Gravity-regulated growth of collenchymatous bundle cap cells in the leaf sheath base of the grass Echinochloa colonum

Abstract

Horizontally held flowering stalks of Echinochloa colonum L. bend upwards as a result of cell elongation in the lower side of the leaf sheath base. Changes in the gravity vector are perceived by statocytes. All cells of the leaf sheath base, including the collenchymatous bundle cap fibres can elongate up to threefold in response to geostimulation. Very little growth takes place in upright nodes and in the upper halves of nodes during bending. The thick walls of the collenchymatous cells are uniformly rich in pectin and remain unlignified. Nonelongating collenchymatous cells accumulate electron-dense wall material in infoldings of the plasmalemma. Elongating cells become thin walled, unincorporated wall material disappears, and a narrow, electron-dense layer of wall forms. The walls thin evenly, although under polarized light the structure of the pits may give an appearance of uneven stretching. When the gravitational stimulus is removed, growth ceases, the walls rethicken, and the electron-dense layer disappears.