Limited photosynthetic plasticity in Sedum pulchellum Michx

Carbon dioxide exchange pattern, carboxylating enzyme specific activity and titratable acidity were measureed for field collected individuals of Sedum pulchellum Michx., a winter annual of xeric cedar glade communities in central Tennessee. Evidence of a weak-CAM mode of photosynthesis was found on 4 of 27 sampling dates during a three year period. Three of these dates occurred during 1978, the driest study season, and nocturnal CO₂ uptake contributed a maximum of 10% of the plants daily CO₂ uptake. Weak CAM in the field was associated with elevated phosphoenol-pyruvate (PEP) carboxylase activity and, to a lesser degree, with decreased ribulose bisphosphate (RuBP) carboxylase activity. In contrast, carboxylating enzyme specific activities of the sympatric, obligate CAM species Opuntia compressa, a dominant perennial of the cedar glades, were highly stable.Weak CAM was also induced in S. pulchellum upon water stress in the laboratory. Again, weak CAM was associated with elevated PEP carboxylase and decreased RuBP carboxylase activities and represented a relatively minor CO₂ uptake contribution. The potential ecological significance of this limited photosynthetic flexibility in S. puchellum is not obvious but it may play an important role in successful yearly reproduction in this annual species at a time when water stress is frequent.