BACKGROUND: The biomechanical, morphological and ecophysiological properties of plant seed/fruit structures are adaptations that support survival in unpredictable environments. High phenotypic variability of noxious and invasive weed species such as Raphanus raphanistrum (wild radish) allow diversification into new environmental niches. Dry indehiscent fruits (thick and lignified pericarp [fruit coat] enclosing seeds) have evolved many times independently. METHODS: A multiscale biomechanics and imaging (microscopy, X-ray, finite element stress simulation, puncture force analysis) approach was used to comparatively investigate the indehiscent fruits of R. raphanistrum (global weed), R. pugioniformis (endemic weed) and R. sativus (cultivated radish). RESULTS: The hard pericarp of Raphanus species (Brassicaceae) imposes mechanical dormancy by preventing full phase-II water uptake of the enclosed seeds. The apparently unilocular fruits of Raphanus species develop from two fused valves, pericarp rupture to permit germination is confined to the midvalve regions, and each midvalve region contains a predetermined breaking zone that is biomechanically defined by the internal shape of the seed chambers. Direct biomechanical analysis revealed great variability in within-fruit and between-fruits pericarp resistances. CONCLUSIONS: Variability in pericarp-imposed dormancy provides a bet-hedging strategy to affect soil seed bank persistence and prolong the germinability period.

Department of Biology Botany University of Osnabrück D 49076 Osnabrück Germany

Laboratory of Growth Regulators Faculty of Science Palacký University and Institute of Experimental Botany Czech Academy of Sciences CZ 78371 Olomouc Czech Republic

Resolve Biosciences 40789 Monheim am Rhein Germany

Seed Biology and Technology Group Department of Biological Sciences Royal Holloway University of London TW20 0EX Egham UK

seedalive 49076 Osnabrück Germany

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