AbstractThe response of photosynthesis,Fe, to salinity was examined in two clones ofAgrostis stolonifera, one derived from a salt marsh (SM) and the other from an inland site (IL). The response of young and old leaves was examined over one week of salt treatment and changes in leaf water relations and concentrations of Na+, Cl−and K+ions were also monitored. Sodium+and chloride−concentrations rose much more sharply in the older leaves of both clones andFedeclined to ca. zero by Day 4.Fein the young leaves of the SM clone was unaffected by salt whilst the young leaves of the IL clone showed a 20% reduction inFewithin 24 h of salt application. This decline was due to a decline in stomatal conductance and a reduction in the photosynthetic capacity of the mesophyll and was associated with a greater accumulation of Na+and Cl−ions and loss of K+ions than in the SM clone. The significance of the salt exclusion from the young leaves of the SM clone to its growth on the salt marsh is disc