Free radical processes and loss of seed viability during desiccation in the recalcitrant species Quercus robur L.

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Abstract
Loss of moisture in mature seeds of Quercus robur L. was associated with loss of viability, a rise in lipid peroxidation and build-up of free radicals. Radical-initiated damage was largely confined to the embryonic axes as their moisture contents declined to below 47 %. The accumulation of a stable free radical in axial tissue, detected by electron para-magnetic resonance (EPR), was indistinguishable from the EPR response previously shown in a moss on droughting and maize roots on desiccation. A minor higher-field component appeared to represent an intermediate stage in the sequence of free radical reactions associated with loss of water. Using seeds from freshly abscised fruits dried to different moisture contents, protective mechanisms against activated forms of oxygen were monitored in cotyledons and in embryonic axes. The two tissues exhibit distinctly different molecular defences against oxidative attack; that in the cotyledons being predominantly enzymatic, with relatively high and increasing activities of superoxide dismutase and glutathione reductase; that in the axes being largely through the anti-oxidants, ascorbic acid and alpha-tocopherol. We conclude that a decrease in enzymic protection against oxidative attack in the axes, associated with diminishing concentrations of alpha-tocopherol (and depletion of the precursor gamma-tocopherol) as moisture was lost, was directly linked with lipid peroxidation and free radical formation in the axes and that these events taken together may contribute to loss of viability in these recalcitrant seeds.