Abstract: The purpose of this experiment was to compare the differences of leaf photosynthetic capacity and chlorophyll fluorescence characteristics between Paulownia backcross optimal plants (including BC-1(194) and BC-1(192)) and their parent TF33. Three light levels \[100% (L₀), 50% (L₁) and 25% (L₂) of natural sunlight\] were conducted to simulate the light conditions of forest opening, forest gap and understory, respectively. The results showed that: (1) The diurnal variation of net photosynthetic rate (P_n) changed from bimodal pattern to unimodal type curve after shading, and the average daily P_n decreased significantly with increase of shade degrees (P<0.05). P_n of BC-1(194) and BC-1(192) showed no significant advantage under L₀. But the P_n transgressive heterosis of BC-1(192) and BC-1(194) were respectively 2.23% and 3.67% under L₁. BC-1(192) had no remarkable advantage in P_n compared to TF33, while BC-1(194) showed hybrid weakness under L₂. (2) TF33 had the highest maximum net photosynthetic rate (P_max) and light saturation point (LSP), but the lowest light compensation point (LCP) and dark respiration rate (R_d) under L₀ among three clones. Under the condition of L₁, P_max, LSP and apparent quantum yield value (AQY) of BC-1(192) were the highest, while LCP and R_d were the lowest. Although P_max of BC-1(194) and BC-1(192) in L₂ were lower than that of TF33, LCP and R_d were lower, too. (3) The differences in fluorescence parameters were not significant among TF33, BC-1(194) and BC-1(192) under L₀. qP and qN were significantly different among three clones and BC-1(192) had the weakest heat dissipation but strongest photosynthetic capacity under L₁. BC-1(192) had no significant difference in qP with TF33, while BC-1(194) had significant difference with TF33 under L₂. At the same time, BC-1(192) had significant difference in F_v/F_m, F₀, ETR with BC-1(194) and TF33 (P<0.05). Our results indicated that TF33, BC-1(194) and BC-1(192) had internal regulatory mechanisms for responding to weak light to a certain extent, but it is most appropriate for growth under full light and their photosynthetic capacity decreased with the increase of shading degree. Inadequate light could become a decisive limiting factor for the growth of hybrid Paulownia. BC-1(194) and BC-1(192) had photosynthetic superiority compared with their parent TF33 in the low light condition of L₁.

Key words: BioMod2, climate change, ensemble model, Panax ginseng