Allocation of salt contaminants to tissue and cell storage
compartments in foliage
Within the 1-2 mm-long leaf rim samples visualized in cryo-microscopy
and whatever the sampling site, at least Cl (NBG) or both salt ions
(Riga’s sites) showed a homogeneous distribution in all leaf blade
tissues (Fig. 4A-C). At leaf level, the allocation was similar to that
of K but showed differences with that of e.g. Ca (Ca-oxalate
crystals and preferential epidermis location) or Si (cell wall location;
results not shown). The increase in Na and Cl count frequency with
higher foliar contamination of salts formed the main inter-site
difference observed during the microanalytical assessments (Fig. 4D-F).
At cell level (Fig. 4G-K), Na and
Cl were found within vacuolar compartments, thus closely matching the
allocation of K (Fig. 4I-J vs. K). Salt accumulation caused a
change in the ionic composition of cell vacuoles from all leaf blade
tissues within leaf rim segments (Fig. 5). In mesophyll, the Na/Cl mass
fraction at salt contaminated sites reached 3.98/7.29% (palisade
parenchyma) and 3.72/8.04% (spongy parenchyma), thus 4.63 and 12.28
times higher in the case of Cl than at the NBG site (NBG Na
concentrations below the detection limit). In epidermis (E), it amounted
to 3.98/4.30% (upper E) and 3.60/4.97% (lower E), thus 1.21 and 8.42
times more in the case of Cl than at NBG. The mesophyll in leaf rim
samples thus showed higher NaCl accumulation than epidermis. In parallel
with the accumulation of salt, the vacuolar compartments within all
tissues showed a marked decrease of nutrients mass fractions (Fig. 5).
Especially that of K in mesophyll and Ca>K in lower
epidermis were lowered.