A novel case of autogamy and cleistogamy in Dendrobium wangliangii: a rare orchid distributed in the dry-hot valley

Dendrobium wangliangii is an epiphytic orchid distributed in the Jinshajiang dry-hot valley in Luquan County, Yunnan Province, China. Most Dendrobium spp. typically have a low fruit set, but this orchid shows a higher fruit set under natural conditions despite the lack of effective pollinators. The pollination biology of the critically endangered D. wangliangii was investigated in this study. A fruit set rate of 33.33±4.71% was observed after bagging treatment in 2017 and a high fruit set rate (65.72±4.44% in 2011; 50.79±5.44% in 2017) was observed under natural conditions, indicating that D. wangliangii is characterized by spontaneous self-pollination. The anther cap prevented the growing pollinium, thus the pollinium slid down and reached the stigmatic cavity, leading to autogamous self-pollination. Specifically, 51.50% of 162 unopened flowers (total 257 flowers) of this Dendrobium species under extreme water-deficit conditions developed into fruits, suggesting the presence of cleistogamy in D. wangliangii. Here, cleistogamy may represent the primary mode of pollination for this orchid. Spontaneous self-pollination and specific cleistogamous autogamy could represent major adaptions to the drought and pollinator-scarce habitat in the Jinshajiang dry-hot valley.