Effects of environmental factors on seed germination of Bidens pilosa and Bidens bipinnata

Bidens pilosa is one of the noxious alien Asteraceae weeds with strong invasiveness,and is spreading widely in Southern China at present.We studied the reproductive traits and seed germination characteristics of invasive B.pilosa and native B.bipinnata by combining field investigations with laboratory experiments.The results showed that compared to native species,B.pilosa had higher ramification ability and branch numbers.One single mature plant could produce more than 13 thousands of seeds with smaller mass.Seed germinations of the two weed species were affected by environment factors.The optimum germination temperature for B.pilosa and B.bipinnata was 20-30 ℃ and 10-25 ℃,respectively.Both species had high germination percentages,above 92.00%.High temperature(30-40 ℃) caused harms to seed germination of the two species,but the inhibition effect of high temperature on native B.bipinnata was severer than that of B.pilosa.The seeds of two weeds reached maximum germination at full irradiance,and the changes of ambient light had no effects on B.pilosa germination except in the dark,but the germination of B.bipinnata was significantly decreased at 25% irradiance.Light drought(ρ(PEG)≤0.10 g mL-1) had no significant effects on germination of the two weed species while moderate drought(ρ(PEG)= 0.15 g mL-1) could reduce their germination percentage,but the negative effect on B.pilosa was less than that of B.bipinnata.Both weed species germinated in a wide range of pH(3.0-7.0) and only pH 2.0 caused the seed germination to decrease significantly.High concentration of N and P nutrients had negative influences on the germination of both weed species,and excessive N concentration had more inhibitory effect on native B.bipinnata than on B.pilosa.Germination of both species could be inhibited by the buried treatment and no seedlings emerged when seeds were sowed 3.0 cm deep.But on the soil surface,germination rate of B.pilosa was significantly higher than that of B.bipinnata.