Pericarp-mediated chemical dormancy controls the fruit germination of the invasive hoary cress (Lepidium draba), but not of hairy whitetop (Lepidium appelianum)

This study provides a comparative analysis of the dormancy and germination mechanisms of the indehiscent fruits of hoary cress (Lepidium draba L.) and hairy whitetop (Lepidium appelianum Al-Shehbaz) , two invasive weeds of the Brassicaceae. Germination assays comparing isolated seeds (manually removed from the fruits) and intact indehiscent fruits showed that the isolated seeds are non-dormant and provided full germination for both species. In contrast to this, the species differed in the germination properties of their indehiscent fruits, in that L. appelianum fruits were non-dormant, while in L. draba the fruit coat (pericarp) conferred a coat-imposed dormancy. The pericarp L. draba fresh fruits was water permeable and neither mechanical scarification nor surface sterilization affected the germination, supporting that the pericarp-mediated dormancy was not due to water-impermeability or mechanical constraint. Washing of L. draba fruits with water, after-ripening (dry storage) and treatment with gibberellin (GA3) stimulated the germination of this species which is indicative of physiological dormancy. Analyses of endogenous abscisic acid (ABA) and gibberellin levels combined with treatment experiments with wash water from fresh and after-ripened L. draba pericarps and with ABA dose response quantification of germination revealed that ABA is a key component of a pericarp-mediated chemical dormancy in this species. Consistent with this, pericarp ABA levels decreased during after-ripening and upon fruit washing, and isolated fresh or after-ripened seeds did not differ in their ABA sensitivities. The possible roles of the ABA-mediated pericarp dormancy for the germination ecophysiology and weed management are discussed.