Crassulacean acid metabolism in three species of the C-4 genus Portulaca

The putative existence of Crassulacean acid metabolism was investigated in three succulent species of the C-4 genus Portulaca: P. grandiflora (a widely cultivated species), P. oleracea (a cosmopolitan weed), and P. mundula (found on rock outcrops throughout the southwestern United States). Patterns of diurnal conductance and CO2 exchange, as well as diurnal tissue malic acid fluctuations, were measured in plants under well-watered and drought-stressed conditions. Net CO2 uptake in both treatments occurred only during the day, although small amounts of nighttime CO2 uptake occurred in a few individuals. Stomatal conductances in well-watered and drought-stressed P. mundula and drought stressed P. oleracea were higher during the night than during the day, though these higher conductances were generally not accompanied by net CO2 assimilation. No significant diurnal malic acid fluctuations occurred under well-watered conditions in any species; however, under drought stress, significant diurnal malic acid fluctuations occurred in the three species. The primary source of carbon for nocturnal malic acid production is presumed to be respiratory CO3, as occurs in C-3 plants that undergo CAM-cycling. This study confirms past reports of CAM acid fluctuations in P. grandiflora and P. oleracea and marks the first report of CAM activity in P. mundula.