Abstract: Tropical rain forest trees often experience several episodes of different light environments during their growth from seedlings into canopy trees, such as in canopy gaps with high irradiance and in forest understorey with very low irradiance. Shade tolerant trees usually have low photosynthetic capacities and low plasticity in photosynthetic acclimation to different light environments. They may suffer from excess light when growing in high light environment, resulting in photoinhibition and photoxidation. Also, when forest understorey plants are suddenly exposed to high irradiance such as canopy gap creation, they may suffer severe photodamage. Plants have two major mechanisms to prevent photodamage, i.e., thermal dissipation through the development of an increased transthylakoid pH gradient and xanthophyll cycle, and antioxidant system to scavenge oxygen free radicals resulting from excess light energy. When growing in high light, plants may increase photosynthetic energy utilization or increase photoprotection capacities or both. This study investigated the effect of growth light level on the capacities of photosynthesis, heat dissipation and antioxidation in seedlings of two tropical rainforest tree species, Pometia tomentosa (Sapindaceae), an important canopy species, and Horsfieldia tetratepala (Myristicaceae), a subcanopy species. The seedlings were cultivated under shading nets with three different light levels, i.e. 100%, 25% and 8% daylight, respectively. The maximum photosynthetic rate (Pmax) of P. tomentosa increased with the growth light level. In contrast, Pmax of H. tetratepala grown in 100% daylight was smaller than that in 25% daylight. Under either 100% or 8% daylight, P. tomentosa had larger Pmax values than H. tetratepala. The values of both photosynthetic light saturation point and chlorophyll content per unit mass increased with growth light level for both species, and under the same light levels, P. tomentosa had greater values than H. tetratepala. Chlorophyll a/Chlorophyll b ratios were not significantly different for both species under the three different light levels; however, P. tomentosa had significantly larger values than H. tetratepala. Compared to the shaded situations, in the fully open site, the maximum photochemical efficiency of photosystem II (Fv/Fm) of both species were reduced, indicating that sustained photoinhibition occurred in the seedlings grown under full sun. When the shaded seedlings were transferred to full sun, on the first day, photoinhibition (as indicated by the decrease of ΔF/Fm′) accelerated from dawn to noon, and the recovery from photoinhibition occurred gradually in the afternoon but was not completed until sunset in P. tomentosa seedlings from the 8% daylight site and in H. tetratepala seedlings from both 8% and 25% daylight sites. The light response curves of non-photochemical quenching showed that heat dissipation capacity of H. tetratepala increased with growth light level. Whereas, P. tomentosa had lower heat dissipation capacity in the fully open site compared to in shaded sites. In the fully open site and in 25% daylight site, P. tomentosa had lower heat dissipation capacity than H. tetratepala. The contents of O-[]·2, H2O2 and activities of SOD and CAT in the seedlings in the open site were the greatest for both species, and these were higher in P. tomentosa than in H. tetratepala under the same light levels. With the pooled data from both species under all light levels, there were significantly correlations among O-[]·2, H2O2 content, and SOD and CAT activities. This indicated that oxygen free radicals in seedlings were scavenged through the enhancement of activities of antioxidative enzymes. The present study shows that the canopy species P. tomentosa has higher photosynthetic plasticity but lower thermal dissipation capacity compared to the subcanopy species H. tetratepala. Under photoinhibitory conditions, the photoprotection of P. tomentosa was more dependent on the enhancement of activities of antioxidative enzymes, while H. tetratepala was more dependent on the non_radiative heat dissipation. H. tetratepala showed poor acclimation to grow in fully open sites.