Abstract: The photosynthetic parameters of Sinocalycanthus chinensis in both overstory and understory, including the diurnal changes of net photosynthetic rate (Pn), intercellular CO₂ concentration (Ci), dark respiration rate (Rd) and stomatal resistance (rs), were measured using a LCA4 Portable Photosynthesis System. The photosynthetic parameters of the accompanying plants such as Boehmeria platanifolia, Rhus chinensis and Weigela japonica var. sinica were also measured for comparison. The results showed that the diurnal changes of photosynthesis of S. chinensis in the overstory took on “double-peak” curve with remarkable midday depression. The midday depression was mainly due to non-stomatal limitations. On the contrary, the diurnal changes of photosynthesis of S. chinensis in the understory took on “one-peak” curve with low Pn. The maximal Pn was only half of that of S. chinensis in the overstory. The correlation analysis showed that the limiting factor for this low Pn might be light intensity (p=0.001). Compared with its accompanying plants, S. chinensis was in a disadvantaged status in the community with low photosynthetic capacity, water use efficiency, and the ratio of photosynthetic rate to respiration rate.

Key words: endangered plant, Sinocalycanthus chinensis, accompanying plant, diurnal change of photosynthesis