Relationships between Photosynthetically Active Radiation, Nocturnal Acid Accumulation, and CO(2) Uptake for a Crassulacean Acid Metabolism Plant, Opuntia ficus-indica.

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 C(3) photosynthesis. The total nocturnal H(+) accumulation and CO(2) 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 CO(2) 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 CO(2) 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-CO(2) ratio of 2.0 was observed. Under progressively drier conditions, both nocturnal H(+) accumulation and CO(2) uptake decreased, but the H(+)-to-CO(2) ratio increased. A ratio of two H(+) per CO(2) is consistent with the H(+) production accompanying the conversion of starch to malic acid, and it apparently occurs for O. ficus-indica when CAM CO(2) uptake is strongly favored over respiratory activity.