Free-space marker studies on the leaves of Saccharum officinarum and Bromus unioloides

Two free-space marker procedures (Prussian blue and lead nitrate) were used on leaves of Saccharum officinarum L. and Bromus unioloides H.B.K. to determine (1) the pathway(s) followed by water and solutes in the transpiration stream after their introduction into the xylem of small and intermediate vascular bundles, and (2) the effectiveness of the suberin lamellae of the chlorenchymatous bundle-sheath (in S. officinarum) and mestome sheath (in B. unioloides) cells as a barrier to the movement of tracer ions (Fe3 + and Pb2 +). Judged from the distribution of Prussian blue crystals and lead deposits, water and tracer ions moved readily from the lumina of the vessels into the phloem apoplast via portions of vessel primary walls not bearing lignified secondary wall thickenings in both species. Prussian blue crystals and lead deposits were abundant on both sides of the suberin lamella of the chlorenchymatous bundle-sheath/mesophyll interface in S. officinarum and the mestome sheath/parenchymatous bundle sheath interface in B. unioloides. Prussian blue crystals and lead deposits were lacking entirely, however, in the suberin lamellae, indicating that the lamellae were fairly Impermeable to the tracer ions. The presence of Prussian blue crystals and lead deposits in the compound middle lamellae of the radial walls of the chlorenchymatous bundle-sheath cells of S. officinarum and the mestome sheath cells of B. unioloides indicates that the compound middle lamella provides an apoplastic pathway for transpirational water and solutes from small and intermediate bundles to the contiguous tissues in both species