Studies On Adaption And Mechanism Of Successive Stamen Movement In Parnassia Wightiana

Successive stamen movement is a kind of floral movements which stamens moved one by one or group by group in a flower,and it is found in all species of genus Parnassia in Saxifragaceae,some species in Tropaeolaceae,Loasaceae and Rutaceae.Stamen movements directly determine pollen fates and mating process by altering the relative positions between male and female organs.However,the adaptions of such movements in terms of pollination are not well understood.In this study,the adaption on pollination and mechanism of successive stamen movement in Parnassia wightiana(Saxifragaceae)are determined.Firstly,to explore the pollination system,floral traits and pollination observation were investigated in P.wightiana.Results showed that the open flowers were visited by a variety of pollinators,most of which were halictid bees and flies,indicating that the pollination type was generalized.Through hand-pollination,we found that it was self-compatible,and seed set was pollen-limited and pollinator-dependent.In our observation,the stamen movement showed a precise spatial arrangement with presenting one fresh anther each day over a period of 5 days.According to the different status of stamen,the stamen movements were divided into three stages:First stage,stamen was in the original position.Second stage,stamen elongated its filament to uplift its anther to flower's central where just overlaid pistil.Third stage,the stamen began bending outwards to clear of pistil and finally descend into the interval between two petals.Secondly,to determine the adaption on pollination of one by one anther dehiscence.Most flowering plant species are visited by a diversity of floral insects,and the amount of pollen wastage is always very high during pollen transfer.Pollen presentation theory(PPT)suggests that staggered pollen presentation can increase male fitness in species of which seed set is pollen-limited and pollinator visits are frequent,but pollinators are inefficient in transporting pollen due to pollen feeding or grooming.Through the estimation of pollination efficiency,we found that honeybee was the most effective pollinator.Then pollinator importance was calculated for each type of pollinators,results showed that halictid bee was the most important pollinator.Although honeybee was the most effective pollinator,its low rate of floral visitation determined it couldn't be the most important pollinator.In the manipulation experiments of presenting two anthers at once,the pollen removal of halictid bee was increased almost twice whereas the pollen deposition was not significantly different as control.Thus the pollen loss was increased nearly twice,so that presenting twice as much pollen to a visitor is not likely to get twice as many of those grains delivered to stigmas.Thus we had proved that one by one anther dehiscence could optimize the delivery by restricting the amount of pollen not being taken too much per visit by a pollinator which supported the pollen presentation theory.Thirdly,to evaluate the effect of movement herkogamy on pollination.P.wightiana is protandrous,and presented mature anthers in the center of flower at the minimal extent of herkogamy.when the flowers entered female phrase,all the five stamen had bent outward and kept the maximal extent of herkogamy.In order to investigate the function of movement herkogamy,we conducted manipulations to change the extent of herkogamy on male and female flowers respectively to test the impacts on visitation rate,percentage that contacting sexual organs,pollen removal of male flowers and seed set of female flowers.Results showed that the visitation rate of halictid bee was decreased significantly while the other visitors were not different much when the extent of herkogamy was manipulated to be maximal in the male phrase flowers.The percentages that contacting anther of all visitors and also pollen removal were decreased dramatically.When the extent of herkogamy was manipulated to be minimal in female phrase flowers,the visitation rate of all insects were not influence much,but the percentages that contacting stigmas of all visitors and seed sets were decreased significantly.These results indicate that pollen removal was negatively correlated with the extent of herkogamy while seed set was positively related with the extent of herkogamy.Thus we conclude that the movement herkogamy of stamen movement promoted both pollen removal and seed set,which both increase male and female fitness.Finally,we investigate the mechanism of stamen movement and detect which factors may influence the movement.In order to identify the mechanism of stamen movement,paraffin section was used to compare the difference of filaments cell length on the three stages of stamen movement.Results showed that the cell length in the elongated filaments were significantly higher than the cell length of the original stamen filaments,so the cell elongation was a key factor that caused filaments extension.In curved stamens,the cell lengths of the abaxial surfaces at base of the filaments were significantly smaller than those of the adaxial surfaces.Both of the cell length of abaxial and abaxial surfaces at base of the curved filaments were longer than those in the elongated filaments.But the cell length at the top of curved filaments where was erect has no significantly difference with those in the elongated filaments.Thus,we conclude that the differential cell growth rates between the adaxial and abaxial surfaces of the base of filaments could account for the stamen bending outward movement,but have no relevance with the cell at the top of filaments.Furthermore,we detected that the time when stamen bending outward could be delay under 24-hour lighting,indicating that the photoperiod was a key factor of regular stamen movement.The stamen movement was not influenced when the flowers were excluded from visitors,indicating that successive stamen movement could not be adjust to insects visitation.In the filaments wounding experiments,we found that the movement of bending outward could be delay when the base of filaments were wounded,while the wounds at top of filament had no influence on such movement.These results were consistent with the conclusions of paraffin sections that the movement of bending outward was related with the cell changes at base of filaments but irrelevant with the cell at the top of filaments.