Ecophysiological Mechanisms Underlying Seed Dormancy in Five Genea of Ranunculaceae Native to the Korean Peninsula

Abstract

The main aim of this study was to explore the diversity of seed germination and seedling emergence strategies in Korean temperate forest herbs and to study the environmental factors regulating germination. Embryo morphology and seed dormancy of two Berberidaceae species (Leontice microrhyncha and Jeffersonia dubia), eight Ranunculaceae species (Adonis amurensis, Aquilegia buergeriana, Pulsatilla tonkangensis, Ranunculus crucilobus, R. franchetii, Thalictrum rochenbrunianum, T. uchiyamai, and T. coreanum), three Melanthiaceae species (Heloniopsis koreana, H. tubiflora, and Trillium tschonoskii), and one Liliaceae species (Erythronium japonicum) were studied, and the types of seed dormancy were classified, and its regulation by phytohormones was discussed. All seeds of 14 species we studied had underdeveloped embryos which occupied about 7-20% of the full seed length at maturity. Most seeds of A. buergeriana and P. tonkangensis had morphological dormancy (MD), whereas the seeds of L. microrhyncha, J. dubia, A. amurensis, R. crucilobus, R. franchetii, E. japonicum, T. tschonoskii, T. coreanum had morphophysiological dormancy (MPD). On the other hand, the seeds of H. koreana, H. tubiflora, T. rochenbrunianum, and T. uchiyamai had both MD and MPD, indicating that there was a different level of dormancy (MD and MPD) within the same seed population examined. Among the 14 species, the types of MPD were classified in seeds of Ranunculaceae. All six species, which had MPD, emerged seedlings mainly in spring after dormancy was broken. However, different dormancy mechanisms to attain this pattern of spring emergence were observed regardless of the time of seed dispersal. Seeds of A. amurensis had a special type of deep simple epicotyl MPD. Embryos grew at relatively warm temperatures in autumn with split pericarp, and seeds germinated after warm followed by cold temperature sequences. GA did not overcome the dormancy. Seeds of R. crucilobus had deep complex MPD. Embryo growth occurred at cold temperature (5oC). Although germination occurred at cold temperature, it was more promoted at warm temperatures after cold stratification. GA did not overcome the dormancy. On the other hand, seeds of R. franchetii had deep simple epicotyl MPD. Embryos in the seeds grew at relatively warm temperature in early autumn, and germination occurred in late autumn. Germinated seeds required cold stratification for seedling emergence. In three Thalictrum species, seeds had non-deep simple MPD. Although embryo growth occurred at low temperatures, it was more promoted when the seeds were transferred from low to high temperatures. Seeds required only a cold stratification to break dormancy, and GA substituted for cold stratification. We collected data of seed dormancy types in Ranunculaceae from available information and compared with congeners in different countries or continents. From the data, we found that there was a wide variety of dormancy types in five genera in Ranunculaceae. However, there were same types of MPD in eastern Asian - North American congeners in the genus Thalictrum with an Arcto-Tertiary distribution pattern. This indicates that identical types of dormancy are evidence that this type of dormancy is at least as old as the Tertiary. MD and MPD were regulated by phytohormones (ABA and GAs). In A. buergeriana seeds (MD), ABA content and sensitivity decreased rapidly, and GA content and sensitivity increased rapidly after burial. On the other hand, in A. amurensis seeds (MPD), ABA content decreased drastically after burial, but GA content did not increase before the seeds experienced temperature changes from high temperatures in summer to medium temperatures in autumn in the natural environment. When underdeveloped embryos grew rapidly in autumn, ABA was non-detectable and GA content increased. After embryo maturation, ABA content increased and GA content decreased at the same time, and thus, the seeds remained ungerminated during cold season in winter. When the seeds started to germinate after cold period in winter, GA content increased rapidly. GA4 played a key role in stimulating embryo growth and germination in both MD and MPD. The changes of GA/ABA ratio were similar to the changes of embryo growth and germination in the buried seeds. These results indicate that MD or MPD in the basal angiosperm taxa also could be controlled by a hormone balance model.