Variations In Floral Traits Among Populations And The Contributions To Pollination Adaption In Two Pedicularis Species

As global climate is rapidly changing,plant living conditions vary dramatically,thus how plants cope with the changing environment is one of the hot spots in plant evolutionary ecology.Plant sexual reproduction,pollinator composition and abundance may change with varying environmental conditions,therefore leading to a series of evolutionary and ecological consequences.This study compared among-population variation in pollination system and reproductive/floral traits and their interaction,in order to generalize the variation in breeding system with environment change.We used two alpine louseworts,Pedicularis siphonantha and P.rhinanthoides;which are both strictly pollinator-dependent.Field investigations with artificial pollination or plant transplantation were carried out to detect the variation in flowering strategies,mating system and pollination services among natural/artificial populations under changed environments.ANOVA/ANCOVA,non-parametric analysis,Redundancy analysis(RDA)and Regression analysis were used to detect the floral divergence among populations and the interrelation with changing pollination environment.The main results and conclusions are as follow:(1)In 2012-2013 we investigated four P.siphonantha populations situated at different altitudes(3260m,3420m,3860m and 4300m).High altitude populations decreased in both bumble bee species and their foraging activities,and thus seed production was obvious pollinator-limited.Pollinator-mediated selfing i.e.,geitonogamy,was the main assurance for sexual reproduction.Also,plants at high altitudes had longer floral duration,larger floral display,shorter corolla tubes with bigger anther-stigma distance,and increased pollen production.These flowering strategies are well adapted to the poor pollination condition,in accordance with the shift in mating system.(2)We selected and transplanted some P.siphonantha plants from high altitudes(3860m and 4300m)to low altitudes(3420m)in 2012 and carried out field investigations and artificial pollination among natural/transplanted populations in 2013.Plants in transplanted populations were well adapted to the local pollination environment at low altitude,thus performing better than those at high altitudes.Seed production increased and for the mating system,outcross-fertilization was favoured while selfing was limited.In addition,some floral traits,such as flower longevity,floral display and corolla tube length,changed rapidly and were consistent with the local population at low altitude.Phenotypic plasticity in floral traits allowed plants to survive the change in pollination environment.(3)From 2012-2014,we investigated 26 P.rhinanthoides populations:12 in wet meadow and 14 in shrub,to study factors influencing population differentiation and plant-pollinator interaction among populations of different habitat types.RDA results indicated that pollinators and habitat influence population differentiation,which perform to be variations in sexual reproduction and diversity of floral traits.And Regression analysis indicated that plant-pollinator interaction and pollinator-mediated selection on floral traits varied among populations of different habitats.Compared to meadow populations,shrub populations were more pollinator-dependent:Some floral traits,such as flower longevity,floral display and anther-stigma distance,were significant correlated with pollinator visiting frequency.(4)In 2013,we investigated P.rhinanthoides in two meadows sites(S and X),which were different in pollinator composition(mainly the proportion).Bombus friseanus was more efficient in pollen transfer than B.festivus.Whereas,half of the pollinators in site S for P.rhinanthoides were B.Festivus,B.friseanus predominated in site X.Higher visiting frequency in site S,lead to higher fruit set.However,decreased pollinator efficiency(stigmatic pollen load)lead to a decrease in seed production per flower.Flowering strategies varied considerably,compare to site X,plants in site S had short flower longevity,decreased pollen production but bigger anther-stigma distance.(5)In 2013,we also investigated populations in three different flowering stages(early,peak and late)in three shrub sites.The pollinator foraging activities varied among flowering stages,thus sexual reproduction fluctuated within a flowering season.Pollen limitation was serious at early flowering stage,while both pollination service and sexual reproduction achieved their best at peak flowering stage.The spatio-temporal variations in pollination may also have influenced the flowering strategies.Plants at peak flowering stage usually had the shortest flowering duration,smallest floral display and anther-stigma distance,and slightly increased pollen production per flower,and thus exhibited enhanced outcross-fertilization with decreased geitonogamy.In conclusion,pollen limitation was ubiquitous across populations of P.siphonantha and P.rhinanthoides,and the sexual reproduction was constrained by both pollinators and environmental conditions.The plant-pollinator interaction also varied in different habitats and was challenge by natural or human-induced environment change.In case of pollinators scarcity or absence,self-fertilization was enhanced and geitonogamy as well;and the flowering strategies tended to increase pollinator attraction and decrease the competition between self and outcross pollen.Comparatively,when pollinators were affluent,the flowering strategies tended to encourage outcross-fertilization while selfing i.e.,geitonogamy,was constrained.This pollinator-mediated selection on floral traits and plasticity in mating system may help plants' survival in unstable environmental conditions.This research adds weight to the increasing knowledge of how plants are likely to cope with rapidly changing climate and thus provide information that may be important for biodiversity conservation.