AN ELECTRON-PARAMAGNETIC-RESONANCE SPIN-PROBE STUDY OF MEMBRANE-PERMEABILITY CHANGES WITH SEED AGING

We developed an electron paramagnetic resonance spin-probe technique t o study changes in the barrier properties of plasma membranes in wheat (Triticum aestivum L.) seeds during aging under dry storage. The esti mation of these barrier properties was based on the differential perme ability of membranes for the stable free radical 4-oxo-2,2,6,6-tetrame thyl-1-piperidinyloxy and the broadening agent ferricyanide. The line- height ratio between the water and lipid components in the electron pa ramagnetic resonance spectra of 4-oxo-2,2,6,6-tetramethyl-1-piperidiny loxy (R value) allowed for the quantitative assessment of the plasma m embrane permeability in small samples, enabling separate studies of th e axis, scutellum, aleurone layer, and starchy endosperm tissue. High R values corresponded to low permeability and vice versa. Starchy endo sperm cells had completely permeable plasma membranes even in mature, viable seeds. The loss of germinability with aging coincided with a co nsiderably increased plasma membrane permeability of the embryo axis c ells, but not of the scutellum and aleurone layer cells. The threshold R value for the individual axes associated with viability loss was es tablished 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 contr ibutions from all individual cells, each of them characterized by a di fferent permeability. The loss of viability, therefore, corresponds to the accumulation of cells having permeability above a critical level.