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 CO
2 fixation rates of 28 mol 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 CO
2 fixation and the increases in
qn 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.
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