臺灣亞熱帶高山湖泊兩種水生植物（東亞黑三稜和水毛花）穩定性碳同位素比值和氮含量變化

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

東亞黑三稜和水毛花是鴛鴦湖生態保護區內兩種優勢水生單子葉植物，本文探討其所使用的光合作用途徑和其生理生態學。本文首先分析東亞黑三稜（沉水植株、成熟植株葉挺水部份和沉水部份、根）植體和水毛花挺水部份（稈）及根部穩定性碳同位素比值（δ13C），比較同種水生植物其植體不同部位是否有不同的δ13C；其後比較這兩種水生植物成熟植株挺水部份其δ13C和氮含量的月變化。主要目的在確定這兩種水生植物其所使用的光合作用途徑；同時探討限制這兩種水生植物光合作用速率的因子。分析結果發現這兩種水生植物其植體各部位δ13C值均落在C3型植物範圍內，顯示其使用C3光合作用途徑。兩種植物其挺水部份δ13C和氮含量有季節性變化，春季高於夏季，且兩者δ13C值和氮含量呈線性正相關，顯示兩者δ13C之季節性變化和二氧化碳固定作用（carboxylation）效率有關。在年內同一月份，東亞黑三稜挺水葉之δ13C值和氮含量都高於水毛花挺水稈。東亞黑三稜有較高的氮含量，因此可能有較高的二氧化碳固定作用效率，以致其挺水葉的δ13C值高於水毛花挺水稈。

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 δ13C 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 δ13C 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 δ13C values was found. Spring leaves (or culms in S. mucronatus) had significantly higher leaf nitrogen content and more positive δ13C values than summer leaves (or culms). Consequently, there are significantly positive correlations between δ13C and leaf nitrogen content in both species. The result suggests that changes in photosynthetic capacity might contribute to the seasonal variation in δ13C in both species. In comparison between two species, S. fallax had significantly more positive δ13C 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 δ13C values than S. mucronatus.