Abstract: To explore the photosynthetic adaptation of the endangered species Ulmus elongata to different light conditions, two-year-old seedlings were planted under different shading treatments, i.e. 100% of natural light (I₁₀₀), 40% of natural light (I₄₀), and 10% of natural light (I₁₀). We analyzed the effects of different light intensities on the photosynthetic characteristics of U. elongata. The results showed that: (1) The diurnal variations of the net photosynthetic rate (P_n), stomatal conductance (G_s), and transpiration rate (T_r) of U. elongata under all the three light intensities presented “singlepeak” curves, with a peak at about 12:00. In contrast, the diurnal variation of the intercellular CO₂ concentration (C_i) exhibited a “Vshaped” pattern. The daily mean values of P_n,G_s, and water use efficiency (WUE) under three light intensities ranked as I₁₀₀>I₄₀>I₁₀, while those of C_i ranked as I₁₀>I₁₀₀>I₄₀. The daily mean value of T_r under I₁₀ treatment was significantly lower than that under the I₁₀₀ and I₄₀ treatments. (2) The maximum net photosynthetic rate (P_nmax), light saturation point (LSP), light compensation point (LCP), and dark respiration rate (R_d) of U. elongata seedlings uniformly occurred under I₁₀₀ treatment. Both the maximum carboxylation rate (V_cmax) and triose phosphate utilization rate (TPU) ranked as I₁₀₀>I₄₀>I₁₀. The maximum electron transfer rate (J_max) under I₁₀ treatment was significantly lower than that under I₁₀₀ and I₄₀ treatments. (3) Shading increased PSII maximum photon yield (F_v/F_m) and PSII actual photon yield (F_v′/F_m′). The non-photochemical quenching coefficient (NPQ) and photochemical quenching coefficient (q_p) under I₁₀₀ treatment were significantly higher than those under I₄₀ and I₁₀ treatments. Light energy absorbed by the leaves of U. elongata seedlings in the fulllight environment was dissipated mainly in the form of antenna heat. The majority of light energy in leaves under the shading treatment was distributed to the photochemical reaction and non-photochemical reaction dissipation part, with little remained for antenna heat dissipation. Collectively, there was no obvious photoinhibition in U. elongata under strong light and high temperature, and no photosynthetic “midday break” phenomenon. The carbon assimilation efficiency of U. elongatawas high. These results revealed the characteristics of heliophilous plants. Insufficient light may be a factor inhibiting the natural regeneration of U. elongata seedlings.

Key words: Ulmus elongata, light intensity, photosynthesis parameter, light response, CO₂ response, chlorophyll fluorescence parameter.