Priming relieves dormancy in lettuce seeds independently of changes in osmotic constituents

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Abstract
The relationship was studied between germination and dormancy of lettuce seeds (Lactuca sativa L. cv. Musette) and both soluble amino nitrogen metabolism and osmotic potential. Germination at 15°C in darkness coincided with a rise in the levels of free amino acids and total soluble amino nitrogen compounds and in the activity of glutamine synthetase (GS, EC nr. 6.3.1.2). In further experiments GS activity was used as indicator of soluble amino nitrogen metabolism. GS activity increased after the start of growth indicated by an increasing intolerance to desiccation. At 30°C seeds did not germinate, unless dormancy was broken beforehand during incubation at 2° or 15°C (priming). The alleviation of dormancy occurred much earlier than the rise in the activity of GS. Priming at 15°C in polyethylene glycol instead of water retarded the breaking of dormancy and at –1.28 MPa even stimulated the induction of secondary dormancy, but did not prevent a continued rise in the activity of GS. GS activity was also not reduced during induction of secondary dormancy by dehydration of primed seeds, which antagonized the beneficial effect of priming. Psychrometric measurements showed that osmotic potential (Ψπ) of the seeds remained constant during prolonged priming in polyethylene glycol at 15°C. During incubation in water, Ψπ increased both prior to and after the moment of germination to less negative values. It is concluded that changes in the level of dormancy in lettuce seeds occur independently of soluble amino nitrogen metabolism and of changes in Ψπ.