Ecotypic variation for seed dormancy contributes to the success of capeweed (Arctotheca calendula) in Western Australia

Abstract

The seed dormancy characteristics of 2 capeweed [Arctotheca calendula (L.) Levyns] ecotypes from Western Australia were studied to determine aspects of seed dormancy that contribute to the success of this species in southern Australia. Short- and long-term dormancy pattern of buried and soil surface seed, effect of summer temperatures on afterripening, and effect of temperature on seed germination were investigated using seed produced in a common environment. There were large differences in the seed dormancy pattern of the 2 ecotypes studied. On the soil surface, >95% of seed of the Mt Barker ecotype became non-dormant and germinated in the first year, the remainder germinating the following season. In contrast, only 5% of Mullewa seed germinated in the first year, with 75% germinating in the second year and 20% of seed remaining dormant after 2 years. Cycling of dormancy was observed for buried seed of both ecotypes, with periods of non-dormancy corresponding with the likely timing of the break of the season. Dormancy cycling was also apparent in seed stored under constant conditions in the laboratory. Burial prevented germination of both ecotypes; however, the ability to resist germination while buried was lost in 30% of the Mt Barker seed in the second season. Differences in the duration of dormancy of soil surface and buried capeweed seed have evolved as an adaptation to the different environments likely to be experienced by plants at their site of collection. All seeds possessed primary dormancy at maturity, with any afterripening during the first year occurring by the end of summer. Afterripening was enhanced by exposure to typical soil surface temperatures, providing some protection against germination during early summer rainfall. Protection from late summer rains is insured by the inability of seed to germinate at temperatures >30°C and a relatively slow rate of germination. These features of capeweed seed dormancy, combined with the ability to evolve genetically distinct populations suited to particular environments, help explain why capeweed is so widespread and abundant across southern Australia.