Growth of the Maize Primary Root at Low Water Potentials

Primary roots of maize (Zea mays L. cv WF9 x Mo17) seedlings growing in vermiculite at various water potentials exhibited substantial osmotic adjustment in the growing region. We have assessed quantitatively whether the osmotic adjustment was attributable to increased net solute deposition rates or to slower rates of water deposition associated with reduced volume expansion. Spatial distributions of total osmotica, soluble carbohydrates, potassium, and water were combined with published growth velocity distributions to calculate deposition rate profiles using the continuity equation. Low water potentials had no effect on the rate of total osmoticum deposition per unit length close to the apex, and caused decreased deposition rates in basal regions. However, rates of water deposition decreased more than osmoticum deposition. Consequently, osmoticum deposition rates per unit water volume were increased near the apex and osmotic potentials were lower throughout the growing region. Because the stressed roots were thinner, osmotic adjustment occurred without osmoticum accumulation per unit length. The effects of low water potential on hexose deposition were similar to those for total osmotica, and hexose made a major contribution to the osmotic adjustment in middle and basal regions. In contrast, potassium deposition decreased at low water potentials in close parallel with water deposition, and increases in potassium concentration were small. The results show that growth of the maize primary root at low water potentials involves a complex pattern of morphogenic and metabolic events. Although osmotic adjustment is largely the result of a greater inhibition of volume expansion and water deposition than solute deposition, the contrasting behavior of hexose and potassium deposition indicates that the adjustment is a highly regulated process.