Combinational dormancy in winter annual Fabaceae

Among 14 species of herbaceous Fabaceae, all six winter annuals show a marked non-deep physiological dormancy of the embryo in addition to physical dormancy. This physiological dormancy is apparent at 23 degrees C, but not at lower temperatures of 10 degrees C and 5 degrees C and disappears after 3 months of dry storage. These results corroborate the hypothesis that combinational dormancy is a double safety mechanism for delaying germination during summer: physical dormancy postpones germination, and even in early softened seeds germination is prevented by physiological dormancy of the embryo. Softened, swollen seeds of Medicago arabica tolerate a subsequent desiccation and remain viable even after five cycles of dehydration and rehydration. The rate of natural softening of M. arabica seeds increases exponentially at higher temperatures, with a Q(10) between 3.4 and 5.1, and obeys the Arrhenius equation. This indicates that a chemical reaction might be involved in breakdown of physical dormancy. Winter annuals with hard seeds show similar properties as winter annuals with permeable seeds: the need for afterripening and requirement of lower temperatures delay germination until autumn. Only one species, Vicia sativa, loses physical dormancy during dry storage. Drying during summer might be a supplementary cue for germination in autumn.