Turgor regulation and expansion growth of maize roots during adjustment to water stress

Transient responses of turgor (P) and growth to water stress were studied in maize roots to evaluate mechanisms that enable plants to adjust growth under limited water supply. Using a cell pressure probe, P was measured across the cortex throughout the expansion zone (3 to 10 mm behind the root tip). Upon exposure of roots to hyperosmotic solutions (KCl or mannitol), P declined rapidly due to water loss to the medium and elongation stopped as soon as P dropped below the yield threshold of growth (Y). However, elongation resumed within minutes, and continuous P measurements during this period indicated two mechanisms that enabled the recovery of growth: a) a reduction of Y from initially 0.6 MPa to values as low as 0.3 MPa, and b) an increase of P during the period of zero growth, presumably due to solute accumulation (transport). The rate of P increase declined with increasing osmotic pressures of the medium (π_m), suggesting that solute accumulation was turgorsensitive. P recovery was confined to younger parts of the elongation zone at π_m= 0.6 MPa, which was associated with a shortening of the elongation zone from initially 10 to 7 mm. The results indicate that solute accumulation could become a rate-limiting process for growth under water stress.