(Halimione) portulacoides is a halophytic, C₃ shrub. It is virtually confined to coastal salt marshes, where it often dominates the vegetation. The aim of this study was to investigate its growth responses to salinity and the extent to which these could be explained by photosynthetic physiology. The responses of young plants to salinity in the range 0–700 mol m⁻³ NaCl were investigated in a glasshouse experiment. The performance of plants was examined using classical growth analysis, measurements of gas exchange (infrared gas analysis), determination of chlorophyll fluorescence characteristics (modulated fluorimeter) and photosynthetic pigment concentrations; total ash, sodium, potassium and nitrogen concentrations, and relative water content were also determined. Plants accumulated Na⁺ approximately in proportion to external salinity. Salt stimulated growth up to an external concentration of 200 mol m⁻³ NaCl and some growth was maintained at higher salinities. The main determinant of growth response to salinity was unit leaf rate. This was itself reflected in rates of CO₂ assimilation, which were not affected by 200 mol m⁻³ but were reduced at higher salinities. Reductions in net photosynthetic rate could be accounted for largely by lower stomatal conductance and intercellular CO₂ concentration. Apart from possible effects of osmotic shock at the beginning of the experiment, salinity did not have any adverse effect on photosystem II (PSII). Neither the quantum efficiency of PSII (Φ_PSII) nor the chlorophyll fluorescence ratio (F_v/F_m) were reduced by salinity, and lower mid-day values recovered by dawn. Mid-day F_v/F_m was in fact depressed more at low external sodium concentration, by the end of the experiment. The growth responses of the hygro-halophyte A. portulacoides to salinity appear largely to depend on changes in its rate of photosynthetic gas exchange. Photosynthesis appears to be limited mainly through stomatal conductance and hence intercellular CO₂ concentration, rather than by effects on PSII; moderate salinity might stimulate carboxylation capacity. This is in contrast to more extreme halophytes, for which an ability to maintain leaf area can partially offset declining rates of carbon assimilation at high salinity.

Sarcocornia fruticosa (L.) A.J. Scott is found in coastal marshes of south‐west Spain, growing under a very wide range of interstitial soil salinity from 10 mM up to nearly 1000 mM. A glasshouse experiment was designed to investigate the effect of this range of salinities on the morphology and the photosynthetic apparatus of S. fruticosa by measuring growth rate, photosynthetic and non‐photosynthetic area, atrophy of distal branch ends, water status, chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. The long‐term effects of salinity on the growth of S. fruticosa were mainly determined by the extent of photosynthetic area rather than the variations in net photosynthetic rate. Photosynthetic area was reduced at 1030 mM as a result of a decrease in the length of the photosynthetic portions. This was induced by fewer internodes and, at salinities lower than 510 mM, smaller internode diameter. Net photosynthetic rate increased as the quantum efficiency of photosystem II decreased in the different salinity treatments, which means that the plant could be increasing photorespiration and/or using cyclic electron transport as additional photoprotective mechanisms. The recorded drop in net photosynthetic rate at higher salinities appeared to be due to a reduction in stomatal conductance. The results indicate that S. fruticosa is capable of tolerating very high and continued exposure to salt, showing its greatest growth rate at 510 mM NaCl.