Abstract
The transition from seed dormancy to germination is triggered by environmental factors, and in pomegranate (Punica granatum) seeds higher germination percentages are achieved by warm + cold stratification rather than by cold stratification alone. Our objective was to define the pattern of internal oxidative changes in pomegranate seeds as dormancy was being broken by warm + cold stratification and by cold stratification alone. Embryos isolated from seeds after 1–42 days of warm stratification, after 56 days of warm stratification + 7, 28 or 56 days of cold stratification, and after 1–84 days of cold stratification alone, were used in biochemical tests. Hydrogen peroxide (H2O2), nitric oxide (NO), proline, lipid peroxidation, protein carbonylation, and activities of the scavenging enzymes superoxide dismutase (SOD), hydrogen peroxide enzyme and peroxidase in the embryos were assessed by colorimetric methods. Our results indicated that warm + cold stratification had a stronger dormancy-breaking effect than cold stratification (85% versus 50% germination), which may be attributed to a higher yield of H2O2, NO, lipid peroxidation and protein carbonylation in warm + cold stratification. Furthermore, warm + cold stratification-induced H2O2 change led to greater changes (elevation followed by attenuation) in activities of the scavenging enzymes than that induced by cold stratification alone. These results indicated that restriction of the level of reactive oxygen species change within a positive and safe range by such enzymes promoted seed germination. In addition, a relatively strong elevation of proline during warm + cold stratification also contributed to dormancy breakage and subsequent germination. In conclusion, the strong dormancy alleviating effect of warm + cold stratification on pomegranate seeds may be attributed to the corresponding active oxidative change via H2O2, NO, proline, malondialdehyde, protein carbonylation and scavenging enzymes.
Highlights
Germination sensu stricto includes all the events from the start of water uptake by dry quiescent seeds to elongation of the embryonic axis and protrusion of the radicle (Bewley 1997)
Two-way ANOVAs showed that germination, nitric oxide (NO) level, concentration of proline, MDA and protein carbonyl groups, activity of CAT and activity of POD were significantly affected by stratification treatment and stratification period
Two-way ANOVA indicated that activity of superoxide dismutase (SOD) and H2O2 level were significantly affected by stratification period instead of stratification treatment
Summary
Germination sensu stricto includes all the events from the start of water uptake (imbibition) by dry quiescent seeds to elongation of the embryonic axis and protrusion of the radicle (Bewley 1997). Non-deep physiological dormancy can be broken by: (i) dry storage of seeds (after-ripening) under ambient laboratory conditions, (ii) warm (moist at !15 C) or cold (moist at 0–10 C) stratification or (iii) warm stratification followed by cold stratification (Bewley and Black 1994; Bewley 1997; Koornneef et al 2002; Donohue et al 2005; Finch-Savage and Leubner-Metzger 2006; Baskin and Baskin 2014)
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