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 e xtension of root pressure and therefore depends on the root environmen t. Solutions of the electrolyte KCl (0.10 osm) reduced the hypocotyl P -x transiently (biphasic response), while the P-x reduction by mannito l solutions was sustained. The amplitudes of the induced P-x reduction depended directly, and the degree of P-x restoration after stress rel ease depended indirectly, on the size of the initial positive P-x indi cating that mannitol released the root pressure by a mechanical rather than osmotic mechanism. Mannitol treatment and other means of root pr essure reduction revealed two separate growth responses in the affecte d cucumber hypocotyls. Only steep P-x drops (following root excision o r 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 del ay characterizes the growth response as an indirect consequence of the P-x change. Pea seedlings, which lacked root pressure and had a negat ive 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 an d release of root pressure.