[Effects of light intensity on photosynthetic capacity and light energy allocation in Panax notoginseng.]

To explore the effects of light intensity on photosynthetic characteristics and light adaptation of the shade-demanding species Panax notoginseng, the responses of photosynthesis to photosynthetic photon flux density, CO₂ and sunflecks in the two-year-old Panax notoginseng were investigated under different levels of light intensity (29.8%, 9.6%, 5.0%, 1.4% and 0.2% of full sunlight). Meanwhile, chlorophyll a fluorescence parameter and light energy partitioning were also recorded and calculated in the responsive process. P. notoginseng grown under 29.8% full sunlight (FL) had a lower apparent quantum yield (AQY), potential photochemical quantum yield (F_v/F_m) and potential photochemical activity (F_v/F_o), however, it had a higher maximum net photosynthetic rate (P_{n max}), maximum electron transport rate (J_max), F/F_m', electron transport rate (ETR), photochemical quenching (q_P) and the proportion of light energy allocated to photochemistry dissipation (ΦPS₂), but the non-photochemical quenching (NPQ) was not the highest. There were no significant differences in P_{n max}, light compensation point (LCP), light saturation point (LSP), dark respiration rate (R_d) among 9.6%FL and 5.0% FL treatments, but these treatments had relatively higher values of NPQ, AQY, carboxylation efficiency (CE), maximum carboxylation rate (V_{c max}), F_v/F_m and F_v/F_o. In addition, the P_{n max}, CE, V_{c max}, J_max, ETR,F/F_m', q_P, NPQ and Φ_PSII decreased with the decrease in light intensity from 5.0%FL to 0.2%FL, and the proportion of light energy allocated to fluorescent dissipation (Φ_f,d) were increased. Under 500 μmol·m^-2·s^-1 light-flecks inducting, the Φ_PSⅡof P. notoginseng increased slowly with the extension of time except for the treatment of less than 5.0%FL, and under the circumstance of 1.4%FL and 0.2%FL, Φ_PSⅡreached significantly a perfect result, moreover, Φ_f,dincreased rapidly. These results suggested that the enhancement in photosynthetic electron transport to use the light energy and the moderate photoinhibition of PS2 might avoid the irreversible oxidative damage of photosynthetic organization in P. notoginseng under high levels of light intensity. Moderate shading was beneficial to maintain its higher non-photochemical quenching ability. However, its photosynthetic capacity depressed and the proportion of light energy allocated to non-photochemical pathway increased obviously in excessive shading, and it easily caused a light oxidative damage.