OSMOTIC POTENTIAL PROPERTIES OF SOLUTES COMMON IN THE SOIL-PLANT SOLUTION CONTINUUM.

In this article, we report on the osmotic potential properties of NaCl, KCl, NH4Cl, CaCl2, NaNO3, and MgSO4, common to soil solutions, and sucrose, d-fructose, and d-mannitol, found in plant solutions. The osmotic potentials of the solute solutions were calculated as the sum of three components: that due to the spacing difference of the free solution water molecules in the theoretical absence of solutes as compared with pure water, the loss of potential of the free solution water to keep solutes in solution, and its loss of potential to keep water firmly held to the solute particles in solution. These factors varied significantly between solutes. At the wilting level of plants, the spacing effect on the free solution water of the plant solution organic solutes ranged from 61 to 82% of their total osmotic potentials. This was only 41 to 49% for the soil solution inorganic solutes, due mainly to the loss of potential to keep water firmly held to ions in solution. The calculations indicate that the water-holding characteristics of soil solution ions would influence absorption by plants. The lesser amounts of water held to the organic solutes suggests that solution flow in the sieve tubes of the phloem is largely a function of the change in the spacing of the free water molecules of the solution. The analysis provides a new insight to solution flow through the soil-plant continuum.