Protogyny and delayed autonomous self‐pollination in the desert herb Zygophyllum macropterum (Zygophyllaceae)

Protogyny and delayed selfing are two mechanisms in facilitating outcrossing and increasing female and male function in self-compatible plant species. However, both mechanisms are rarely reported within a species. Here, we investigated pistil and stamen development, breeding system, and pollination biology of the desert herb Zygophyllum macropterum C. A. Mey. by use of field observations and manipulation experiments. Our primary aim was to assess whether protogyny and delayed autonomous self-pollination occurred in this species. Zygophyllum macropterum is a spring flowering species with a floral longevity of approximately 3days, including the female stage and the bisexual stage. Movement herkogamy occurred during both stages. The stigma was receptive when it was exposed from the corolla. The time of first anther dehiscence was approximately 12h after stigma exposure. These results indicated that Z. macropterum is protogynous. Different pollination treatments showed that this species is self-compatible and can self-pollinate autonomously. Time of first dehiscence of anther overtopping the stigma was approximately 16h after stigma exposure, suggesting that autonomous self-pollination can occur in middle-late anthesis, and thus can be defined as delayed autonomous self-pollination. Anthophora plumipes Pallas. was the only pollinator of Z. macropterum. The unfavorable spring desert environment and the small corolla mouth diameter at the female stage would limit pollinator activity and thus lead to failure of outcrossing and occurrence of delayed self-pollination. Therefore, both protogyny and delayed selfing play an important role in enhancing the opportunity for outcrossing and reproductive assurance of Z. macropterum in the spring desert environment.