| Summary: | <i>Clematis</i> is a perennial ornamental vine known as the “Vine Queen” for its gorgeous floral color and rich flower shape. <i>Clematis tientaiensis</i>, an endangered plant, is a key protected wild plant and a rare breeding parent of <i>Clematis</i> because of its extremely high ornamental value. Light environment is one of the important environmental factors affecting the space distribution and the size of <i>C. tientaiensis</i> population. One-year-old homogenous and healthy potted <i>C. tientaiensis</i> plants were grown under four different light intensities (T1: 1800 ± 30/0 µmol m<sup>−2</sup> s<sup>−1</sup>; T2: 1500 ± 30/0 µmol m<sup>−2</sup> s<sup>−1</sup>; T3: 1200 ± 30/0 µmol m<sup>−2</sup> s<sup>−1</sup>; T4: 900 ± 30/0 µmol m<sup>−2</sup> s<sup>−1</sup>). This study analyzed the potential adaptive mechanism of <i>C. tientaiensis</i> in response to irradiance by investigating the photosynthesis, rapid light curve, chloroplast ultrastructure, Rubisco activase enzyme (RAC), Rubisco enzyme, amino acids, and gene expression under four irradiance treatments. High light caused the leaves chlorosis and yellowing, reduced the net photosynthetic rate (Pn), stomatal conductance (Gs), RAC and Rubisco enzyme activity; the quantum yield of unregulated energy dissipation [Y(NO)], and increased the content of eight amino acids content. The expression of psbA, psbB, psbC, and Psb(OEC) were down-regulated with decreasing irradiance. The results showed that <i>C. tientaiensis</i> plants grown under T1 (1800 ± 30 µmol m<sup>−2</sup> s<sup>−1</sup>) irradiance were in danger of absorbing more light energy than they could use for photosynthesis, while they exhibited good adaptability to the T3 (1200 ± 30 µmol m<sup>−2</sup> s<sup>−1</sup>) irradiance, and the PSII reaction center and Rubisco and RCA enzymes could be the key points in response to high light stress, which also emphasized the importance of appropriate light management practices for promoting the growth and population expansion of <i>C. tientaiensis</i>.
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