Stress tolerance mechanisms in Juncus: responses to salinity and drought in three Juncus species adapted to different natural environments.

Comparative studies on the responses to salinity and drought were carried out in three Juncus species, two halophytes (Juncus maritimus Lam. and Juncus acutus L.) and one more salt-sensitive (Juncus articulatus L.). Salt tolerance in Juncus depends on the inhibition of transport of toxic ions to the aerial part. In the three taxa studied Na+ and Cl- accumulated to the same extent in the roots of salt treated plants; however, ion contents were lower in the shoots and correlated with the relative salt sensitivity of the species, with the lowest levels measured in the halophytes. Activation of K+ transport at high salt concentration could also contribute to salt tolerance in the halophytes. Maintenance of cellular osmotic balance is mostly based on the accumulation of sucrose in the three species. Yet, neither the relative salt-induced increase in sugar content nor the absolute concentrations reached can explain the observed differences in salt tolerance. In contrast, proline increased significantly in the presence of salt only in the salt-tolerant J. maritimus and J. acutus, but not in J. articulatus. Similar patterns of osmolyte accumulation were observed in response to water stress, supporting a functional role of proline in stress tolerance mechanisms in Juncus.