1GURALNICK, LONNIE J.*, 2MAURICE SB KU, 2GERALD E. EDWARDS, 2VINCENT R. FRANCESCHI, AND 3BRANDON HOCKEMA. 1-Department of Biology, Western Oregon University, Monmouth, OR 97361, 2-School of Biological Sciences, Washington State University, PO Box 644236, Pullman, WA 99164, 3-University of Florida IFAS, Citrus Res and Education Ctr, 700 Experiment Station Rd, Lake Alfred, FL 33850. - Induction of the CAM pathway in the C4 plant,Portulaca grandiflora.
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