Some deleterious effects of drought, soil salinity and other abiotic stresses are mediated by the generation of oxidative stress through an increase in reactive oxygen species (ROS) that damage cellular membranes, proteins and DNA. In response to increased ROS, plants activate an array of enzymatic and non-enzymatic antioxidant defences. We have correlated the activation of these responses with the contrasting tolerance to salinity and drought of three species of the genus Juncus, viz. J. maritimus, J. acutus (both halophytes) and J. articulatus (salt-sensitive). Both stresses were given for 8 weeks to 6-week-old seedlings in a controlled environment chamber. Each stress inhibited growth and degraded photosynthetic pigments in the three species with the most pronounced effects being in J. articulatus. Salt and water stress also generated oxidative stress in all three taxa with J. articulatus being the most affected in terms of accumulation of malondialdehyde (a reliable oxidative stress marker). The apparent lower oxidative stress in halophytic J. maritimus and J. acutus compared with salt-sensitive J. articulatus is explained by a more efficient activation of antioxidant systems since salt or water deficiency induced a stronger accumulation of antioxidant phenolic compounds and flavonoids in J. maritimus and J. acutus than in J. articulatus. Qualitative and quantitative differences in antioxidant enzymes were also detected when comparing the three species and the two stress treatments. Accordingly, glutathione reductase and superoxide dismutase activities increased in the two halophytes under both stresses, but only in response to drought in J. articulatus. In contrast, ascorbate peroxidase activity varied between and within species according to treatment. These results show the relative importance of different antioxidant responses for stress tolerance in species with distinct ecological requirements. The salt-sensitive J. articulatus, contrary to the tolerant taxa, did not activate enzymatic antioxidant responses to salinity-induced oxidative stress.
Keywords: Antioxidant enzymes; Juncus; antioxidant phenolics; ecological adaptation; malondialdehyde (MDA); photosynthetic pigments; salt stress; water deficiency stress.