Hydration offers an effective means for raising seed performance in many crop species. The objective of this study was to evaluate the effect of vermiculite hydration on germinability and several physiological activities related to vigor in artificially aged watermelon seeds differing in ploidy. Aging was achieved by incubating the seeds at 45°C and 79% relative humidity for 6 days, then the seeds were air‐dried to their original moisture level (4.7%). Hydration was achieved by mixing the untreated and aged seeds with moist vermiculite No. 3 at 25°C for 24 h. The partially hydrated seeds were air‐dried at 25°C for 36 h to 4.7% moisture level. Significant differences existed between unaged and aged seeds, with lower germination percentage and slower germination speed in the latter. Aging also increased lipid peroxidation and reduced the activity of peroxide‐scavenging enzymes. The germinability of aged watermelon seed was restored partially by vermiculite hydration. The activities of protein synthesis and peroxide‐scavenging enzymes in axis and cotyledon portions of the seeds were also increased by hydration treatment. The changes in germination and related physiological responses in relation to aging and hydration are similar in seeds differing in ploidy, despite differences in their germination performance, seed leakage, extent of lipid peroxidation and activities of peroxide‐scavenging enzymes.

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