The genus Phillyrea includes woody shrub species of Mediterranean
maquis which can be found occasionally in
saline soils or areas exposed to salt aerosols. To test salinity tolerance
in
Phillyrea latifolia L., plants were grown
in 1-l containers and exposed to different levels of salinity in a glasshouse
over two growing seasons. Plants
survived exposure to NaCl concentrations of up to 500 mM NaCl
for 4
months. Analysis of the growth response
to increasing salinity showed that shoot elongation was inhibited by 50%
at
123 and 135 mM external NaCl after
31 and 123 d of salt treatment, respectively. Growth parameters were completely
inhibited by treatment with
300 mM external NaCl for 64 d. High salt tolerance in Phillyrea
plants was the result of both exclusion and
secretion mechanisms. Secretion of ions occurred via salt glands present
mainly
on the abaxial surface of the leaf.
Salt glands occurred alone or in pairs and were typically formed by a highly
vacuolated collecting cell, a stalk
cell and a group of 10–16 secretory cells. The mean diameter of the
glands was 54 μm and the average density was
6·1 mM−2 (abaxial side). Peaks of Cl,
K, Ca,
Na, Si, Mg, Fe and Cu were detected in salt crystals near glands by
X-ray microprobe analysis. This is the first report of salt glands in the
Oleaceae family and of their role in salinity
tolerance in Phillyrea species.