Photosynthetic and anatomical characteristics in the C4–crassulacean acid metabolism-cycling plant Portulaca grandiflora

Abstract

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001.

Portulaca grandiflora (Lind.) is a succulent species with C₄ photosynthesis and crassulacean acid metabolism (CAM) cycling in leaves, and CAM-idling type photosynthesis in stems. We investigated the level and localization of carbon fixation enzymes and photosynthetic activity of leaves and stems of P. grandiflora under well-watered and drought conditions. As CAM activity increased during water stress, the leaf water-storage tissue collapsed, presumably transferring water to the bundle sheath and mesophyll cells, and so maintaining the C₄ photosynthetic pathway. Tissue prints indicated an increase in phosphoenolpyruvate carboxylase (PEPC) in the water-storage tissue of leaves and the cortex of stems. Immunoblot analyses after 10 d of water stress showed that leaves had a slight decrease in the proteins of the C₄-CAM pathway, while at the same time a new isoform of NADP-malic enzyme (NADP-ME) appeared. In contrast, the stem showed increases in proteins of the CAM pathway when water stressed. Under water stress, diurnal fluctuation in acidity in leaves was not accompanied by a net gain or loss of CO₂ at night, and there was sustained, but decreased, fixation of CO₂ during the day, characteristic of CAM cycling. High gross rates of O₂ evolution were maintained during the day under water stress, suggesting induction of alternative electron sinks. With induced diurnal fluctuations in acidity in stems, there was no net carbon gain during the day or night. These results demonstrate, for the first time, that the stem of P. grandiflora is an inducible CAM-idling tissue. Our results also indicate that the C₄ and CAM pathways operate independently of one another in P. grandiflora.