Physical dormancy (PY) is typically induced by seed coat impermeability that develops once the moisture content of seeds drops below a species-specific threshold. Considering this, we utilized Albizia julibrissin (Fabaceae) to ask (i) whether seeds that mature on the outer branches of trees (directly exposed to sunlight) are more likely to be impermeable than seeds matured under canopy cover; (ii) whether this difference might be explained by the maternal environment in which the seeds mature; and (iii) which conditions impose secondary dormancy following dispersal? Temperature was tracked in both shaded and sun-exposed seed pods throughout the growing season using data-loggers. Temperatures remained lower in pods under canopy cover than those exposed to direct sunlight. Consequently, the moisture content of seeds collected from sun-exposed branches were significantly lower than seeds matured under canopy cover, thereby producing a higher percentage of impermeable seeds. A dispersal-mimicking experiment revealed that seeds matured in sun-exposed branches and subsequently dispersed to an open site for 4 months were more likely to develop impermeability (i.e. secondary dormancy). The opposite was found to be true for seeds matured in shaded branches and subsequently dispersed to a canopy-covered site. We conclude that the microclimate of both the maternal environment in which seeds mature, and the site to which they disperse, determines the development of primary and secondary dormancy, respectively.

中文翻译：

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空气和土壤温度在合欢（豆科）初级和次级物理休眠发展中的关键作用

物理休眠 (PY) 通常是由种皮不透性引起的，一旦种子的水分含量低于特定物种的阈值，就会出现这种不透性。考虑到这一点，我们利用合欢（豆科）询问（i）在树的外枝（直接暴露在阳光下）成熟的种子是否比在树冠覆盖下成熟的种子更可能不透水；(ii) 这种差异是否可以用种子成熟的母体环境来解释；(iii) 哪些条件会在散布后造成二次休眠？使用数据记录器在整个生长季节跟踪遮蔽和暴露在阳光下的种子荚的温度。在树冠覆盖下的豆荚中的温度仍然低于那些暴露在阳光直射下的豆荚。因此，从暴露在阳光下的树枝中收集的种子的水分含量明显低于在树冠覆盖下成熟的种子，从而产生更高百分比的不透水种子。分散模拟实验表明，种子在暴露在阳光下的枝条中成熟并随后分散到空旷地点 4 个月更有可能发展不透水性（即二次休眠）。发现在阴影树枝中成熟并随后分散到树冠覆盖的地点的种子情况正好相反。我们得出的结论是，种子成熟的母体环境和它们散布的地点的小气候分别决定了初级和次级休眠的发展。