Overcoming Seed Thermodormancy in a Heat Sensitive Romaine Lettuce by Seed Priming1

Seeds of ‘Valmaine’ romaine lettuce (Lactuca sativa L.) were very sensitive to high temperature during laboratory germination. Priming seeds in −5.1 bar K 3 PO 4 or polyethylene glycol osmotic solutions was more effective for increasing the germination rate over nonprimed seeds at 30°C than priming seeds in water. When planted in the field in 1978 and 1979, primed ‘Valmaine’ lettuce seeds emerged more rapidly and uniformly than nonprimed seeds. At harvest, plants from priming treatments which had emerged sooner, were at a more advanced and uniform stage of maturity than plants from nonprimed seeds.