C4-like photosynthesis and the effects of leaf senescence on C4-like physiology in Sesuvium sesuvioides (Aizoaceae).

Sesuvium sesuvioides (Sesuvioideae, Aizoaceae) is a perennial, salt-tolerant herb distributed in flats, depressions or disturbed habitats of southern Africa and the Cape Verdes. Based on carbon isotope values it is considered a C4 species, despite a relatively high ratio of mesophyll to bundle sheath cells (2.7:1) in the portulacelloid leaf anatomy. Using leaf anatomy, immunocytochemistry, gas exchange measurements and enzyme activity assays, we sought to identify the biochemical subtype of C4 photosynthesis used by S. sesuvioides and to explore the anatomical, physiological as well as biochemical traits of young, mature and senescing leaves, with the aim to elucidate the plasticity and possible limitations of the photosynthetic efficiency in this species.Assays indicated that S. sesuvioides employs the NADP-malic enzyme as the major decarboxylating enzyme. The activity of C4 enzymes, however, declined as leaves aged and the proportion of water-storage tissue increased while air space decreased. These changes suggest a functional shift from photosynthesis to water-storage in older leaves. Interestingly, S. sesuvioides demonstrated CO2 compensation points ranging between C4 and C3-C4 intermediate values and immunocytochemistry revealed labeling of Rubisco large subunit in mesophyll cells. We therefore hypothesize that S. sesuvioides represents a young C4 lineage with C4-like photosynthesis in which C3 and C4 cycles are running simultaneously in the mesophyll.