The effect of light levels on daily patterns of chlorophyll fluorescence and organic acid accumulation in the tropical CAM tree Clusia hilariana

Sandy plains are characteristic of the coastal region of Brazil. We investigated the diel patterns of changes in organic acid levels, leaf conductance and chlorophyll a fluorescence for sun-exposed and shaded plants of Clusia hilariana, one of the dominant woody species in the sandy coastal plains of northern Rio de Janeiro state. Both exposed and shaded plants showed a typical CAM pattern with considerable diel oscillations in organic acid levels. The degradation of both malic and citric acids during the midday stomatal closure period could lead to potential CO₂ fixation rates of 28 μmol m-² s-¹ in exposed leaves. Moreover, exposed leaves exhibited large increases in total non-photochemical quenching (q(N)) accompanied by a substantial decrease in effective quantum yield during the course of the day. However, these potential high rates of CO₂ fixation and the increases in q(N) of exposed plants were not enough to maintain the primary electron acceptor of photosystem II (Q(A)) in a low reduction state, similar to that of shaded plants. As a result, there was a moderate increase in the reduction state of Q(A) throughout the day. Most of the decline in photochemical efficiency of exposed leaves of C. hilariana was reversible, as evidenced by the high levels of pre-dawn potential quantum yields (F(v)/F(m)) and their rapid recovery after sunset. However, the depletion of the organic acid pool in the afternoon resulted in an accentuated subsequent drop in F(v)/F(m), suggesting that prolonged periods of water stress accompanied by high irradiance levels may expose plants of C. hilariana in unprotected habitats to the danger of photoinhibition.