The morphology, ultrastructure, element distribution and motion behaviour in pollen of Ginkgo biloba L.

Abstract

The morphology, structure, element distribution and motion of pollen grains in Ginkgo biloba are beneficial for pollination and germination in the pollination drop. The morphology and structure of pollen grains are important for pollination, and hence reproductive success in gymnosperms. To examine the role of pollen structures associated with pollination and germination in Ginkgo biloba L., scanning and transmission electron microscopy were used to observe ultrastructure features of pollen. Compared with pollen grains before dispersal, the aperture area was sunken, spinules on the pollen surface disappeared, and the tectum and intine were thickened during pollen dispersal. However, after pollen hydration, the aperture area and spinules recovered and the thickness of tectum and intine were similar to those of pollen before dispersal, suggesting that G. biloba pollen grains could facilitate pollination and germination through changing their microstructure. In addition, calcium was mainly distributed in the intine. Calcium levels decreased significantly after ethylene glycol tetraacetic acid treatment, resulting in a low germination rate compared with untreated pollen. These results indicate that calcium could play an important role in pollen germination. Importantly, the use of both in vivo and in vitro pollen experiments revealed that after the pollen grains landed on the ovule, they hydrated and swelled on the surface of the pollination drop. This was accompanied by air bubble release at the exposed part of the aperture area. The viable Ginkgo pollen could get into the ovule easier than less-viable pollen. These results showed that the effectiveness of hydration and sinking as a selection mechanism is a result of the ability of G. biloba to exclude less-viable pollen grains and preferentially concentrate on viable pollen inside ovules.