The Ecological Mechanism Of The Long Corolla Tube Maintenance In Roscoea Praecox

The long corolla tube or nectar spur is considered to be an adaptation to long-mouth pollinators.Generally,Roscoea plants(Zingiberaceae)have long corolla tube structure and secrete more volume nectars,were explained by the relationship between floral traits and pollinators successfully in the Himalayas.However,in the Hengduan Mountain,an interesting doubt that why do Roscoea species plants maintain longer corolla tubes until today under the generalist pollinators service.Therefore,explored the ecological mechanism of the long corolla tube maintenance in Roscoea species was an attractive issue in the Hengduan Mountains.Two hypotheses have been summarized for explaining the ecological meaning of long corolla tube existence according to previous researches: 1.Long corolla tube could increase flower display height from the ground,and enhance the attraction to pollinators;2.Long corolla tubes accompanied long styles and provided a longer "competitive track" to pollen competition.Here,we controlled the pollinator phenotypic selection experiment,reduced corolla tube,and flower display height manipulation experiment and compared pollen tube growth rate between different addition pollen(come from different pater)treatment groups to verify the above hypotheses in a natural population of Roscoea praecox.The primary results are as follows:1.Bombus trifasciatus were main pollinator of R.praecox during observing period in 2019 and 2020 flowering seasons.Supplemental pollination treatment increased seed number significantly with open pollination per flower(2019: P < 0.001;2020: P =0.016),the pollen limitation index was 0.368 and 0.317(2019 and 2020 respectively)in the natrual population.Phenotypic selection experiments results showed that the corolla tube length did not obtained net directional selection pressure significantly(2019: P =0.653;2020: P = 0.847),and also did not be exerted by pollinator's directional selection pressure significantly(2019: P = 0.845,2020: P = 0.168).In addition,decreased corolla tube or flower display height artificially did not affected the pollinator's visition frequence(2019: 76.6% ± 0.022 in natural group and 74.0% ± 0.012 in experimental group,P = 0.341;2020: 61.7% ± 0.032 in natural group and 60.3% ± 0.009 in experimental group,P = 0.707);and similarly affect pattern on seeds number per flower(2019: 6.595 ± 0.685(n = 42)in natural group,6.974 ± 0.864(n = 39)in experimental group,P = 0.730;2020: 5.225 ± 0.840(n = 40)in natural group,5.020 ± 0.840(n = 50)in experimental group,P = 0.893).2.Different hand-pollinated group(self-pollination,1:1mixed self-and outcross-pollination,out-pollination)influenced seeds number per flower significantly.However,there was no difference in seed number between 1:1 mixed pollination and outcross-pollination group both 2019 and 2020 year(P = 0.086,P = 0.061 respectively).Interesting,mixed pollination way increased seeds number significantly than self-pollination in both 2019 and 2020 year(P = 0.005,P = 0.040 respectively).Moreover,the pollen tube growth rate showed a significant difference between the self-pollination and cross-pollination group after pollination 8h(P = 0.035).Only 20%of samples of pollen tubes arrived ovary position after self-pollination 18 h,while 80%of samples arrived ovary position in the outcross-pollination group.After pollination24 h,80% of samples of pollen tubes in the self-pollination group arrived ovary position,and 100% of samples in cross-pollination arrived ovary position.The similarly results in2020 also showed that the pollen tubes growth rate have no significant difference between self-pollination and cross-pollination treatment after 2-8h,but after pollination10-14 h,the outcross group showed a faster pollen tubes growth rate than self(P < 0.05),and more samples in the outcross-pollination group reached ovary after pollination22-24 h.All the pollen tubes arrived ovary position after pollination 48 h.These results did not support the hypothesis of long corolla increase attractiveness to pollinators.However,the results of seeds number per flower demonstrated successfully that outbred pollen had a higher advantage in competing the ovules indicated as the cross pollen enhanced pollen tubes growth rate than self.Therefore,it is supported the hypothesis of long corolla tube accompanied long styles,and provided a longer "competitive track" to pollen competition.In a conclusion,this study could be beneficial to understand the ecological mechanism of the long corolla tube maintenance in alpine species under the specialist pollinator lacking environments.