SUMMARYLoss 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 °o-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 giutathione reductase; that in the axes being largely through the antioxidants, 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.

SUMMARYOn germination, maize seeds rapidly become intolerant of desiccation. Germinating seeds exposed to dehydrating conditions for 24 h show a significant rise in lipid peroxidation and the accumulation of an electron paramagnetic resonance-detectable organic radical, in parallel with the development of desiccation intolerance. Throughout the 5-d-long germination phase and the period of loss of desiccation tolerance, the radical scavengers a-and ytocopherol and glutathione accumulate in response to episodes of dehydration. However dehydration suppresses, or delays, increases in activity of superoxide dismutase, peroxidase and glutathione reductase, major components in the protection against activated forms of oxygen. We present evidence of a sequence of events in which activated oxygen plays an early and probably causative role in loss of tolerance of desiccation in these seeds.

~~ ~ ~~~ ~~~~~~~When germinating Zea mays 1. seeds are rapidly desiccated, free radical-mediated lipid peroxidation and phospholipid deesterification is accompanied by a desiccation-induced buildup of a stable free radical associated with rapid loss of desiccation tolerance. Comparison of the eledron paramagnetic resonance and eledron nuclear double resonance properties of this radical with those of the radical in dried, desiccation-intolerant moss showed that the two were identical. At the subcellular level, the radical was associated with the hydrophilic fraction resulting from lipid extraction. lsolated mitochondria subjected to drying were also found to accumulate an identical radical in vitro. When increasing concentrations of cyanide were used, a significant positive correlation was shown between rates of respiration and the accumulation of the radical in desiccation-intolerant tissues. Another positive correlation was found when rates of O2 uptake by radicles at different stages of germination were plotted against free radical content following desiccation. This indicates that free radical production is closely linked to respiration in a process likely to involve the desiccation-induced impairment of the mitochondrial eledron transport chain to form thermodynamically favorable conditions to induce accumulation of a stable free radical and peroxidized lipids. Modulation of respiration using a range of inhibitors resulted in broadly similar modulation of the buildup of the stable free radical. One site of radical generation was likely to be the NADH dehydrogenase of complex I and probably as a dired consequence of desiccation-impaired electron flow at or close to the ubiquinone pool.Most vascular plants can withstand little desiccation except during the later stages of seed development and the early

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Lysed chloroplasts prepared from droughted wheat plants generate O2- on illumination as detected by electron spin resonance spectroscopy, the amplitude of the signal increasing with the severity of water deprivation. Following a similar time-course as radical formation and chlorophyll destruction, there was a significant increase in the accumulation of iron in the droughted shoots to reach an estimated concentration in the cell sap of about 2.5 mM. The evidence suggests that superoxide generated as a result of impaired electron transport in the chloroplasts reacts with the high concentration of accumulating iron resulting in the formation of hydroxyl radical, the probable cause of the primary pathologies observed in droughted plants.