Plant Physiology 71:71-75 (1983)
© 1983 American Society of Plant Biologists
Articles
Relationships between Photosynthetically Active Radiation, Nocturnal Acid Accumulation, and CO2 Uptake for a Crassulacean Acid Metabolism Plant, Opuntia ficus-indica1
Park S. Nobel and
Terry L. Hartsock
Department of Biology, University of California, Los Angeles, California 90024,
Laboratory of Biomedical and Environmental Sciences, University of California, Los Angeles, California 90024
The influences of photosynthetically active radiation (PAR) and water status on nocturnal Crassulacean acid metabolism (CAM) were quantitatively examined for a widely cultivated cactus, Opuntia ficus-indica (L.) Miller. When the total daily PAR was maintained at 10 moles photons per square meter per day but the instantaneous PAR level varied, the rate of nocturnal H+ accumulation (tissue acidification) became 90% saturated near 700 micromoles per square meter per second, a PAR level typical for similar light saturation of C3 photosynthesis. The total nocturnal H+ accumulation and CO2 uptake reached 90% of maximum for a total daily PAR of about 22 moles per square meter per day. Light compensation occurred near 0 moles per square meter per day for nocturnal H+ accumulation and 4 moles per square meter per day for CO2 uptake. Above a total daily PAR of 36 moles per square meter per day or for an instantaneous PAR of 1150 micromoles per square meter per second for more than 6 hours, the nocturnal H+ accumulation actually decreased. This inhibition, which occurred at PAR levels just above those occurring in the field, was accompanied by a substantial decrease in chlorophyll content over a 1-week period.
A minimum ratio of H+ accumulated to CO2 taken up of 2.5 averaged over the night occurred for a total daily PAR of 31 moles per square meter per day under wet conditions. About 2 to 6 hours into the night under such conditions, a minimum H+-to-CO2 ratio of 2.0 was observed. Under progressively drier conditions, both nocturnal H+ accumulation and CO2 uptake decreased, but the H+-to-CO2 ratio increased. A ratio of two H+ per CO2 is consistent with the H+ production accompanying the conversion of starch to malic acid, and it apparently occurs for O. ficus-indica when CAM CO2 uptake is strongly favored over respiratory activity.
1 Supported by Department of Energy Contract DE-AM03-76-SF00012.
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