Water Relations of Growing Maize Coleoptiles : Comparison Between Mannitol and Polyethylene Glycol 6000 As External Osmotica for Adjusting Turgor Pressure

Water relations of growing segments of maize (Zea mays L.) coleoptiles were investigated with osmotic methods using either mannitol (MAN) or polyethylene glycol 6000 (PEG) as external osmotica. Segments were incubated in MAN or PEG solutions at 0 to - 15 bar water potential (Psi(o)) and the effects were compared on elongation growth, osmotic shrinkage, cell sap osmolality (OC), and osmotic pressure (pi(i)). The nonpenetrating osmoticum PEG affects pi(i) in agreement with Boyle-Mariotte's law, i.e. the segments behave in principle as ideal osmometers. There is no osmotic adjustment in the Psi(o) range permitting growth (0 to -5 bar) nor in the Psi(o) range inducing osmotic shrinkage (-5 to -10 bar). Promoting growth by auxin (IAA) has no effect on the osmotic behavior of the tissue toward PEG. In contrast to PEG, MAN produces an apparent increase in pi(i) accompanied by anomalous effects on segment elongation and shrinkage leading to a lower value for Psi(o) which establishes a growth rate of zero and to an apparent recovery from osmotic shrinkage after 2 hours of incubation. These effects can be quantitatively attributed to uptake of MAN into the tissue. MAN is taken up into the apoplastic space and the symplast as revealed by a large temperature-dependent component of MAN uptake. It is concluded that MAN, in contrast to PEG, is unsuitable as an extemal osmoticum for the quantitative determination of water relations of growing maize coleoptiles.