Acid-induced wall loosening is confined to the accelerating region of the root growing zone

The aims of this study were to quantify developmental differences in acid growth along the root axis and to determine whether these differences were due to alterations in cell turgor or cell wall properties. The apoplast pH of maize roots growing in hydroponics was altered from pH 7.0 to pH 3.4 using 2 mol m−3 citrate-phosphate buffer or unbuffered solutions. Whole root elongation rate rapidly increased and measurement of the local growth profile indicated that this increase in growth occurred in young cells in the accelerating zone (apical 0–4 mm) while more proximal growing cells were unaffected. Unbuffered solutions of identical pH produced qualitatively similar results. Single cell turgor pressures were unchanged between pH treatments both longitudinally and radially in the root tip. This suggests that the rapid acid-induced changes in growth rate were due to an increase in cell wall loosening. Single cell osmotic pressure and water potential were not significantly different between pH treatments. Acid pH caused net solute import at the root tip to increase 3- to 4-fold, which, coupled with the maintenance of turgor and osmotic pressure, indicated that solute import was not limiting expansion. Thus, acidic solutions cause an increase in growth in accelerating but not decelerating regions. It has been shown for the first time that acid growth in intact, growing roots is not due to differences in turgor, assigning these changes to cell wall properties. Possible cell wall biochemical alterations are discussed.