An electron paramagnetic resonance spin-probe study of membrane-permeability changes with seed aging

We developed an electron paramagnetic resonance spin-probe technique to study changes in the barrier properties of plasma membranes in wheat (Triticum aestivum L.) seeds during aging under dry storage. The estimation of these barrier properties was based on the differential permeability of membranes for the stable free radical 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy and the broadening agent ferricyanide. The line-height ratio between the water and lipid components in the electron paramagnetic resonance spectra of 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (R value) allowed for the quantitative assessment of the plasma membrane permeability in small samples, enabling separate studies of the axis, scutellum, aleurone layer, and starchy endosperm tissue. High R values corresponded to low permeability and vice versa. Starchy endosperm cells had completely permeable plasma membranes even in mature, viable seeds. The loss of germinability with aging coincided with a considerably increased plasma membrane permeability of the embryo axis cells, but not of the scutellum and aleurone layer cells. The threshold R value for the individual axes associated with viability loss was established at 5 to 6, with the total ranging from 0 to more than 12. We suggest that the R value of an individual axis is the result of contributions from all individual cells, each of them characterized by a different permeability. The loss of viability, therefore, corresponds to the accumulation of cells having permeability above a critical level.