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Responses of carbon acquisition traits to irradiance and light quality in Mercurialis annua (Euphorbiaceae): evidence for weak integration of plastic responses¹

²Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1; ³Science Department, Royal Botanical Gardens, P.O. Box 399, Hamilton, Ontario, Canada L8N 4H4; ⁴Department of Botany, University of Toronto, 25 Willcocks Street, Toronto, Ontario, Canada M5S 3B2

It is often suggested that traits will be integrated, either because of pleiotropy or because natural selection may favor suites of integrated traits. Plant responses to different environments can provide evidence of such integration. We grew Mercurialis annua plants in high-density stands in high irradiance, in neutral shade, and in high red to far-red (R:FR) shade, resulting in environments of high irradiance, low R:FR; low irradiance, low R:FR; and low irradiance, high R:FR. We measured gas exchange, leaf morphology, stem elongation, and biomass traits and tested the prediction that traits within each functional group would show higher trait integration, as evidenced by high correlations among traits within environments, higher correlations of trait plasticity, and lower plasticity of trait correlations. Overall, we found evidence of only moderate integration for some groups of traits. Functionally related groups of traits, or pairs of traits, could be strongly integrated by one criterion but weakly integrated by another of the criteria. Stem elongation traits, though often observed to be strongly integrated in other taxa, showed little evidence of integration. Internode traits exhibited a novel pattern of responses to low R:FR, with increased elongation of the hypocotyl, decreased elongation of the first internode, and no change in the second internode. We propose that these responses to light are more likely to be the result of natural selection than the consequence of constraints imposed by pleiotropy.

Key Words: androdioecy • gas exchange • phenotypic plasticity • plasticity integration • R:FR • stem elongation