To examine the role of gibberellins in plasticity to foliage shade, we characterized the reaction norms of gibberellin-insensitive and deficient mutants of Arabidopsis thaliana to variation in photosynthetically active radiation (PAR) and in the ratio of red : far red light (R:FR). We asked: (1) Do mutations in the gibberellin (GA) signaling system alter the phenotypic plasticity of A. thaliana to foliage shade? (2) Do GA-deficient mutants at distinct loci differ in their effects on reaction norms when compared to a GA-insensitive mutant? (3) If a mutation in GA signaling affects plasticity to foliage shade, does it affect resource-mediated plasticity to reduced PAR, or phytochrome-mediated plasticity to the R:FR cue? Mutations at GA signaling loci altered the reaction norms of A. thaliana, but mostly in their height, not in the degree of plasticity. There were clear quantitative differences in reaction norms among GA loci, but the gibberellin-insensitive mutant was not phenotypically distinguishable from the gibberellin-deficient ones. The only effect of mutations on the shape of reaction norm was detected for fruits production, implying the existence of other traits affected by GA that we did not measure and which mediate the role of these hormones in response to light quantity but not quality. Certain GA mutants dramatically increased reproductive fitness relative to the wild type under the favorable conditions and unlimited growing season encountered in the greenhouse. While this fitness advantage might not occur under the stricter selective regime imposed by field conditions, it does demonstrate that mutations at major regulatory loci can dramatically and positively affect fitness, depending on the environmental circumstances.

Key words: fitness, GA, irradiance, phenotypic plasticity, phytochromes, R:FR