Tetrameles nudiflora is a representative species of the Southeast Asian rainforest and as its seedlings rarely survive in the understory, it was included in the protection category. Previous investigations found that the Tetrameles nudiflora seedling rarely survived in the understory, therefore it is hard to explain why the adults could grow into the top layer of the canopy in mature rainforests. In this study, we discussed the photosynthetic characteristics of the Tetrameles nudiflora seedling which varied under different light and water conditions. Our purpose was to confirm whether Tetrameles nudiflora was a heliophilous plant or not, and how it responded to drought stress, which would be very important for the in-situ and ex-situ protection of Tetrameles nudiflora in monsoon climate areas.
In this experiment, Tetrameles nudiflora seedlings have been treated under three different water supplies: water-saturated treatment (ST), (32.2±8.3)% soil volumetric water content (VWC); control treatment (CT), (24.6±7.7)%; and drought treatment (DT), (19.6±7.6)%. The results showed that Tetrameles nudiflora seedlings had a higher light saturation point (LSP) (726-1080 μmol m^-2 s^-1) and maximum photosynthetic rate (P_max) (5.481-9.708 μmol CO₂ m^-2 s^-1) than other species from later stages of succession, under all water gradients. The P_n response to PAR changed as the V-curve when the light changed from saturating to a more limited level. The net photosynthetic rate of water-saturated treatment ((9.515±3.216) μmol CO₂ m^-2 s^-1) was higher than others in stable light (1500 μmol m^-2 s^-1), but the time requirements of the net photosynthetic rate to stabilize from darkness to 1500 μmol m^-2 s^-1 were (6.9±0.7) min (ST), (6.6±2.2) min (CT) and (7.5±1.4) min (DT). The time requirements of net photosynthetic rate to stabilize from 1500 μmol m^-2 s^-1 to 100 μmol m^-2 s^-1 were (5.6±0.8) min (ST), (5.9±0.5) min (CT) and (4.7±1.7) min (DT). The response time of photosynthetic acclimation to light changes had little change over all water gradients.
In conclusion, the high LSP and P_max of Tetrameles nudiflora supports our hypothesis that it is a heliophilous species. The Tetrameles nudiflora seedling could grow faster than other local species in both the rainy and dry seasons with sufficient sunlight, and photosynthetic acclimation characteristics to light changes indicated this species was probably an early successional species. The Tetrameles nudiflora seedling rarely survived under a closed canopy, therefore the adults in a closed forest may have been established in the very early stages of community succession, long ago. The regeneration of the Tetrameles nudiflora population probably depends on the existence of large canopy gaps and secondary bare land. The higher photosynthesis and growth characteristics enable Tetrameles nudiflora to quickly close the forest and form the understory environment under which the Tetrameles nudiflora adults may contribute to the establishment of late successional species.