The Stress Gradient Hypothesis (SGH), which predicts increasing ratios of facilitative:competitive interactions with increasing stress, has long been a guiding framework for conceptualizing plant-plant interactions. Recently, there has been a growing recognition of the roles of microbes in mitigating or exacerbating environmental stress for their plant hosts. As such, we might predict based on the SGH that beneficial microbial effects on plant performance should be positively associated with stress. Specifically, we hypothesized that support for the SGH would depend on the host plant's habitat specialization such that species that specialize in high stress habitats and thus likely coevolved with the resident microbes would exhibit stronger support for the SGH than non-specialist plant species. We further hypothesized that support for the SGH would vary with germination frequency, since boosting germination of low-frequency germinators is one effective means by which microbes can benefit plant species performance. Here, we explore whether plant-microbial interactions support the SGH, using 12 plant species native to the Florida rosemary scrub. We conducted factorial experiments that manipulated the presence of microbes in nine soils collected along an elevational stress gradient, and recorded germination frequency and biomass. Microbes increased the germination frequency of four species, all of which had relatively low germination rates. Furthermore, we found support for the SGH in nearly half of the species examined, with soil microbes facilitating germination with increasing stress for five of the 12 species tested, and none of the species exhibiting the opposite trend. Support for the SGH was not predicted by either the plant hosts' habitat specialization or germination frequency. In contrast to germination, biomass results showed little support for the SGH, with four of 12 species refuting and one species supporting SGH predictions. Taken together, our study documents that interactions between the soil microbial community and plant species along a stress gradient can support the SGH, but emphasizes that these effects are life history stage-dependent. This work also identifies a common mechanism (germination facilitation) by which microbes can benefit plant species in stressful habitats.

中文翻译：

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植物-微生物相互作用是否支持应力梯度假说？

压力梯度假设 (SGH) 预测促进性：竞争性相互作用的比率随着压力的增加而增加，长期以来一直是概念化植物 - 植物相互作用的指导框架。最近，人们越来越认识到微生物在减轻或加剧植物宿主的环境压力方面的作用。因此，我们可以根据 SGH 预测，微生物对植物性能的有益影响应该与压力呈正相关。具体来说，我们假设对 SGH 的支持将取决于寄主植物的栖息地专业化，因此专门在高压力栖息地并因此可能与常驻微生物共同进化的物种将比非专业植物物种对 SGH 表现出更强的支持。我们进一步假设对 SGH 的支持会随着发芽频率的变化而变化，因为促进低频发芽者的发芽是微生物有益于植物物种性能的一种有效手段。在这里，我们使用佛罗里达迷迭香灌木的 12 种原生植物来探索植物与微生物的相互作用是否支持 SGH。我们进行了因子实验，操纵沿海拔应力梯度收集的九个土壤中微生物的存在，并记录发芽频率和生物量。微生物增加了四种物种的发芽频率，所有这些物种的发芽率都相对较低。此外，我们发现近一半受检物种支持 SGH，土壤微生物促进发芽，并在 12 个受试物种中的 5 个物种中增加压力，没有一个物种表现出相反的趋势。植物宿主的栖息地专业化或发芽频率均未预测对 SGH 的支持。与萌发相反，生物量结果几乎不支持 SGH，12 个物种中有 4 个驳斥，1 个物种支持 SGH 预测。总之，我们的研究证明，土壤微生物群落和植物物种之间沿着压力梯度的相互作用可以支持 SGH，但强调这些影响是生命史阶段依赖性的。这项工作还确定了一种常见的机制（促进发芽），通过该机制，微生物可以使压力生境中的植物物种受益。与萌发相反，生物量结果几乎不支持 SGH，12 个物种中有 4 个驳斥，1 个物种支持 SGH 预测。总之，我们的研究证明，土壤微生物群落和植物物种之间沿着压力梯度的相互作用可以支持 SGH，但强调这些影响是生命史阶段依赖性的。这项工作还确定了一种常见的机制（促进发芽），通过该机制，微生物可以使压力生境中的植物物种受益。与萌发相反，生物量结果几乎不支持 SGH，12 个物种中有 4 个驳斥，1 个物种支持 SGH 预测。总之，我们的研究证明，土壤微生物群落和植物物种之间沿着压力梯度的相互作用可以支持 SGH，但强调这些影响是生命史阶段依赖性的。这项工作还确定了一种常见的机制（促进发芽），通过该机制，微生物可以使压力生境中的植物物种受益。但强调这些影响取决于生活史阶段。这项工作还确定了一种常见的机制（促进发芽），通过该机制，微生物可以使压力生境中的植物物种受益。但强调这些影响取决于生活史阶段。这项工作还确定了一种常见的机制（促进发芽），通过该机制，微生物可以使压力生境中的植物物种受益。