The objectives of the present work were to investigate whether loss of sunflower (Helianthus annuus L.) seed viability was affected by the embryo moisture content (MC) during seed pretreatment at 35°C, and was related to changes in energy metabolism and in the antioxidant defence system. Non-dormant seeds were equilibrated at MC of the embryonic axis ranging from 0.037 to 0.605 g H 2 O g 21 dry matter (DM) for 1 day at 15°C, and they were then placed at 35°C for various durations up to 14 days before the germination assays at 15°C. As expected, the higher the MC, the faster was seed deterioration. There existed a negative linear relationship between the time taken for germination to drop to 50% (P 50 ) and the embryonic axis MC ranging from 0.108 and 0.438 g H 2 O g 21 DM. In dry seeds, adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate represented 6.3, 14.8 and 70.9% of the adenylate pool, respectively, and the energy charge (EC) was very low (0.14). ATPand ADP levels and EC increased sharply during the first day of equilibrium of seeds at a MC above 0.158 g H 2 O g 21 DM. Subsequent controlled deterioration at 35°C resulted in a decrease in the adenylate pool, and consequently in ATP level. The higher the energy metabolism during ageing, the lower was seed viability. Loss of seed viability was associated with an accumulation of H 2 O 2 , and then of malondialdehyde (MDA) suggesting that lipid peroxidation was not the only cause of seed deterioration. When there was a sublinear relationship between H 2 O 2 content in the embryonic axis and seed viability, MDA accumulation only occurred when 50% of the seed population died within 7 days, i.e. when MC was higher than 0.248 g H 2 O g 21 DM. Ageing was associated with a decrease in the activity of superoxide dismutase, catalase and glutathione reductase, the main enzymes involved in cell detoxification. The involvement of seed MC, as key factor of ageing is discussed with regards to energy metabolism and the regulation of active oxygen species accumulation.Abbreviations -ADP, adenosine diphosphate; AMP, adenosine monophosphate; AOS, active oxygen species; ATP, adenosine triphosphate; CAT, catalase; EC, energy charge; FW, fresh weight; GR, glutathione reductase; MC, moisture content; MDA, malondialdehyde; PCD, programmed cell death; SD, standard deviation; SOD, superoxide dismutase. IntroductionSensitivity of seeds to high temperatures is strongly dependent on their water content, loss of viability being faster with increasing moisture content (MC) (McDonald 1999, Priestley 1986, Roberts and Ellis 1989. The rate of loss of viability in orthodox seeds (i.e. seeds which are desiccation tolerant) is a positive function of water content. However, the application of the viability equation originally developed for barley Roberts 1980a, 1980b) has a lower limit, which coincides with a critical MC between 2 and 6% fresh weight (FW) basis, and an upper one for water content ranging from 15 to 28% FW, depending on whether the seeds are o...

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