Demonstration of respiration-dependent water uptake and turgor generation in Vigna hypocotyl

Respiration-dependent water uptake and turgor change were observed by the xylem perfusion technique. Immediate and reversible shrinkage with anoxia were repeatedly demonstrated under appropriate osmotic stress in elongating cow pea hypocotyl segments. Such shrinkage and re-elongation were always preceded by reversible inhibition and re-activation of the electrogenic xylem pump, respectively. In mature zone segments where cell wall extensibility had been shown to be practically null by means of the turgor jump method, anoxia and reaeration caused elastic shrinkage and expansion, respectively. The extent of respiration-dependent turgor was calculated from the amplitudes of the elastic volume change induced by pressure jump and anoxia. In such segments, the direction of water flow across the xylem-symplast interface should be determined solely by the cell wall elasticity and the change in apoplasmic concentration of osmotica controlled by the xylem pump activity, irrespective of any change in water conductivity or cell wall extensibility.