Growth at reduced turgor: irreversible and reversible cell-wall extension of maize coleoptiles and its implications for the theory of cell growth.

The relationship between steady-state elongation rate (G) and turgor pressure (P; G/P curve) was investigated using isolated segments of maize (Zea mays L.) coleoptiles incubated in osmotic solutions of a water potential range of 0 to -10 bar (polyethylene glycol 6000 as osmoticum). Short-term elongation measurements revealed curvilinear G/P curves with a steep slope at high turgor and a shallow slope at low turgor. Owing to a decrease of osmotic pressure and turgor, there was a tendency for straightening of the G/P curves during long-term elongation. An elongation rate of zero was adjusted by lowering the turgor by 4.5 bar at a constant osmotic pressure of 6.7 bar. Auxin increased - whereas abscisic acid decreased - the slope of the G/P curve but these hormones had no effect on the threshold turgor of growth (Y = 2.2 bar). It is concluded that extensibility of the growing cell walls represented by the 'yielding coefficient' of Lockhart's growth equation is turgor-dependent and therefore decreases to a very low value as the turgor approaches Y. When the turgor was kept at Y, a constant segment length was maintained over at least 6 h. However, separation of reversible (lrev) and irreversible (lirr) components of total (in vivo) length (ltot = lrev + lirr) W measuring segment length before and after freezing/thawing revealed that lirr increased continuously and lrev decreased continuously at constant ltot. After a step-down in turgor the segments grew in lirr although they shrank in ltot over the whole turgor range of 0<P<Y. This phenomenon is termed 'cryptic growth'. It is concluded that Y represents the turgor at which irreversible cell extension and decrease of the reversible length fraction of the cell walls cancel each other out and therefore net extension becomes zero. Cryptic growth at P = Y is accompanied by a decrease in cell-wall elasticity. The finding that auxin promotes cryptic growth supports the notion that a wall-stiffening process is a constitutive component of the mechanism by which the hormone induces irreversible cell elongation.