WARM PLUS COLD STRATIFICATION REQUIREMENT FOR DORMANCY BREAK IN SEEDS OF THE WOODLAND HERB CARDAMINE CONCATENATA (BRASSICACEAE), AND EVOLUTIONARY IMPLICATIONS

At maturity in mid-May, the physiologically dormant seeds of Cardamine concatenata have fully developed embryos; 59.2% of those sown in a nonheated greenhouse in May germinated the following late February to mid-April. Maximum germination occurred from 7 to 14 March, when mean daily maximum and minimum air temperatures were 14.8 and 3.3 degrees C, respectively. No additional seeds germinated in two subsequent springs, indicating lack of ability of this species to form a persistent seed bank. Seeds receiving only a cold stratification period germinated to 98% after 28 weeks at 5 degrees C, whereas those given a warm-stratification pretreatment of 4 weeks at 30:15 degrees C (max-min) germinated to 100% after only 18 weeks at 5 degrees C. In the field, warm stratification during summer and autumn would reduce the length of the cold-stratification period required to break dormancy, and thus allow dormancy to be broken during winter. This is only the second report of a woodland herb having physiologically dormant seeds with fully developed embryos that require warm plus cold stratification to germinate in spring. We hypothesize that this requirement for dormancy break could have evolved from ancestors whose seeds required only a period of warm stratification to emerge from dormancy.