Pericarp structure of Glebionis coronaria (L.) Cass. ex Spach (Asteraceae) cypselae controls water uptake during germination

Glebionis coronaria (L.) Cass. ex Spach is a common Mediterranean weed producing distinctive central and peripheral dormant cypselae with a hard fruit coat, which was previously hypothesized to impose physical dormancy. Analysis of water uptake in cypselae and in naked seeds showed that it preferentially takes place at the basal end of the fruit; however, seeds within an intact pericarp do not fully imbibe when compared with naked seeds. Germination was not significantly different between the two heteromorphs, and afterripening or cold stratification did not increase germination, while warm stratification at 35/20°C, as revealed by logistic regression, resulted in a significant improvement. However, loss of viability was also rapid at these high temperatures. Central and peripheral cypselae generally showed very low germination. In both heteromorphs, faster and higher germination (60-70%) was reached only after extensive scarification of pericarp tissue, and full germination was observed only after complete removal of pericarp tissue. Although the pericarp significantly reduced water uptake, no palisade layer(s) of macrosclereids could be observed. Xylem-vessel elements were found running through the basal end of the pericarp and forming the main point of water entry. We reject the hypothesis that G. coronaria cypselae have physical dormancy. Instead, water uptake and germination are impeded by: (1) directed water uptake, mainly through a pericarp-spanning channel-like structure; and (2) mechanical constraint on embryo growth exerted by the hard pericarp. The channel-like structure forms the principal system for controlling seed germination.