Photosynthetic responses to light variation in rainforest species
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Photosynthetic induction under constant and fluctuating light conditions was investigated in intact leaves of Alocasia macrorrhiza and Toona australis, two species native to Australian rainforests. When leaves were exposed to saturating light following a long period at low light intensity, an induction period of 25–40 min was required before steady-state photosynthesis was achieved. A long induction period was required regardless of plant growth conditions (high vs. low light) and ambient CO2 concentrations during mesurement. In low-light grown A. macrorrhiza, the initial slope of the relationship between assimilation and internal CO2 pressure increased 7-fold from 30 s following illumination to the end of the induction period. Both stomatal and carboxylation limitations play a role in photosynthetic induction, but carboxylation limitations predominate during the first 6–10 min. In both species, leaf induction state increased 2 to 3-fold during a sequence of five 30-or 60-s lightflecks separated by 2 min of low light. Rates of induction during 60-s lightflecks and during constant illumination were similar. Induction loss in low-light grown leaves of Alocasia macrorrhiza required more than 60 min of continuous exposure to low light conditions. These results suggest that, under forest understory conditions, leaves are at intermediate induction states for most of the day. The ability to utilize sunflecks may therefore be strongly influenced by the ability of leaves to maintain relatively high states of induction during long periods of low light.
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