Photosynthetic characteristics of Flindersia brayleyana and Castanospermum australe from tropical lowland and upland sites

Photosynthetic responses to temperature, light and carbon dioxide partial pressure were studied in two-year-old Flindersia brayleyana F. Muell. and Castanospermum australe Cunn. & C. Fraser ex Hook. growing on coastal lowland and upland rainforest sites in tropical Queensland, Australia. Climatic conditions ranged from moist and cool (17-19 degrees C) to dry and warm (22-24 degrees C). The optimum temperature for photosynthesis was 23.7-25.6 degrees C for C. australe and 21.2-24.6 degrees C for F. brayleyana. Mean maximum rate of electron transport for each species did not differ between sites but was higher (60-62 micromoles m-2 s-1) in F. brayleyana than in C. australe (42-44 micromoles m-2 s-1). Ribulose-bisphosphate carboxylation rate did not differ significantly between sites or species. Maximum rates of photosynthesis at 1000 microPa Pa-1 CO2 did not differ significantly between sites for each species, but did differ significantly between species. At 350 microPa Pa-1 CO2, photosynthetic light use efficiencies of F. brayleyana and C. australe were 0.05 and 0.015, respectively, at the upland site, and the corresponding values at the lowland site were 0.025 and 0.05. In C. australe, these differences were reflected in significantly greater maximum rates of photosynthesis at 350 microPa Pa-1 CO2 at the lowland site than at the upland site (5.2 versus 3.3 micromoles m-2 s-1).