Glucose-6-phosphate Dehydrogenase Activity under Conditions of Water Limitation: a Possible Model System for Enzyme Reactions in Unimbibed Resting Seeds and its Relevance to Seed Viability

A model system has been used to measure glucose-6-phosphate dehydrogenase (G-6-PDH) activity when the water contents of the reactants are comparable to the water contents of dry resting seeds. The activity of G-6-PDH is reduced by 102−103 when the water content is limited to between 1.5 and 25 per cent. G-6-PDH activity is affected by temperature and by the protein content of the model system. The glucose 6-phosphate (7.03 nmol g−1 embryo) and the NADP+ (25.0 nmol g−1 embryo) contents of barley embryos were measured. Using these measurements, together with the measurements in the model system of G-6-PDH activity at low water concentrations, an estimate is made of the G-6-PDH activity in resting barley embryo. A cor-relation between estimated G-6-PDH activity at different water contents and the periods for which seeds remain viable is indicated. The limitations of the model system are discussed.