priming; rice seed; SEM; embryo; starch granule; α -amylase; sugar

An experiment was carried out to find out the priming effects of rice seeds, Oryza sativa L. (cv. Ilpumbyeo) on. the seedling establishment and early emergence under excess soil moisture conditions. Seeds were primed by soaking in -0.6 MPa polyethylene glycol (PEG) solution at 25~circC for 4 days. The primed seeds were sown in soils with various soil moistures (60, 80, 100, 120, and 140% field capacity) at 17 and 25~circC , respectively. Germination and emergence rates, plumule height, and radicle length of primed seeds were higher than those of untreated seeds at any soil moisture and temperature examined. The time from planting to 50% germination (T50 ) of primed seeds was less than that of untreated seeds by 0.9～3.7 days. Germination rate, emergence rate, plumule height, and radicle length were highest at the soil moisture of 80% field capacity among the soil moistures. Priming effects of rice seeds on germination and emergence rates were more prominent under the unfavorable soil moistures (60, 100, 120, and 140% field capacity) than those under the optimum soil moisture condition (80% field capacity). However, priming effects on seedling growth were greater at near optimum soil moisture compared with too lower or higher soil moistures. Therefore, these findings suggest that priming of rice seeds may be a useful way for better seedling establishment under the adverse soil conditions.

To find out the effects of humidification and hardening of rice (Oryza sativa L. cv. Ilpumbyeo) seeds on the germination, both normal and artificially aged seeds with 60% germination rate were humidified at 40, 60, and 80% RHs and 25~circC for five weeks or hydrated for 6, 12, 18, and 24 hours at 25~circC from one to five cycles for hardening. Relative humidity and duration of humidification did not affect the germination rate of normal seeds, while reduced the time to get 50% germination rate (T50 ) by 1.0～1.6 days compared to that of normal seeds at 25~circC . Aged seeds humidified at 40 and 60% RHs did not affect the germination rate and T50 regardless of relative humidity and duration of treatment, while at 80% RH, the germination rate decreased and T50 increased significantly with the duration of humidification. Hardening of normal seeds reduced T50 by 0.7～1.1 days without changes in the germination rate. However, the germination rate and T50 of aged seeds soaked in water for 6, 12, and 18 hours were similar regardless of soaking/drying cycles, while hardening of artificially aged seeds with 1～4 cycles in 24 hours soaking increased the germination rate by 11-16% and reduced T50 by 1.4～2.0 days.

Experiments were carried out to find out the optimum water potential, temperature, and duration for the priming of rice seeds, Oryza sativa L. (cv. Ilpumbyeo) for better germination at sub-optimal temperatures. Seeds were primed in 0, -0.2, -0.4, -0.6, -0.8, and -1.0 MPa PEG (polyethylene glycol) solutions at 25~circC . The optimum water potential for seed priming, the highest water potential at which rice seeds did not germinate, was -0.6 MPa. To find out optimum priming temperature and duration rice seeds were primed in -0.6 MPa PEG solution and 0 MPa (water as a control) for various durations at 15 and 25~circC and the seeds were germinated at 17, 20, and 25~circC . Considering germination rate and speed, the optimum priming time in water (0 MPa) was 4 days at 15~circC and 1 day at 25~circC , while 4 days was the optimum priming time in a -0.6 MPa PEG solution, regardless of the priming temperature. Priming reduced the actual time of germination, especially at sub-optimal temperatures. Priming did not affect germination rate in -0.6 MPa PEG solution at 15~circC , but overpriming reduced the final germination rate in water at 15~circC and in -0.6 PEG solution at 25~circC . Total sugars and α -amylase activity induced during the seed priming were negatively correlated with the final germination rate and there was no noted relationship with the speed or uniformity of germination.