Direct seeding could be an attractive alternative to transplanting of rice (Balasubra- manian and Hill 2002), but poor germination, uneven crop stand, and high weed infestation are among the main constraints to its adoption (Du and Tuong 2002). Seed priming is an effective technique for rapid and uniform seed germination of several cereal crops (Basra et al 2004, Farooq et al 2004).
Seed hardening, also called wetting and drying or hydration/dehydration, is done by repeated soaking and drying of seeds in water (Pen Aloza and Eira 1993). Osmoconditioning is a special type of seed invigoration that has been used for slow hydration of seeds in aerated, low-water-potential solution (Bradford 1986). In earlier studies, hardening (Basra et al 2003) and osmopriming (Lee and Kim 1999) were found to be effective invigoration tools in rice. Seeds are hardened in tap or distilled water (Lee et al 1998); furthermore, seed priming is performed in a single cycle of wetting and drying (Lee and Kim 1999). These studies aimed to explore the possibility of rice seed priming by two cycles of wetting and drying (osmohardening) like seed hardening in low-water-potential solutions.