Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

The positive xylem pressure (P(x)) in cucumber hypocotyls is a direct extension of root pressure and therefore depends on the root environment. Solutions of the electrolyte KCI (0.10 osm) reduced the hypocotyl P(x) transiently (bipbasic response), while the P(x) reduction by mannitol solutions was sustained. The amplitudes of the induced P(x) reduction depended directly, and the degree of P(x) restoration after stress release depended indirectly, on the size of the initial positive P(x), indicating that mannitol released the root pressure by a mechanical rather than osmotic mechanism. Mannitol treatment and other means of root pressure reduction revealed two separate growth responses in the affected cucumber hypocotyls. Only steep P(x) drops (following root excision or root pressure release in mannitol) directly cause a rapid, transient drop in growth rate (GR). Both rapid and slow (after root incubation in KCN or NEM) decreases in root pressure, however, led to a sustained growth inhibition of cucumber hypocotyls after about 30 min. This delay characterizes the growth response as an indirect consequence of the P(x) change. Pea seedlings, which lacked root pressure and had a negative P(x) throughout, showed extremely small changes in epicotyl P(x) and GR after root incubation in mannitol. It is apparent that the higher sensitivity of cucumber growth to mannitol depended on the presence and release of root pressure.