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Photosynthetic Acclimation to Different Growth Light Environments in Seedlings of Three Tropical Rainforest Syzygium Species

Light is one of the most important factors that affect the regeneration and succession of tropical rain forests. Tropical trees with different successional status have different acclimation capacities to different growth light environments. In the present study, gas exchange and diurnal variation of chlorophyll fluorescence parameters were measured in the seedlings of three tropical rainforest Syzygium species, which are early (S. szemaoense), middle (S. cumini) and late (S. latilimbum) successional species. The seedlings were cultivated under three different light regimes (fully open site, 42% and 14% daylight). Their leaf mass per unit area (LMA) and chlorophyll content were also determined. The early successional species had the highest light- saturated photosynthetic rate (P max) and photosynthetic acclimation capacity to the variation of growth light environments, the next was the middle successional species. The late successional species had the lowest P max values and its photosynthesis was strongly suppressed when grown in full light. Nevertheless, grown under both the full light and 42% daylight, all the three species did not suffer from irreversible photoinhibition, as indicated by their dawn values of maximal photochemical efficiency of PS Ⅱ (F v/F m) larger than 0.8, and their φPSⅡ was able to recover on the same day. Dynamic (reversible) photoinhibition occurred in all species, the degree of which increased with growth light level. Among species, the degrees of dynamic photoinhibition were similar. The non-photochemical fluorescence-quenching rate (NPQ) increased with the growth light level for all species. However, in the fully open site, the NPQ rates in the morning were higher in the late and middle successional species than in the early successional species. This compensated for the lower photosynthetic rates of the former two species, and probably plays a role in preventing photodamage to these species under full light. Leaf dark respiration rate per unit area (R d) and leaf mass per area (LMA) tended to increase with the growth light level. Under the same light regimes, R d was higher in early than in middle and late successional species. The LMA values of late successional species were higher than for the other two species. This could be favorable for this shade-tolerant species to prevent herbivory. For all of the three species grown in 14% daylight, area-based P max, photosynthetic light saturation point, light compensation point, R d and LMA were reduced, while chlorophyll content per unit dry weight were increased compared to those in the other two light regimes. This showed their favorable acclimation to low light. Nevertheless, under 14% daylight the seedlings of both the early and middle species died, only the seedlings of the late successional species were still alive. This indicates that this late successional species has high capacity to acclimate to low light.