Water availability is a critical driver of population dynamics in arid zones, and plant recruitment is typically episodic in response to rainfall. Understanding species' germination thresholds is key for conservation and restoration initiatives. Thus, we investigated the role of water availability in the germination traits of keystone species in an arid ecosystem with stochastic rainfall. We measured seed germination responses of five arid species, along gradients of temperature and water potential under controlled laboratory conditions. We then identified the cardinal temperatures and base water potentials for seed germination, and applied the hydrotime model to assess germination responses to water stress. Optimum temperatures for germination ranged from 15 to 31 °C under saturated conditions (0 MPa), and three species had low minimum temperatures for germination (<3 °C). A small proportion of seeds of all species germinated under dry conditions (Ψ ≤ -1 MPa), although base water potential for germination (Ψ b50) ranged from -0.61 to -0.79 MPa. Species adhered to one of two germination traits: (i) the risk-takers which require less moisture availability for germination, and which can germinate over a wider range of temperatures irrespective of water availability (Casuarina pauper and Maireana pyramidata), and (ii) the risk-avoiders which have greater moisture requirements, a preference for cold climate germination, and narrower temperature ranges for germination when water availability is low (Atriplex rhagodioides, Maireana sedifolia and Hakea leucoptera). High seed longevity under physiological stress in H. leucoptera, combined with a risk-avoiding strategy, allows bet-hedging. The hydrotime model predicted lower base water potentials for germination than observed by the data, further supporting our assertion that these species have particular adaptations to avoid germination during drought. This study provides insights into the complex physiological responses of seeds to environmental stress, and relates seed germination traits to community dynamics and restoration in arid zones.

中文翻译：

---

冒险者和回避者：在降雨量不可预测的干旱地区，发芽对水分胁迫的敏感性。

水资源供应是干旱地区种群动态的关键驱动因素，植物的补充通常是根据降雨而间歇性进行的。了解物种的发芽阈值是保护和恢复计划的关键。因此，我们研究了随机降雨的干旱生态系统中水的可用性对关键物种发芽性状的作用。我们在受控实验室条件下沿着温度和水势梯度测量了五种干旱物种的种子发芽反应。然后，我们确定了种子发芽的基本温度和基础水势，并应用水时间模型来评估发芽对水分胁迫的反应。饱和条件下（0 MPa），最适发芽温度为15~31°C，其中3个物种的最低发芽温度较低（<3°C）。尽管发芽基水势 (Ψ b50) 范围为 -0.61 至 -0.79 MPa，但所有物种的一小部分种子在干燥条件下 (Ψ ≤ -1 MPa) 发芽。物种遵循两个发芽特征之一：(i) 需要较少水分来发芽的风险承担者，并且可以在更宽的温度范围内发芽，而与水的可用性无关（Casuarina pauper 和 Maireanapyrama），以及 (ii)风险规避者，对水分要求较高，偏爱寒冷气候发芽，当水分利用率较低时，发芽温度范围较小（滨藜、Maireana sedifolia 和 Hakea leucoptera）。白翅螟在生理胁迫下的种子寿命较长，结合风险规避策略，可以进行对冲。水时间模型预测的发芽基础水势比数据观察到的要低，进一步支持了我们的断言，即这些物种具有避免在干旱期间发芽的特殊适应性。这项研究深入了解了种子对环境胁迫的复杂生理反应，并将种子发芽性状与干旱地区的群落动态和恢复联系起来。