Come, D. 1996, Changes in malondialdehyde content and in superoxide dismutase, catalase and glutahione reductase activities in sunflower seeds as related to deterioration during accelerated aging. -Physiol. P!am.97: 104-110, Sunflower (Helianthus annuus L.) seeds progressively lost their ability to germinate at 25°C, the optimal temperature for germination, after accelerated aging was carried out at 45°C (a temperature too high to permit germination) in water or at 76 or 100% relative humidity (RH), The deleterious effects of the high-temperature treatment increased with increasing seed moisture content. Incubation of seeds at 45 °C in water resulted in electrolyte leakage, which indicated a loss of membrane integrity, A relationship between leakage and loss of seed viability could not be assumed, since no increase in electrolyte efflux occurred after aging at 100% RH. Accelerated aging induced accumulation of malondialdehyde, suggesting that seed deterioration was associated with lipid peroxidation. However, there was no direct relationship between lipid peroxidation and deterioration in membrane integrity. Loss of seed viability was also associated with a decrease in superoxide dismutase, catalase and giutathione reductase activities. Finally, the results obtained suggest that sunflower seed deterioration during accelerated aging is closely related to a decrease in the activities of detoxifying enzymes and to lipid peroxidation.

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...

Two mechanisms have been suggested as being responsible for dormancy in barley grain: (i) ABA in the embryo, and (ii) limitation of oxygen supply to the embryo by oxygen fixation as a result of the oxidation of phenolic compounds in the glumellae. The aim of the present work was to investigate whether hypoxia imposed by the glumellae interferes with ABA metabolism in the embryo, thus resulting in dormancy. In dormant and non-dormant grains incubated at 20 degrees C and in non-dormant grains incubated at 30 degrees C (i.e. when dormancy is not expressed), ABA content in the embryo decreased dramatically during the first 5 h of incubation before germination was detected. By contrast, germination of dormant grains was less than 2% within 48 h at 30 degrees C and embryo ABA content increased during the first hours of incubation and then remained 2-4 times higher than in embryos from grains in which dormancy was not expressed. Removal of the glumellae allowed germination of dormant grains at 30 degrees C and the embryos did not display the initial increase in ABA content. Incubation of de-hulled grains under 5% oxygen to mimic the effect of glumellae, restored the initial increase ABA in content and completely inhibited germination. Incubation of embryos isolated from dormant grains, in the presence of a wide range of ABA concentrations and under various oxygen tensions, revealed that hypoxia increased embryo sensitivity to ABA by 2-fold. This effect was more pronounced at 30 degrees C than at 20 degrees C. Furthermore, when embryos from dormant grains were incubated at 30 degrees C in the presence of 10 microM ABA, their endogenous ABA content remained constant after 48 h of incubation under air, while it increased dramatically in embryos incubated under hypoxia, indicating that the apparent increase in embryo ABA responsiveness induced by hypoxia was, in part, mediated by an inability of the embryo to inactivate ABA. Taken together these results suggest that hypoxia, either imposed artificially or by the glumellae, increases embryo sensitivity to ABA and interferes with ABA metabolism.

Seeds of bean (Phaseolus vulgaris cv. Vernel) were collected throughout their development on the plant and dried at 15 degrees C and 75% relative humidity to a final moisture content of about 16% (fresh weight basis) to determine whether the onset of tolerance to this drying condition was related to changes in soluble sugars or the activities of the main antioxidant enzymes, namely superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR). Measurements of soluble sugars and enzyme activities were made after drying the seeds, and drying tolerance was evaluated by the ability of dried seeds to germinate and to produce normal seedlings. Seeds became tolerant to drying at 45 d after anthesis, a time marking physiological maturity. At physiological maturity, the moisture content of seeds was about 50-55% (fresh weight basis) and seed dry matter reached about 190 mg per seed. Seed vigour, evaluated by controlled deterioration and conductivity measurements, continued to increase after seed mass maturity, but decreased when seeds remained thereafter for more than 7 d on the plant. Acquisition of drying tolerance was coincident with an accumulation of raffinose and stachyose. Dried-tolerant seeds were also characterized by a high amount of sucrose, the most abundant sugar, and by a low content of monosaccharides. The (raffinose+stachyose)/sucrose ratio increased during seed filling, reaching a value close to 1 when all the seeds became tolerant to drying, and maintaining this proportion during the final stages of maturation. Acquisition of drying tolerance was also related to a reorientation of the enzymatic antioxidant defence system. Drying-tolerant dried seeds displayed high CAT and GR activities and low SOD and APX activities, while the opposite condition was observed in immature dried seeds. The shift in antioxidant enzymes corresponded to the beginning of the maturation-drying phase. These results suggest that oligosaccharide metabolism and enzymatic antioxidant defences may be involved in acquisition of drying tolerance during bean seed development, but are not related to seed vigour.

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