GROWTH, WATER RELATIONS AND SOLUTE ACCUMULATION IN OSMOTICALLY STRESSED SEEDLINGS OF THE TROPICAL TREE COLOPHOSPERMUM MOPANE

Root and hypocotyl elongation, water status and solute accumulation we re studied in osmotically stressed seedlings of the tropical tree, Col ophospermum mopane (Kirk ex Benth.) Kirk ex J. Leonard, which grows in hot arid areas of southern and central Africa. Seeds were imbibed for 24 h and then subjected to a polyethylene-glycol-generated osmotic st ress of -0.03 (control), -0.2, -0.8, -1.6 or -2.0 MPa for 60 h. Seedli ngs subjected to moderate water stress (-0.2 MPa) had higher root grow th rates (2.41 +/- 0.24 mm h(-1)), greater final root lengths (111 +/- 3.8 mm) and longer cells immediately behind the root elongation zone than control seedlings (1.70 +/- 0.15 mm h(-1) and 93 +/- 3.9 mm, resp ectively). Root lengths of seedlings in the -0.8 and -1.6 MPa treatmen ts were similar to those of control seedlings, whereas the -2.0 Mpa se edlings had significantly shorter roots. Both root and hypocotyl tissu es exhibited considerable osmotic adjustment to the external water pot ential treatments. Seedlings in the -0.03, -0.2, and -0.8 MPa treatmen ts had similar cell turgor pressures (0.69 +/- 0.10, 0.68 +/- 0.07 and 0.57 +/- 0.04 MPa, respectively), whereas the -2.0 MPa treatment lowe red cell turgor pressure to 0.17 +/- 0.04 MPa. Root vacuolar osmotic p ressures were generally similar to sap osmotic pressures, indicating t hat the increased root elongation observed in moderately water-stresse d seedlings was not caused by increased turgor pressure difference. Ne utral-fraction solute concentrations, including the osmoticum pinitol, increased approximately two-fold in root sap in response to a low ext ernal water potential. In hypocotyl sap of seedlings in the -2.0 MPa t reatment, pinitol more than doubled, sucrose increased from about 2 to 75 mol m(-3) but glucose and fructose remained unchanged and, as a re sult, total sugars increased only slightly. The benefits of rapid earl y root elongation and osmoticum accumulation under conditions of water stress are discussed in relation to seedling establishment.