Dormancy of Rice Seed

The mean dormancy period is defined as the mean time taken from harvest for the individual seeds of a population to attain the ability to germinate under normal conditions. Evidence is presented which shows that the distribution of dormancy periods among the individual seeds of a pure line is normal. Since germination usually reaches 100 per cent., and because a normal distribution is symmetrical, a good estimate of the mean dormancy period can be obtained by observing the time taken from harvest to the point when 50 per cent. of the population is capable of germination as shown by a series of germination tests. The main error involved is due to the fact that there is no obvious way of deciding, to within a few days, when grain is ripe for harvest. Harvesting the seed prematurely tends to speed up the processes leading to loss of dormancy whereas late harvesting has little influence on the date on which the mean dormancy period is achieved; it follows that when estimating the mean dormancy period the error is much smaller if seed is harvested too early rather than too late. Contrary to some previous reports, no correlation has been detected between ‘duration’ (period from sowing to modal flowering plus 30 days) and mean dormancy period among different varieties.