Histochemical localization of nucleoside triphosphatase activity in assimilate conducting plant tissue

Summary For the histochemical localization of nucleoside triphosphatases at the electron microscopic level, prefixed tissues were incubated with lead nitrate in addition to substrate (GOMORI reaction). While ATP and UTP as substrates gave electron-dense reaction products at the plasmalemma of sieve tubes, companion cells and phloem parenchyma cells, and at plasmodesmata in primary pitfields, AMP gave reaction products only at the tonoplast of parenchyma cells. Since electron-dense deposits also occur in cell walls and vacuoles, energy dispersive X-ray microanalysis was used to distinguish between lead deposits and lead-phosphate deposits. The latter were restricted to the symplast. Among the three plant species used, the leaf bundle phloem ofHordeum distichon showed ATPase activity largely restricted to the phloem cells, except for the thickwalled sieve tubes. Some activity also bordered the chloroplasts of the bundle sheath cells. In the C₄ plantGomphrena globosa, ATPase and UTPase activities appeared to be the greater in phloem parenchyma cells than in sieve tubes. In the phloem of youngMonstera deliciosa roots, ATPase occurred not only at the plasmalemma of sieve tubes, but also around sieve-tube plastids. When compared with AMP as substrate, it appears that nucleoside triphosphates are the natural substrates of the enzyme(s) in the plasmalemma of sieve tubes and phloem parenchyma cells.