Par and Temperature Influences on CO2 Uptake by Desert CAM Plants

Abstract

The influences of photosynthetically active radiation (PAR) and tissue temperature on chlorenchyma activities were examined for three desert succulents exhibiting Crassulacean acid metabolism (CAM): Agave deserti, Ferocactus acanthodes, and Opuntia ficus-indica. Total daily PAR predicted by computer simulation for clear days averaged 15 mol m−2 day−1 for leaves of A. deserti on an equinox and was uniformly distributed over the rosette. Measurements throughout the year indicated that PAR averaged 16 mol m−2 day−1 for leaves of A. deserti and 22 mol m−2 day−1 for unshaded stem surfaces facing in the four cardinal directions for F. acanthodes and O. ficus-indica. Both nocturnal CO2 uptake and acid accumulation by these desert succulents were 90% PAR saturated near 20 mol m−2 day−1, indicating that such plants require clear days and unshaded conditions for maximal CAM metabolism. The optimal temperature for nocturnal CO2 uptake averaged about 14°C when the plants were grown at day/night air temperatures of 20°C/10°C. This optimum shifted upward to 20°C when the growth temperatures were increased to 40°C/30°C. An analogous acclimation occurred for the tolerance of chlorenchyma tissues to extreme temperatures; the low temperature tolerated (temperature leading to 50% inhibition in cellular uptake of a vital stain) increased from an average of −11°C to −6°C upon shifting from 10°C/0°C to 50°C/40°C and the high temperature tolerated increased from 53°C to 65°C for this shift in growth temperature. The latter temperature represents an extremely great heat tolerance and helps explain why A. deserti, F. acanthodes, and O. ficus-indica can be so successful in a hot desert environment.