Abstract
Different mechanisms may contribute to halophytes' tolerance to abiotic stress in nature, but their relative ecological relevance is largely unknown. We studied the responses to changing environmental conditions of five halophytes (Sarcocornia fruticosa, Inula crithmoides, Plantago crassifolia, Juncus maritimus, J. acutus) in a Mediterranean salt marsh. PCA was used to correlate soil and climatic data with changes in the plants' contents of biochemical markers of conserved stress responses - ions, osmolytes, MDA (a marker of oxidative stress) and antioxidant systems. Tolerance in S. fruticosa, I. crithmoides and P. crassifolia (succulent dicots) depends mostly on the transport of ions to the aerial parts and the synthesis of specific osmolytes, whereas both Juncus (monocots) avoid accumulation of toxic ions, maintaining a relatively high K+/Na+ ratio. For the most salt-tolerant taxa (S. fruticosa, I. crithmoides), seasonal variations of Na+, Cl- and glycine betaine did not correlate with environmental parameters associated to salt or water stress, suggesting that their tolerance mechanisms are constitutive and relatively independent of external conditions. Proline levels were too low to possibly have any osmotic effect but, except for P. crassifolia, Pro may play a role in stress tolerance as 'osmoprotectant'. No correlation was found between abiotic stress and MDA or antioxidants, indicating that these halophytes are not subjected to oxidative stress in their natural habitat
