Time course study of aluminum-induced callose formation in barley roots asobserved by digital microscopy and low-vacuum scanning electron microscopy

To clarify the mechanism(s) involved in the short-term inhibition of root e longation by Al, we monitored the morphological changes of barley roots by digital microscopy. Within 30 min after exposure to 37 mu M Al, the surface of the root epidermis in the region of a distance of 1.5 mm from the root tip became rough and began to show signs of damage. After 38 min, callose w as rapidly excreted from the junction between the root cap and the root epi dermis, and formed a spherical lump approximately 60 mu m in diameter. The fine structure of the callose deposits on the root surface was analyzed by low-vacuum scanning electron microscopy. After 50 min, there was a signific ant increase in the callose contents in the distal 0.6 mm part. At the same time, root elongation stopped completely. Fluorescence staining indicated that callose was localized on the surface of the cell elongation area (the elongation zone of primary roots and root hairs), but not on the surface of the meristem. The root growth reduction associated with Al treatment may b e due to the use of sugar substrates for callose formation instead of cellu lose formation.