© 1996 Heron Publishing—Victoria, Canada
Growth, water relations and solute accumulation in osmotically stressed seedlings of the tropical tree Colophospermum mopane
J. M. Johnson (1, 2), J. Pritchard (3, 4), J. Gorham (1) and A. D. Tomos (3)
1. Centre for Arid Zone Studies, Prifysgol Cymru, Bangor, Gwynedd LL57 2UW, U.K. / 2. School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, U.K. / 3. Ysgol Gwyddorau Bioleg, Prifysgol Cymru, Bangor, Gwynedd LL57 2UW, U.K. / 4. School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K. / Received October 18, 1995
Summary
Root and hypocotyl elongation, water status and solute accumulation were studied in osmotically stressed seedlings of the
tropical tree, Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard, 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 stress of –0.03 (control), –0.2, –0.8, –1.6 or –2.0 MPa
for 60 h. Seedlings subjected to moderate water stress (–0.2 MPa) had higher root growth 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, respectively). Root lengths of seedlings in the –0.8 and –1.6 MPa treatments were similar to those of control
seedlings, whereas the –2.0 MPa seedlings had significantly shorter roots. Both root and hypocotyl tissues exhibited considerable
osmotic adjustment to the external water potential treatments. Seedlings in the –0.03, –0.2, and –0.8 MPa treatments 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 lowered
cell turgor pressure to 0.17 ± 0.04 MPa. Root vacuolar osmotic pressures were generally similar to sap osmotic pressures,
indicating that the increased root elongation observed in moderately water-stressed seedlings was not caused by increased
turgor pressure difference. Neutral-fraction solute concentrations, including the osmoticum pinitol, increased approximately
two-fold in root sap in response to a low external water potential. In hypocotyl sap of seedlings in the –2.0 MPa treatment,
pinitol more than doubled, sucrose increased from about 2 to 75 mol m–3 but glucose and fructose remained unchanged and, as a result, total sugars increased only slightly. The benefits of rapid
early root elongation and osmoticum accumulation under conditions of water stress are discussed in relation to seedling establishment.
Keywords:
osmotic stress, pinitol, root elongation, turgor pressure, solute accumulation, Zimbabwe.
