Life history plasticity and water use trade-offs associated with drought resistance in a clade of California jewelflowers

Water limitation is a primary driver of plant geographic distributions and
individual plant fitness.  Drought resistance is the ability to
survive and reproduce despite limited water, and numerous studies have
explored its physiological basis in plants. However, it is unclear how
drought resistance and trade-offs associated with drought resistance
evolve within plant clades. We quantified the relationship between water
availability and fitness for 13 short-lived plant taxa in the Streptanthus
clade that vary in their phenology and the availability of water in the
environments where they occur. We derived two parameters from these
relationships: plant fitness when water is not limiting, and the water
inflection point (WIF), the watering level at which additional water is
most efficiently turned into fitness.  We used phylogenetic
comparative methods to explore trade-offs related to drought resistance
and trait plasticity, and the degree to which water relationship
parameters are conserved. Taxa from drier climates produced fruits at the
lowest water levels, had a lower WIF, flowered earlier, had shorter
lifespans, more plastic water use efficiency (WUE), and lower fitness at
non-limiting water.  In contrast, later-flowering Streptanthus
taxa from less xeric climates experienced high fitness at non-limiting
water but had no fitness at the lowest water levels. Across the clade, we
found a trade-off between drought resistance and fitness at high water,
though a single ruderal species was an outlier in this relationship. Our
results suggest that drought escape trades off with maximal fitness under
non-limiting water, and both are tied to phenology. We also found that
variation in trait plasticity determines how different plant species
produce fitness over a water gradient.

水分限制是决定植物地理分布与个体植株适合度的核心驱动因素。抗旱性是指在水分受限条件下仍能存活并完成繁殖的能力，已有大量研究探讨了植物抗旱性的生理基础。然而，目前尚不清楚在植物演化支（clade）内，抗旱性及其相关权衡关系是如何演化的。本研究针对Streptanthus演化支内的13个短命植物类群，量化了其所在生境的水分可获得性与适合度间的关联；这些类群在物候特征与所处环境的水分可获得性上均存在差异。我们从上述关联中推导得到两个参数：一是水分不受限时的植株适合度，二是水分拐点（water inflection point, WIF）——即额外水分可最高效地转化为适合度的灌水水平。本研究采用系统发育比较方法（phylogenetic comparative methods），探究了与抗旱性及性状可塑性相关的权衡关系，以及水分关联参数的演化保守程度。来自较干旱生境的类群可在最低水分水平下结实，其WIF更低、开花更早、寿命更短，水分利用效率（water use efficiency, WUE）的可塑性更强，且在水分不受限条件下的适合度更低。与之相反，来自非干旱生境的晚开花Streptanthus类群，在水分不受限条件下可获得较高适合度，但在最低水分水平下则无适合度可言。在整个演化支范围内，我们发现抗旱性与高水分条件下的适合度之间存在权衡关系，不过有1种杂草类物种在该关联中表现为异常值。本研究结果表明，避旱（drought escape）策略与水分不受限条件下的最大适合度之间存在权衡关系，且二者均与物候特征密切相关。此外，本研究还发现，性状可塑性的差异决定了不同植物物种在水分梯度上产生适合度的模式。