Osmotic Stability of Mannitol and Polyethylene Glycol 20,000 Solutions Used as Seed Germination Media1

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Abstract
Solutions of mannitol and high molecular weight polyethylene glycols (PEG) of different osmotic potentials are often used to simulate water stress in seed germination studies. However, little has been reported about the osmotic stability of these solutions. The objectives of this study were to determine the osmotic stability of mannitol and PEG 20,000 solutions, to examine the biological significance of any observed instabilities, and to compare osmotic vs. matric water potential effects on germination of winter wheat (Triticum aestivum L., ‘Nugaines’). Four solutions of mannitol, PEG, and KC1 (standard) with water potentials ranging from −3.5 to −18.0 bars were incubated at 10, 20, and 30 C and water potentials were measured periodically with thermocouple psychrometry. In addition, comparisons were made of the percent and rate of germination of wheat seeds placed in 28‐day‐old and freshly prepared solutions of mannitol and PEG and the percent and rate of seedling emergence from a watered soil media. The osmotic potential of different mannitol solutions and of the −9.1‐ and −17.4‐bar PEG solutions did not change with time. However, the osmotic potential of the −4.0‐ and −6.4‐bar PEG solutions decreased 1.0 bar. Percent and rate of germination of wheat was the same in the 28‐day‐old and in freshly prepared mannitol and PEG solutions; but at equal potentials, the germination rate was most rapid in the mannitol solutions. Wheat emergence rate from watered soil was linearly related to the germination rate in PEG, but not in mannitol solutions.Hence the slight instability of PEG solutions appears to be of no biological consequence in seed germination studies, and wheat seed emergence rates at different soil matric potentials can be approximated from germination rates in PEG osmotic solutions.