Osmopriming is a pre-soaking treatment that improves seed germination performance as well as stress tolerance. In this study, we use spinach (Spinacia oleracea cv. Bloomsdale) as the model to study cellular mechanisms contributing to the improved stress tolerance during post-priming germination. We first determined an optimal osmopriming protocol for `Bloomsdale' spinach, i.e. priming seeds with -0.6 MPa PEG 8000 at 15 yC for 8 d. This protocol also improved germinating seeds' tolerance to temperature (sub- and supra-temperature) and desiccation stress. To explore the biochemistry contributing to the priming-induced stress tolerance, we examined two extensively studied stress-responsive components: antioxidant system and dehydrins. An update of `antioxidant system' was found during osmopriming and early germination stage, as manifested in the repression of antioxidants involved in seed protection during dry storage, and enhancement of those related to seed germinability. Possibly, this system update was resulted from the transition of seeds from dry to imbibing / germinating state. Osmopriming might provide a `head-start' for this transition, and thus resulted in a more robust antioxidant system in primed seeds and increased their germination potential. Consequently, primed seeds exhibited improve tolerance to chilling and desiccation stress. Our study of dehydrin accumulation, on the other hand, suggests an alternative strategy for osmopriming to improve stress tolerance in germinating seeds. Several dehydrins (30, 26, and 19-kD dehydrins, and CAP85) transiently accumulated during osmopriming at both protein and transcript levels. These dehydrins also re-accumulated in primed seeds in response to chilling and desiccation stress, and thus may be associated with the improved stress tolerance. It is possible that osmopriming imposed mild osmotic stress in seeds and induced accumulation of stress responses (dehydrins) to confer cross-tolerance that rendered primed seeds more tolerant to subsequent stress exposures. We assumed that osmopriming might use the above-described two strategies (i.e. increasing seed germination potential and inducing cross-tolerance in seeds) that act together to enhance seed stress tolerance during post-priming germination.