δ13C and N Contents of Two Aquatic Plants, Spaganium fallax and Schenoplectus mucronatus, in a Subtropic Mountainous Lake

Abstract

Spaganium fallax and Schoenoplectus mucronatus subsp. robustus are emergent monocot plants dominating a subtropical mountainous lake, the Yuanyang Lake (YYL), in Taiwan. The photosynthetic pathways and the ecophysiology of these two species were studied in this study. I first analyzed δ^13C of fractions of S. fallax (submerged leaves and roots of seedlings, and submerged and emergent parts of leaves, and roots of mature plants) and S. mucronatus (emergent culms and roots of mature plants) to identify their photosynthetic pathway. I then investigated monthly variation in δ^13C and nitrogen content of emergent parts of the two species during 2003 to evaluate the integrated photosynthetic response. The leaf carbon isotope ratio of both emergent plants was within the range of most C3 plants indicating that they used C3 photosynthetic pathway. Similar pattern of seasonal variation in leaf nitrogen content and δ^13C values was found. Spring leaves (or culms in S. mucronatus) had significantly higher leaf nitrogen content and more positive δ^13C values than summer leaves (or culms). Consequently, there are significantly positive correlations between δ^13C and leaf nitrogen content in both species. The result suggests that changes in photosynthetic capacity might contribute to the seasonal variation in δ^13C in both species. In comparison between two species, S. fallax had significantly more positive δ^13C and higher leaf nitrogen content than S. mucronatus during most of the sampling months. The higher leaf nitrogen content found in S. fallax might also contribute to its more positive δ^13C values than S. mucronatus.