Pattern of Leaf epidermal characters in Cycas and its significance of taxonomy and paleoecology
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Abstract:
Cycas is a group of gymnosperms with a long evolutionary history. This study is to investigate the relationship between the leaf epidermal morphology of Cycas and the environment to provide fundamental evidences for palaeoenvironmental reconstructions. In this study, we compared the epidermal characters and stomatal parameters of 27 Cycas species in the garden culturing environment and then analyzed the phylogenetic signals of stomatal parameters. The present study mainly found that: 1) the epidermal characters of Cycas were generally conservative within species and it has taxonomic significance. These 27 Cycas species could be classified into four types according to their epidermal cells pattern and other characters of stomata apparatus. The above result indicated that leaf epidermal characters can be used for identifying extant or fossil Cycas species; 2) Four leaf epidermal types were related to their natural habitats; therefore, the combination of different leaf epidermal characters may be a good indicator for palaeoenvironment; 3) Species included in this study varied in stomatal parameters despite they grew under the same cultivating environment. The variation of Stomatal Index (SI) within Cycas was associated with phylogeny: the SI presented with significant phylogenetic signal, whereas the phylogenetic signals of other stomatal parameters were weak. This study indicated that, when using the stomatal density/index method to reconstruct palaeo-atmospheric CO2 concentration, it should be cautiously considered to choose nearest living species as proxies, especially these with similar morphology, habitat and close phylogenetic relationship. In addition, the relationship between stomatal parameters and CO2 partial pressure (pCO2) should be clarified within a genus so as to improve the effectiveness of this method.
