Water Content and Cell Elongation in Protruding and Growing Roots

Abstract

Vicia faba minor seeds germinate only by cell elongation which begins at first in the hypocotyl and continues in the basal the radicle cells at a water content (WC) of 72-73% fr.w. Two physiological mechanisms preceding the initiation of cell elongation develop in imbibing axial organs. The first one (accumulation of osmotic substances) develops at a WC of 65-68%, the second one (activation of a proton pump resulting in cell-wall loosening and “acid” growth) at a WC of 68-72%. If water uptake is prevented and the WC of axial organs is maintained at 65 and 68% levels, no root protrusion occurred for 30h, although in control seeds it began after 6 and 3h respectively. It is a water uptake up to certain critical levels that triggers both the physiological mechanisms and the initiation of elongation. Water loss does not cause any irrevercell elongation. Water loss does not cause any irreversible effects since rehydrated cells resume their processes of cell elongation. Elongating axial organs are also insensitive to dessication until the beginning of mitotic activity. The elongation of the cell is of primary importance to successful root growth after root protrusion but this does not apply to cell division. Cell elongation provides more rapid growth and contact with soil water. Moreover, after water loss, elongating cells resume their growth rate but dividing cells cannot recover.