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The effects of site of origin, time of seed maturation, and seed age on germination behavior, in terms of final percentage of germination and time for 50% of final germination were investigated in the widespread weed Portulaca oleracea L. Seeds were collected in August from one population at each of three geographical regions of the Old and New Worlds representing temperate (Southern Ontario, Canada), Mediterranean (Tanta, Egypt), and subtropical (Al-Ain, United Arab Emirates (U.A.E.)) climates. The freshly harvested seeds were incubated in both light and dark at 15, 25, and 40°C. Seeds of the U.A.E. site showed little dormancy and germinated faster under a wider range of incubation conditions than did seeds from the Canadian site. To assess the effect of time of seed maturation on germination behavior, seeds were collected during the different seasons (November, February, May, and August) of the U.A.E. population. The percentage of germination was significantly higher for seeds collected in May and November and germination was significantly faster for those collected in May and August. Response of germination to light and temperature varied according to site of origin and time of seed maturation. Seeds from the Canadian site and those that matured during winter from the U.A.E. site required light and high temperature for optimal germination. The effect of seed age on percentage of germination and rate was significant at the Canadian site but not at the Egyptian and U.A.E. sites. Seed stored for 5 months at room temperature germinated significantly faster and to a higher percentage than freshly harvested seeds. Results are discussed in light of the adaptation of seed dormancy and time of germination to the prevailing environmental conditions.Key words: dormancy, geographical regions, germination, germination rate, Portulaca oleracea, seed age, site of origin, time of seed maturation.