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Because plants depend on light for growth, their development and physiology must suit the particular light environment. Plants native to different environments show heritable, apparently adaptive, changes in their response to light 1, 2. As a first step in unraveling the genetic and molecular basis of these naturally occurring differences, we have characterized intraspecific variation in a light-dependent developmental process—seedling emergence. We examined 141 Arabidopsis thaliana accessions for their response to four light conditions, two hormone conditions and darkness. There was significant variation in all conditions, confirming that Arabidopsis is a rich source of natural genetic diversity. Hierarchical clustering revealed that some accessions had response patterns similar to known photoreceptor mutants, suggesting changes in specific signaling pathways. We found that the unusual far-red response of the …