lab 4 addon - Laboratory on Seed Germination The Effect of...

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Laboratory on Seed Germination: The Effect of Light on Lettuce Seed Germination Plant species occur on almost every continent, and the species of plants in any given ecosystem often defines what type of animals and other organisms can survive there. In each species, natural selection has resulted in evolution of seeds that germinate when conditions are most favorable for growth in that species’ particular environment, and which remain dormant when conditions would not be favorable. Domesticated crops, however, have been artificially selected to germinate reliably when the farmer decides to plant them (or else he or she might not plant that variety again). Thus, most domesticated crops whose wild ancestors once had seed dormancy have lost it. A case in point is lettuce ( Lactuca sativa ). The wild ancestors of lettuce were probably colonizers of disturbed areas in temperate climates of Europe and Asia. In a forest, the seeds that did best would lie dormant in the soil until a light gap opened above them. Buried seeds that were able to delay germination until the soil was disturbed, bringing buried seeds to the surface, also had a selective advantage. In addition, germination had to be timed so that the young plants could complete most of their growth in the cool, rainy weather of spring, not in the fall (when frost might kill a young plant) or in the summer (when it might be too hot and dry for optimal growth). Ancestral wild lettuce thus evolved a combination of light- and temperature-mediated seed dormancy. The classic studies demonstrating that dormancy could be regulated by red and far-red light were performed using one of the last varieties of modern lettuce to retain ancestral seed dormancy, Lactuca sativa ‘Grand Rapids’ (Borthwick et al. 1954). Since then, under relentless selection for “uniform germination,” most strains of Grand Rapids no longer show seed dormancy.
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