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Background and Aims: Many angiosperms exhibit cleistogamy, the production of both cleistogamous flowers (CL), which remain closed and obligately self-pollinated, and chasmogamous flowers (CH), which are potentially open-pollinated. The CH proportion can be plastic. Plasticity is adaptive if environmental changes can be reliably assessed and responded to with an appropriate phenotype and if plastic genotypes have higher fitness in variable environments than non-plastic ones., Methods: We studied the plastic response of four natural populations from northern and southern France of an annual cleistogamous plant, Lamium amplexicaule, to predictable seasonal variation. Plants were grown in a semi-controlled environment in spring and in autumn. We assessed the variation in flower number, phenology and cleistogamy-related traits, which were all plastic with respect to season. The CH proportion was higher in spring than in autumn in all four populations., Key Results: We showed significant stabilizing selection for cleistogamy traits, with higher optimal CH proportions and more pronounced stabilizing selection in spring than in autumn. Observed CH proportions were close to the predicted optimal CH proportions in each season except in autumn for southern populations, which do not experience the autumnal growing season in nature., Conclusions: These results are consistent with adaptive plasticity across seasons of cleistogamy in L. amplexicaule.We propose that adaptive plasticity of cleistogamy could be driven by pollination environment variation, with CL flowers providing reproductive assurance when pollinators are scarce and CH flowers reducing the inbreeding depression in offspring when pollinators are abundant., (© The Author 2016. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Plants display various forms of phenotypic plasticity in anticipation of changing conditions, many of which are influenced by information obtained from neighbouring plants. Here, we tested the hypothesis that cleistogamic Lamium amplexicaule plants can adaptively modify production of chasmogamous (CH) and cleistogamous (CL) flowers based on the perception of conspecific neighbours.The production and proportion of CH and CL flowers was examined in individual L. amplexicaule grown at varying densities or treated with root leachates from plants grown at different densities.When growing at high density or treated with root leachates from high‐density pots, L. amplexicaule increased production of more expensive, potentially outcrossing CH flowers. In contrast, single plants or plants treated with root leachates from empty pots or single‐source plants predominantly developed cheaper, self‐pollinated CL flowers.The results demonstrate a novel root‐based neighbour‐perception modality that enables plants to adaptively adjust production of CH and CL flowers in response to the presence of potential reproductive partners. Further research is needed to explore the broader ecological implications of this novel interplant cueing on reproductive bet‐hedging and plasticity in natural settings, as well as to identify the involved cues and their mode of operation. [ABSTRACT FROM AUTHOR]