Abstract
Aims
Osmolytes, used for maintaining osmotic balance and as
osmoprotectants, are synthesized in plants as a general, conserved
response to abiotic stress, although their contribution to
stress-tolerance mechanisms remains unclear. Proline, the most
common osmolyte, accumulates in many plant species in parallel
with increased external salinity and is considered a reliable biochemical
marker of salt stress. We have measured proline levels
in two halophytic, closely related Juncus species under laboratory
and field conditions to assess the possible relevance of proline
biosynthesis for salt tolerance and therefore for the ecology of
these two taxa.
Methods
Proline was quantified in plants treated with increasing NaCl concentrations
and in plants sampled in two salt marshes located in
the provinces of Valencia and Alicante, respectively, in southeast
Spain. Electrical conductivity, pH, Na+ and Cl− concentrations
were measured in soil samples collected in parallel with the plant
material.
Important Findings
Treatment with NaCl inhibited growth of J. acutus plants in a
concentration-dependent manner, but only under high salt conditions
for J. maritimus. Salt treatments led to proline accumulation in both
species, especially in the more salt-tolerant J. maritimus. The results,
obtained under laboratory conditions, were confirmed in plants sampled
in the field. In all the samplings, proline contents were significantly
lower in J. acutus than in the more tolerant J. maritimus growing
in the same area. No direct correlation between soil salinity and proline
levels could be established, but seasonal variations were detected, with
increased proline contents under accentuated water deficit conditions.
Our results suggest that proline biosynthesis is not only an induced,
general response to salt stress but also an important contributing factor
in the physiological mechanisms of salt tolerance in Juncus, and that it
therefore correlates with the ecology of both species.
