LIGHT AS A FACTOR INFLUENCING THE DORMANCY OF LETTUCE SEEDS

Lettuce seeds of the highest vitality may fail to germinate when tested in a closed chamber germinator. Seed analysts have observed that germination may be induced in many of these dormant lettuce seeds by presoaking them in cold water for 2 hours. However, such increase in germination is due not to the soaking in water but to the action of light on the wet seeds.2 Recent investigations by the writer show that it is even unnecessary to soak the seeds in cold water since the dormant condition can be broken merely by placing the seeds in an atmosphere saturated with water vapor and giving a longer exposure to light. Lettuce seeds of the Grand Rapids variety which were placed in a humid atmosphere and exposed to light for 10 hours and then placed on moist blotters at 200 C. in the dark germinated 98 per cent. A duplicate test which had received the same treatment but which was not exposed to light germinated only 8 per cent. The period of exposure to light can be shortened to 2 minutes or perhaps less by allowing the seeds to take up moisture from the air before exposing them to light. This germination response is of importance because it shows that water in the form of a film surrounding the seed is not essential to the breaking of the dormant condition by light. Whether or not lettuce seeds in which the dormant condition has been broken by exposure to light will remain non-dormant when dried depends upon the physiological condition of the seed, the period of exposure to light, the temperature, and the manner in which the drying process is conducted. Lettuce seeds of the Big Boston variety which were placed in a humid atmosphere and exposed to light for a period of 24 hours at 230 C. and, without drying, placed on moist blotters at 20° C. in the dark, germinated 98 per cent. A similar lot of seeds which had received the same treatment but which was dried in the dark germinated 35 per cent. The failure of lettuce seeds to retain the non-dormant state when dried appears to be a characteristic response of lettuce seeds which require a temperature of 22° C. or lower for germination. It has been determined that if the seeds are set over water, exposed to light for 6 hours, and then placed in a refrigerator at a temperature of 60 C. for 10 days, they can then be dried and still retain the non-dormant state. Furthermore, it has been observed that