[Response of photosynthesis and antioxygenic enzymes in seedlings of three tropical forest tree species to different light environments].

This paper studied the response of photosynthesis, chlorophyll a fluorescence and 3 antioxygenic enzyme activities in the seedlings of a pioneer species (Mallotus barbatus) and two endangered sub-canopy tree species (Litsea pierrei var. szemaois and L. dilleniifolia) in Xishuangbanna tropical rain forest to different light environments. Three light regimes (50%, 25% and 8% full sunlight) were designed for the seedlings. The largest values of maximum photosynthetic rates (Pmax) of L. pierrei var. szemaois and L. dilleniifolia were documented at 25% and 8% sunlight, respectively, which showed that the two sub-canopy species had poor ability in photosynthetic acclimation to high light. In contrast, the Pmax of M. barbatus increased with light level. During a day, the maximum photochemical efficiency (Fv/Fm) of the photosystem II in all the three species grown under three light levels showed minimum values at noon, and the lowest Fv/Fm values were shown in seedlings at 50% sunlight. Under 50% sunlight, the Fv/Fm values at dawn were lower in L. dilleniifolia than in the same species under other two light levels, indicating that sustained photoinhibition occurred in this case. The contents of malondialdehyde (MDA), H2O2, and activities of ascorbate peroxidase (APX), superoxid dismutase (SOD), catalase (CAT) of both L. pierrei var. szemaois and M. barbatus increased with stronger irradiance, which revealed that these two species were able to alleviate photodamage under high light stress by increasing their antioxigenic activities. But, high content of MDA revealed that L. dilleniifolia was probably suffered from membrane damage. In conclusion, our data revealed that the pioneer species M. barbatus had a high capacity to acclimate to different light environments, particularly high light, while the two subcanopy Litsea species had poor ability in acclimation to high light. Based on the findings of the present study, we suggest that the small populations and endangered status of both L. pierrei var. szemaois and L. dilleniifolia in natural environment could be partially due to their poor flexibility in response to changed light environment resulted from habitat fragmentation.