Growth of the Maize Primary Root at Low Water Potentials. III. Role of Increased Proline Deposition in Osmotic Adjustment

Seedlings of maize (Zea mays L. cv WF9 × Mo17) growing at low water potentials in vermiculite contained greatly increased proline concentrations in the primary root growth zone. Proline levels were particularly high toward the apex, where elongation rates have been shown to be completely maintained over a wide range of water potentials. Proline concentration increased even in quite mild treatments and reached 120 millimolal in the apical millimeter of roots growing at a water potential of -1.6 megapascal. This accounted for almost half of the osmotic adjustment in this region. Increases in concentration of other amino acids and glycinebetaine were comparatively small. We have assessed the relative contributions of increased rates of proline deposition and decreased tissue volume expansion to the increases in proline concentration. Proline content profiles were combined with published growth velocity distributions to calculate net proline deposition rate profiles using the continuity equation. At low water potential, proline deposition per unit length increased by up to 10-fold in the apical region of the growth zone compared to roots at high water potential. This response accounted for most of the increase in proline concentration in this region. The results suggest that osmotic adjustment due to increased proline deposition plays an important role in the maintenance of root elongation at low water potentials.