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.