Rice seed dormancy

Rice seeds that do not sprout when planted in favorable soil and atmospheric conditions are either dead or dormant. Wild and traditional rice cultivars have higher degrees of dormancy than modern ones. Rice seeds may be dormant up to 80 days but most of the cultivated ones have only 2-3 weeks dormancy. A cultivar may be considered slightly dormant (germinates within 15 days from harvest); moderately dormant (germinates within 15-60 days from harvest); or highly dormant (germinates within 60 days from harvest or later). Others, especially many modern varieties, have no dormancy at all. Dormancy is naturally broken through time.

Dormancy can be a plant's natural means to prevent mature seeds in the panicle from sprouting especially during the rainy season in the tropics. For man, it is an advantage when seeds being dried are rained on as these could sprout and lead to losses. However, dormancy becomes a problem when seeds are purposely planted or assessed for germination.

Traditional and modern cultivars

DETECTION OF DORMANCY:

The degree of dormancy of rice seeds are often judged based on previous knowledge about the cultivar and the length of time the seeds have been stored. It can be determined along with the germination rate by sowing mature seeds in moist sand, soil, absorbent paper or cloth. After 5-7 days the seeds are checked and any ungerminated seeds that are not moldy or rotten (easily determined by gently squeezing ungerminated seed) are judged dormant.

BREAKING DORMANCY IN RICE:

Any of the following methods could be used before planting or during germination testing to break dormancy, but their effectivity depends on the cultivar or storage time which determine the degree of dormancy:

· Soak one part seed in 5-10 parts water at 40°C (approximated by allowing boiling water to cool off for 8-10 minutes).

· Heat seeds which have been sundried for 3-5 days (seed moisture content approximately 10%) in an oven at 50-55°C for 7 days.

Soak one part seed in 5-10 parts water

· Soak 1 kg of seeds for 16-24 hrs in 1 liter nitric acid solution (HNO3) prepared by pouring 1 1/4 tsp (6.3 ml) concentrated nitric acid (of 68% purity) into a liter of water (a motor oil can = 1 liter). After soaking, sundry seeds for about 3-7 days.

Soak 1 kg of seeds

Caution: The acid must be poured into the water during the preparation of the solution. Do not pour water into the acid because an explosion could result.

· Soak 1 kg seeds for 24 hrs in 1 liter water containing 1/2 tsp fresh sodium hypochlorite (chlorox, available market grade = 5.25%).

· For a germination test where only a few seeds are used, manually remove hulls. Be careful not to injure the germ or embryo which is the part where the plant develops. This treatment is not applicable to wild rice.

Soak 1 kg seeds for 24 hrs