Portulaca grandiflora is a herbaceous annual of the family Portulacaceae. It exhibits a Kranz anatomy typical of C4 species but Portulaca shows some degree of succulence. Research has shown that Portulaca shows a diurnal acid fluctuation typical of CAM-cycling. Under well-watered conditions, PEP carboxylase was localized in the leaf mesophyll tissue but when water-stressed, PEP carboxylase was found throughout the water storage tissue of the leaf. Stems also showed an increase in the PEP carboxylase signal when water-stressed. During water stress, the water storage tissue collapses, presumably transferring water to the bundle sheath, which remain turgid during the 10 days of water stress. Immunoblot analyses of protein gels were done for NADP-ME, and PEP carboxylase. We found a slight decrease in the proteins of the C4/CAM pathway in the leaves of Portulaca. The stem showed increases in the proteins of the CAM pathway when water stressed which included a new isoform of NADP- ME. These results demonstrate for the first time, that the stems of Portulaca are an inducible CAM tissue. The stem may play a supporting role for the leaves during water stress since our experiments showed that the stem during 10 days of water stress, lost only ~5% of its relative water content. Under water stress conditions, a measure of the true rate of photosynthesis (Photosystem II activity; JO2) showed an increase in the late afternoon when compared to control plants. Two factors may account for this; one, an increase in the CO2 released from deacidification in the water storage tissue giving the leaf an internal source of carbon dioxide, and two, an increase in the Mehler peroxidase reaction. Our results reveal differences between Portulaca grandiflora and other, non-succulent C4 plants, Amaranthus cruentus and Zea Mays, which show a decline in JO2 after 4 days of water stress.

Key words: C4/CAM photosynthesis, CAM-cycling, PEP carboxylase, Portulaca grandiflora, Portulacaceae, water stress