Gas exchange in the salt marsh species Atriplex portulacoides L. and Limoniastrum monopetalum L. in Southern Portugal

Salt marshes are ecosystems subjected to a variety of environmental stresses like high salinity, water deficit, intense radiation or high temperatures. Field measurements were conduced in two halophyte species, Atriplex portulacoides L. and Limoniastrum monopetalum L., in the Reserva Natural do Sapal de Castro Marim, to compare their physiological response, i.e., water potential (psi), net photosynthetic rate (A), stomatal conductance (gs) under natural conditions. Both species demonstrated marked variations in psi throughout the year, with very low values in the summer, the period of higher salinity, drought and temperature. Deficit water potential (Delta psi = psi(midday) -psi(predawn)) was lower in the summer than in other seasons in A. portulacoides but not in L. monopetalum. The highest values for A and gs in L. monopetalum were observed in autumn and for A. portulacoides in winter, presenting both lowest values in spring and summer. A(max) was particularly high for L. monopetalum than for A. portulacoides in summer and autumn, despite gs(max) was similar in both species. Diurnal pattern of A and gs were similar in both species, with higher values in the morning, decreasing throughout the day.