Abstract
The growing cells of hydroponic maize roots expand at constant turgor pressure (0.48 MPa) both when grown in low- (0.5 mol m−3 CaCl2) or full-nutrient (Hoagland’s) solution and also when seedlings are stressed osmotically (0.96 MPa mannitol). Cell osmotic pressure decreases by 0.1–0.2 MPa during expansion. Despite this, total solute influx largely matches the continuously-varying volume expansion-rate of each cell. K+ in the non-osmotically stressed roots is a significant exception — its concentration dropping by 50% regardless of the presence or absence of K+ in the nutrient medium. This corresponds to the drop in osmotic pressure. Nitrate appears to replace Cl− in the Hoagland-grown cells.
Analogous insensitivity of solute gradients to external solutes is observed in the radial distribution of water and solutes in the cortex 12 mm from the tip. Uniform turgor and osmotic pressures are accompanied by opposite gradients of K+ and Cl−, outwards, and hexoses and amino acids, inwards, for plants grown in either 0.5 mol m−3 CaCl2 or Hoagland’s solution (with negligible Cl−). K+ and Cl− levels within both gradients were slightly higher when the ions were available in the medium. The gradients themselves are independent of the direction of solute supply. In CaCl2 solution all other nutrients must come from the stele, in Hoagland’s solution inorganic solutes are available in the medium.
24 h after osmotic stress, turgor pressure is recovered at all points in each gradient by osmotic adjustment using organic solutes. Remarkably, K+ and Cl− levels hardly change, despite their ready availability. Hexoses are responsible for some 50% of the adjustment with mannitol for a further 30%. Some 20% of the final osmotic pressure remains to be accounted for. Proline and sucrose are not significantly involved. Under all conditions a standing water potential step of 0.2 MPa between the rhizodermis and its hydroponic medium was found. We suggest that this is due to solute leakage.
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Abbreviations
- EDX:
-
energy dispersive X-ray microanalysis
- Ψ:
-
water potential
- πi :
-
cell osmotic pressure
- P:
-
turgor pressure
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Pritchard, J., Fricke, W., Tomos, D. (1997). Turgor-regulation during extension growth and osmotic stress of maize roots. An example of single-cell mapping. In: Anderson, H.M., Barlow, P.W., Clarkson, D.T., Jackson, M.B., Shewry, P.R. (eds) Plant Roots - From Cells to Systems. Developments in Plant and Soil Sciences, vol 73. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-5696-7_2
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DOI: https://doi.org/10.1007/978-94-011-5696-7_2
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