of contribution from anemophily to reproductive success were similar in the two G.montanum populations,and both were about 20%.Last,we calculated aerodynamic parameters to reveal the mechanism of anemophily in G.montanum.Results showed that the terminal settling velocity(Ut)of G.montanum was 3.95cm/s,in line with the velocities range of 2-6cm/s of airborne pollen in literature.Besides,we constructed a mathematical and physical model,based on Ut,wind speeds and the distance between female and male plants in these two populations,to measure the minimum threshold of the male plant height for successful anemophily.The model indicated that male plant thresholds were 0.81m in Hainan G.montanum population,and 2.97m in Shenzhen population.According to our field observations,the general heights of male plants were higher than the above thresholds,indicating that G.montanum possesses the aerodynamic basis of wind pollination.