Gas exchange and chlorophyll fluorescence of C3 and C4 saltmarsh species

Spartina maritima (Curtis) Fernald, Spartina densiflora Brong, Arthrocnemum perenne (Miller) Moss, and Arthrocnemum fruticosum (L.) Moq are very frequent halophytes on the coasts of SW Europe. The first two are perennial Gramineae with C-4 metabolism; the last two are perennial Chenopodiaceae with C-3 metabolism. Controlled garden experiments were carried out with the four species to compare their physiological response, i.e., water potential (Psi), net photosynthetic rate (P-N), transpiration rate (E), stomatal conductance (g(s)), intercellular CO2 concentration (C-i), and chlorophyll fluorescence of photosystem (PS) 2 under saline and non-saline conditions. S. maritima behaves as an osmoconformer species, the other three as osmoregulators. in the four species, P-N, E, and g(s) improved following freshwater irrigation. The variations in P-N might be related with biochemical changes (which appear not to affect PS2), but not with significant stomatal fluctuations, which are associated with a lower water use efficiency in the case of Arthrocnemum. The species were segregated into two groups (not depending on their C-3 or C-4 photosynthetic pathway), in relation with the topographic level of this species in natural conditions: the relative responses of P-N in S. maritima and A. perenne were lower than those of S. densiflora and A. fruticosum. The salt-tolerance index supports such segregation. S. densiflora demonstrated the best competitive possibilities against salt-tolerant glycophytes, with its more flexible response in saline or brackish environments, which explains its spreading along the rivets draining into the estuaries of the SW Iberian Peninsula.