Combinational dormancy in winter annualFabaceae

Abstract

AbstractAmong 14 species of herbaceousFabaceae, 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°C, but not at lower temperatures of 10°C and 5°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 ofMedicago arabicatolerate a subsequent desiccation and remain viable even after five cycles of dehydration and rehydration. The rate of natural softening ofM. arabicaseeds increases exponentially at higher temperatures, with a Q10between 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.