Effects Of Environmental Factors On The Germination Of Achenes Of Bidens Frondosa

Bidens frondosa L.(Asteraceae),native to North America,is an annual exotic species.It has distributed in many regions and affected local agriculture and biodiversity.But the invasive mechanisms about this weed are rarely reported.In this paper,achene germination of B.frondosa,its congeners and invasive populations(including populations of Liaoning,Hebei,Jiangsu,Jiangxi and Guangxi)under different lights,temperatures and water stress were studied,aimed to determine the difference between B.frondosa and its congeners and the difference among invasive populations,the factors influencing achene germination,explore the invasive mechanism,and thus,provide a basis for invasiveness,invasibility and management of Bidens frondosa.The main results are as follows:1.B.frondosa produced three different morphs of achenes within a single capitulum,i.e.central,intermediate and peripheral achene.They were differences in number,color,length and width.The awns of the central achenes were significantly longer than those of the periphery;this made it more easily spread over long distances on a carrier such as animals or human.However,the native congener B.tripartita produced one morphs of achenes within one capitulum.Heteromorphic achenes of B.frondosa were superior in the dissemination and proliferation to the native species with a single form of achene.2.The proportions of each morph of achene in one capitulum were obviously different among invasive populations.For example,the proportion of central achene was the largest in population Hebei and was the smallest in population Guangxi;the proportion of intermediate achene was the largest in population Jiangsu and was the smallest in population Guangxi;the proportion of peripheral was the largest in population Guangxi and was the smallest in population Liaoning.In addition,1000-achene weight was also significantly different among invasive population.It was remarkably heavier in population Hebei and lighter in population Jiangsu than in the other populations.3.Temperature,light and Provenances had significant effects on the germination percentage,germination energy,germination index and germination rate of B.frondosa,and temperature had the greatest effects,light had the second greatest effects and provenances had the third greatest effects.Both peripheral and central achenes exhibited the same rule.4.Under 12-h photoperiod,both morphs from all populations didn't germinate at 15?/10?,the germination percentage increased with temperature increasing at 20 ?/15 ?(25?/20 ?)to 35 ?/30 ?,and had a maximum at 35 ?/30 ?(99.38%?100%for central achenes and 97.50%?99.38%for peripheral achenes),but decreased at 40 ?/35?.The germination percentage of peripheral and central achenes at 40 ?/35 ? was the lowest in population Liaoning,the highest in population Guangxi and was intermediate in population Hebei,Jiangsu and Jiangxi.Under full dark,both morphs from all populations didn't germinate at 15 ?/10 ??25?/20 ?,the germination percentage increased with temperature increasing at 30 ?/25 ??40?/35? and had a maximum at 40 ?/35 ?.The germination percentage of peripheral achene from population Liaoning was significantly higher than that of the other populations at 30 ?/25 ??35 ?/30 ?,but was obviously lower that that of the other population at 40 ?/35 ?.5.Polyethylene glycol(PEG-6000)mass concentration,morphs of achene,provenances and their interactions had significant effects on germination percentage,germination energy,germination index and germination rate,except the interactions PEG mass concentration and morphs of achene on germination index and germination rate.The germination percentage decreased with PEG mass concentration increasing,and few achenes germinated at the 20%concentration of PEG mass.The 5%concentration of PEG mass had no significant effects on the germination percentage of central achenes from all populations,since the germination percentage of central achene for each population was indifferent between CK and the 5%concentration of PEG mass.The germination percentages of peripheral achene for all populations,except for populations Liaoning and Jiangsu,were also not affected by the 5%concentration of PEG mass.Population Guangxi had the highest germination energy in all population at the 5%?20%concentration of PEG mass.Population Liaoning had the highest germination index in all populations at the 5%?10%concentration of PEG mass.The germination rate of periphery achene was the highest in population Guangxi at all PEG mass concentrations.The 5%?10%concentration of PEG mass had no significant effects on the germination percentage of both B.frondosa and its congener,since the germination percentage at those concentrations for each species was similar to their controls.The germination percentage for both species decreased obviously at the 20%concentration of PEG mass,and it was significant lower in B.frodosa than in the native B.tripartita.The germination energy and germination rate for both species were significant lower than that of control.The germination rates of B.frondosa at all PEG concentrations were obviously lower than that of B.tripartita.This indicated that achene germaination of B.tripartita was more tolerant than that of B.frondosa to dry habitat.But these traits were not benefit for population establishment,as dry condition would decrease the opportunity for seedling survival.B.frondosa seldom germinate when water short,which could avoid unfavorable condition and decrease achene pod lost.It might be helpful for the extension of population.6.Achene of B.frondosa rarely germinate at low temperature,however,the germination percentage increased with temperature increasing,thus,we should pay high attention to the expansion of B.frondosa toward tropical region.Management should focus on cleaning up before it set achene.In addition,the achene germination was influenced under dark,so we can control the achene germination by deeply ploughing arable land and covering with soil.