Relaxation and transport phenomena in corn seeds

The low-frequency dynamics in cereal seeds has been studied by measurements of d.c. conductivity, thermally stimulated depolarization current techniques and by broadband a.c. dielectric spectroscopy with the aim to understand its relation to their biological behavior. Such a broad overall frequency and temperature range allows to investigate simultaneously the mobility of water molecules in seeds, influence of water on molecular mobility of seeds constituents and change mobility. Our results indicate, that water in seeds freeze for concentration higher than critical hydration, whereas for lower hydration it undergoes a glass- like transition. Temperature of this transition strongly depends on water content in the sample and shifts by seeds drying from 180 K up to room temperature. D.c. protonic conductivity increases sharply above glass transition temperature. Its dependence on water content indicates percolative transport of protons along threads of hydrogen- bonded water molecules with a percolation thresholding the range of 0.15-0.30 g water/g dry material. The biological implications of these findings will be discussed.