Viability loss and free radical processes during desiccation of recalcitrant Avicennia marina seeds

Abstract

At shedding, the moisture content (MC) of Avicennia marina (Forssk.) Vierh. propagules was 65% (fresh mass basis), and there was no significant difference in the MC of four tissues (hypocotyl, cotyledons, plumule and root primordia). Viability declined as the propagules were dried below 60% MC, so that only 40%of seeds were capable of germination at 54% MC. At 47% MC all the seeds had died. The four tissues dried at the same constant rate of 0.02 g water g dwt-1 h–1 throughout this range of MCs. There was no significant depletion of the free-radical-quenching mechanisms measured. In each tissue an organic free-radical was detected by electron paramagnetic resonance (EPR). In the plumule the amplitude of the signal increased by a further 50% at MCs where viability was lost, but there was no increase in the other tissues. There was a concurrent increase in the amount of tocopherol and the activity of superoxide dismutase in the plumule. Lipid peroxidation, assessed by the amount of thiobarbituric acid-reactive substances, also increased in advance of viability loss, suggesting that propagules were experiencing oxidative stress. However, lipid peroxidation decreased at 54–57% MC, where most seeds lost viability. The results presented are consistent with a propagule reacting to oxidative stress, but overtaken by more catastrophic physical damage.