Lack of divergence in seed ecology of two Amphicarpaea (Fabaceae) species disjunct between eastern Asia and eastern North America

Kelinag Zhang, Jerry M. Baskin, Carol C. Baskin, Xuejun Yang and Zhenying Huang


东亚—北美间断分布的地上下结实物种两型豆在种子生态学方面缺少分化
在洲际间断生物地理学研究中,很多同属物种都是东亚—北美间断分布的。对于东亚—北美间断分布的豆科和(或)种子异型性物种的种子生物学比较研究,尚未见报道。我们比较了东亚—北美间断分布的地上下结实物种两型豆的种子休眠与萌发。调查了采集自中国的两型豆地上和地下种子的生理生态特性,并且根据前人研究结果,比较了和北美两型豆的异同。结果表明:两型豆地上种子的种皮发育完全;地下种子的种皮发育不完全。地上种子存在复合休眠(物理休眠+生理休眠);地下种子存在生理休眠;地上种子的休眠可以通过划破种皮(打破物理休眠),而后低温层积或后熟作用打破;地下种子休眠的打破只能通过低温层积打破。地上种子存在持久土壤种子库,地下种子存在短暂土壤种子库。中国的两型豆和北美两型豆地上种子都存在复合休眠,地下种子都存在生理休眠。两个物种的地下种子都具有脱水敏感性。地上种子传播距离比地下种子远。这些结果表明,东亚和北美的两型豆地上种子和地下种子在长期的适应进化过程中,并没有产生分化。地上与地下种子的休眠与传播策略不符合大多数种子异型性物种的高风险-低风险 (H-H/L-L) 策略,两个物种都表现出了H-L/L-H的适应策略。


Abstract
Premise of the study: Many congeneric species are disjunct between eastern Asia and eastern North America. No previous study has compared the seed biology of closely related disjunct taxa of legumes or of a diaspore-heteromorphic species. Our objective was to compare seed dormancy in two such sister species in the genus Amphicarpaea (Fabaceae).
Methods: We investigated the ecology and ecophysiology of aerial and subterranean seeds of the amphicarpic species Amphicarpaea edgeworthii from China and compared the results to those published for its sister species A. bracteata from eastern North America.
Key results: The seed coat of aerial seeds of A. edgeworthii is well developed, whereas the seed coat of subterranean seeds is not. Aerial seeds have combinational dormancy (physical dormancy [PY] + physiological dormancy [PD]) broken by scarification followed by cold stratification or by after-ripening and scarification; whereas subterranean seeds have PD broken by cold stratification. Aerial seeds formed a persistent soil seed bank, and subterranean seeds a transient soil seed bank. Aerial seeds of A. bracteata also have PY+PD and subterranean seeds PD. Subterranean seeds of both species are desiccation intolerant.
Conclusion: Dormancy in neither aerial nor subterranean seeds of both species has diverged over geological time. Compared to subterranean seeds, aerial seeds of both species dispersed over longer distances. Seed dispersal ability and degree of dormancy of neither species fits the high-risk/low-risk (H-H/L-L) strategy found in many diaspore-dimorphic species. Rather, both species have an H-L/L-H strategy for these two life history traits.
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