THE DYNAMICS OF PHOTOSYNTHETIC ACCLIMATION TO CHANGES IN LIGHT QUANTITY AND QUALITY IN 3 AUSTRALIAN RAIN-FOREST TREE SPECIES

Photosynthetic acclimation was studied in seedlings of three subtropic al rainforest species representing early (Omalanthus populifolius), mi ddle (Duboisia myoporoides) and late (Acmena ingens) successional stag es in forest development. Changes in the photosynthetic characteristic s of pre-existing leaves were observed following the transfer of plant s between deep shade (15% of photosynthetically active radiation (PAR) , selectively filtered to produce a red/far-red (R/FR) ratio of 0.1) a nd open glasshouse (60% PAR and a R/FR ratio of 1.1-1.2), and vice ver sa. The extent and rate of response of the photosynthetic characterist ics of each species to changes in light environment were recorded in t his simulation of gap formation and canopy closure/overtopping. The li ght regimes to which plants were exposed produced significant levels o f acclimation in all the photosynthetic parameters examined. Following transfer from high to low light, the light-saturated rate of photosyn thesis was maintained near pre-transfer levels for 7 days, after which it decreased to levels which closely approximated those in leaves whi ch had developed in low light. The decrease in photosynthetic capacity was associated with lower apparent quantum yields and stomatal conduc tances. Dark respiration was the parameter most sensitive to changes i n light environment, and responded significantly during the first 4-7 days after transfer. Acclimation of photosynthetic capacity to increas es in irradiance was significant in two of the three species studied, but was clearly limited in comparison with that of new leaves produced in the high light conditions. This limitation was most pronounced in the early-successional-stage species. O. populifolius. It is likely th at structural characteristics of the leaves, imposed at the time of le af expansion, are largely responsible for the limitations in photosynt hetic acclimation to increases in irradiance.