Spatial Distribution of Turgor and Root Growth at Low Water Potentials 1

Spatial distributions of turgor and longitudinal growth were compared in primary roots of maize (Zea mays L. cv FR27 x FRMol7) growing in vermiculite at high (-0.02 megapascals) or low (-1.6 megapascals) water potential. Turgor was measured directly using a pressure probe in cells of the cortex and stele. At low water potential, turgor was greatly decreased in both tissues throughout the elongation zone. Despite this, longitudinal growth in the apical 2 millimeters was the same in the two treatments, as reported previously. These results indicate that the low water potential treatment caused large changes in cell wall yielding properties that contributed to the maintenance of root elongation. Further from the apex, longitudinal growth was inhibited at low water potential despite only slightly lower turgor than in the apical region. Therefore, the ability to adjust cell wall properties in response to low water potential may decrease with cell development. Expansive growth of plant organs is driven by protoplast turgor which deforms the cell walls irreversibly at a rate dependent on physical and metabolically regulated wall yielding properties (4, 21). To continue organ expansion at reduced i,62 therefore, the plant may compensate by adjusting cell osmotic potential to maintain turgor and/or by adjusting wall yielding properties. Roots often continue to grow at low i/w that completely inhibit shoot growth (22, 24, 27). In primary roots of maize growing at low 4,w in vermiculite, Sharp et al. (24) showed that elongation was maintained preferentially toward the apex and was unaffected in the apical 2 to 3 mm at iw as low as -1.6 MPa. Substantial osmotic adjustment did occur in this region (23) but compensated only partially for the decrease in vermiculite 46w, suggesting that turgor was greatly decreased. To account for the maintenance of elongation, therefore, it is probable that a large adjustment ofcell wall yielding properties had occurred. The decrease in turgor in the apical region at low 4,w requires confirmation in the form of direct measurements, because the above-mentioned turgor estimates did not account for several effects of unknown magnitude, including decreases in the tissue iw due to growth (2) and, possibly, large concentraSupported by the Food for the 21st Century Program, University of Missouri, Columbia. Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 11,329. 2 Abbreviations: {, water potential. tions of apoplastic solutes (4). Our objective was to use a miniature pressure probe to measure directly the longitudinal and radial distribution of turgor in the elongation zone of intact maize primary roots growing at high or low i,6 in vermiculite. The measurements confirm that turgor was greatly decreased at low t,6 throughout the elongation zone. Thus, a large adjustment of cell wall yielding properties must contribute to root growth maintenance at low 4'N. MATERIALS AND METHODS