De-greened Sp. wallisii leaves showed the blue luminescence
typical of FCCs (see Fig. 2). Green (Fig. 4A) and yellow, senescent
Sw-leaves (Fig. 4B) were thus studied by microscopic analyses. In
senescent Sw-leaves a marked blue fluorescence was observed in
mesophyll cells (palisade parenchyma as well as spongy parenchyma).
Thicker cell walls of tracheids and fibers (Fig. 4B-f) also
showed blue fluorescence (ascribed to cell wall phenolics, lignin
and coumaric acid derivatives [31–33]), as was also seen in green
leaves (Fig. 4A-c). Senescent leaves lacked the red Chl- fluorescence;
only in the guard cells of their stomata were intact chloroplasts
marked by their red fluorescence (Fig. 4B-e and h). Indeed,
stomata remain functional during (early phases of) senescence
[34]. Green Sw-leaves showed bright red Chl-fluorescence in the
whole mesophyll, which lacked the marked blue fluorescence of
the senescent cells