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 prob e 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 dependen ce of tau(R) at all water contents studied followed Arrhenius behavior, wit h 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 depe ndence of tau(R) could also be described by the WLF equation, using constan ts deviating considerably from the universal constants. The temperature eff ect on tau(R) above T-g was much smaller in pea axes, as found previously f or sugar and polymer glasses. Thus, although glasses are present in seeds, the melting of the glass by raising the temperature will cause only a moder ate increase in molecular mobility in the cytoplasm as compared to a huge i ncrease in amorphous sugars.