Pollination Biology of Apocynum venetum L.in the Middle Reaches of Heihe River

We studied the floral dynamics,pollen viability,stigma receptivity,floral visitors and their behaviors,pollen ovule rate(P/O),outcrossing index(OCI) of Apocynum venetum L.in the field of the middle reaches of Heihe River.The main results were as follows:the flowering of A.venetum L.was 4～6 days,the community flowering season is June to August.Pollen viability achieved the maximum at 3 hours later in flowering.After 4 days,the pollen viability reduce to 0.Stigma receptivity achieved the maximum at 2 days later in flowering.Stigma receptivity can last for 4 days.More than 10 species of insects were observed to visit the flowering of A.venetum L.The most common floral visitors were arranged in Hymenoptera,of which Ophion luteus(Linnaeus),Megachile bicolor kagiana Cockerell,Megachile spissula Cockerell,Apis mellifera Linnaeus,Episyrphus balteatus,Pieris rapae(Linnaeus),Aphidencyrtus sp.were the effective pollinators.The pollen ovule rate(P/O) was 36.2 and the outcrossing index(OCI) was 2,fruit set rate of the first and second group were(2.79±0.99)% and(3.41±1.05)%,respectively.The results indicated that facultative autogamy is the main form in breeding system of A.venetum L.,the fruit set rate is low and the floral visiting insects just have partly impact on the process of pollination.