PLANT PHYSIOLOGY , Vol 102, Issue 3 835-841, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Short-Term Regulation of Crassulacean Acid Metabolism Activity in a Tropical Hemiepiphyte, Clusia uvitana
G. Zotz and K. Winter
Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Republic of Panama
Diel courses of net CO2 exchange of leaves were studied in Clusia uvitana
(Clusiaceae), a tropical Crassulacean acid metabolism (CAM) hemiepiphyte,
growing in the crown of a 47-m tall kapok tree on Barro Colorado Island,
Panama. Measurements on days without precipitation showed that net uptake
of atmospheric CO2 occurred at night, a feature of CAM, as well as in the
early morning and late afternoon. During 36 h of almost continuous
rainfall, nocturnal net CO2 uptake was abolished and the diel pattern of
net CO2 exchange became similar to that of a C3 plant. Exposing
well-watered, potted plants of Clusia in the laboratory to temperatures and
photosynthetic photon flux densities similar to those during the tropical
rainstorm also abolished nocturnal net CO2 uptake. In contrast, Kalanchoe
pinnata (Crassulaceae), an obligate CAM plant, still showed net CO2 dark
fixation following the same low-light and moderate-temperature conditions,
albeit at decreased rates. During these 12-h photoperiods, titratable
acidity in Clusia increased slightly above its high level measured at the
end of the previous dark period, whereas in Kalanchoe, the acid content
decreased by about 40%. A survey among outer canopy leaves of Clusia on
Barro Colorado Island showed that leaves that exhibited little or no
nocturnal acidification maintained high levels of H+ at dawn and dusk.
Progressively lower levels of H+ at dusk were accompanied by progressively
higher nocturnal increases in H+. The data suggest that in C. uvitana the
rapid switching between CAM- and C3-type carbon fixation that may occur
within 24 h in response to environmental changes is controlled by the
acidity status of the leaves in the light. Nocturnal CO2 fixation is
enhanced by conditions that decrease the organic acid content during the
light period.
