• Li, Xiaochun
  University of Maine Graduate School
  

Metasequoia glyptostroboides is a useful nearest living relative (NLR) of the Eocene fossil Metasequoio. Research on modern Metasequoia might give us some clues about its fossil counterpart.

During this study the leaf anatomy of Metasequoia, Glyptostrobus, Sequoia and Taxodium was investigated with light microscopy and transmission electron microscopy. Metasequoia exhibits several characteristics of typical sciaphilic plants, such as slightly arched outer cell walls in the adaxial epidermal cells, strongly arched outer cell walls in the abaxial epidermal cells, mesophyll composed of spongy cells, chloroplasts with well-developed grana not only in mesophyll cells but in both the adaxial and abaxial epidermis. Based on comparison of leaf morphology and anatomy, we conclude that Metasequoio is best adapted to low light intensities, Sequoia and Taxodium are intermediate, and Glyptostrobos is adapted to higher light intensities.

The effects of light intensity on mesophyll plastids of Metasequoia leaves were studied with trees grown under different light intensities. Metasequoia had the ability to synthesize chlorophyll under complete darkness and was stressed under high light. These characteristics would provide adaptive advantages for Metaequoia to adapt to low intensity, low angle, polar light at their Eocene high latitude paleo- environments, particularly during the polar spring when light levels are exceedingly low. It provides evidence to explain why Metasequoia was the dominant tree species in Eocene high latitudes.

The thesis is written as an article to be submitted to the American Journal of Botany.