Characterization of molecular mobility in seed tissues: An electron paramagnetic resonance spin probe study

The relationship between molecular mobility (tau(R)) of the polar spin probe 3-carboxy-proxyl and water content and temperature was established in pea axes by electron paramagnetic resonance (EPR) and saturation transfer EPR, At room temperature, tau(R) increased during drying from 10(-11) s at 2.0 g water/g dry weight to 10(-4) s in the dry state. At water contents below 0.07 g water/g dry weight, tau(R) remained constant upon further drying. At the glass transition temperature, tau(R) was constant at similar to 10(-4) s for all water contents studied. Above T-g, isomobility lines were found that were approximately parallel to the T-g curve, The temperature dependence of tau(R) at all water contents studied followed Arrhenius behavior, with a break at T-g. Above T-g the activation energy for rotational motion was similar to 25 kJ/mol compared to 10 kJ/mol below T-g. The temperature dependence of tau(R) could also be described by the WLF equation, using constants deviating considerably from the universal constants. The temperature effect on tau(R) above T-g was much smaller in pea axes, as found previously for sugar and polymer glasses. Thus, although glasses are present in seeds, the melting of the glass by raising the temperature will cause only a moderate increase in molecular mobility in the cytoplasm as compared to a huge increase in amorphous sugars.