| Pomegranate is an important economic crop cultivated in many countries. In recent years, the pomegranate industry has developed rapidly in Xinjiang. However, poor quality and low yield of cultivars are widespread problems that greatly restrict the development of the pomegranate industry. Therefore, elucidation of the dormancy mechanism and germination characteristics of pomegranate seeds would contribute to improving old pomegranate cultivars and developing new ones. In addition, it would also provide a theoretical foundation of seed dormancy and germination characteristics for pomegranate.The primary purpose of the present study was to understand the morphology, dormancy mechanism and germination characteristics of four pomegranate cultivars from Xinjiang Province, China, namely Kashgar Akeqishiliu (I), Yecheng Suanshiliu (II), Hotan CeLel#shiliu (III), and Turpan Suanshiliu (IV), and then to investigate the pattern of internal oxidative changes in pomegranate seeds as dormancy was being broken by warm+cold stratification and by cold stratification alone. The main results are as follows:(1) The color and shape of dry seeds of the four pomegranate cultivars are quite similar; they are milky white and irregular in shape. Length, width and mass of seeds did not differ significantly among I, II, and III, but those of IV differed significantly in all three measures from the other three cultivars. The seed of pomegranate is composed of a seed coat and an embryo. Cotyledons of the embryo in dormancy seeds were tightly closed compared with those during the germination period. Development of the cotyledons of the germinated embryo was normal when treated with distilled water, however, the cotyledons exhibited apparent bending after they were incubated in exogenous H2O2 and NO, indicating a significant influence on development of cotyledons by H2O2 and NO.(2) The kind (class) of dormancy in pomegranate seeds was determined via comparison of the effect of various treatments on germination. Fresh pomegranate seeds incubated on moist filter paper imbibed water, but they germinated to only 16% to 20%. Different treatments, including sulfuric acid scarification, cold stratification and warm followed by cold stratification, significantly increased germination percentages. Among those treatments, seeds soaked in concentrated H2SO4 for 40 minutes followed by cold stratification for 2 months reached 65% germination, and those warm stratified for 1-3 months followed by cold stratification for 2 months germinated to approximately 75% to 80%. An imbibition test showed that pomegranate seeds have a water-permeable seed coat:therefore, it can be concluded that they do not have physical dormancy or the combination of physical and physiological dormancy. Further, the embryo is fully developed, and thus they do not have morphological or morphophysiological dormancy. Since acid scarification (weakens mechanical resistance of seed coat) and GA3 (increases embryo growth potential) promoted germination, it can be concluded that pomegranate seeds have nondeep physiological dormancy.(3) The internal changing oxidative patterns induced by warm+cold stratification and by cold stratification were elucidated in pomegranate seeds. Biochemical tests were carried out on 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. H2O2, NO, proline, lipid peroxidation, protein carbonylation and activities of the scavenging enzymes SOD, CAT and POD in the embryos were assessed by colorimetric methods. 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.(4) Warm+cold stratification-induced H2O2 change led to greater changes (elevation followed by attenuation) in activities of the scavenging enzymes than those induced by cold stratification alone. These results indicated that restriction of the level of ROS change in ROS 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.(5) 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, MDA, protein carbonylation and scavenging enzymes. |
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