Editor: Christina CarusoPremise of research. Evolutionary radiations can be driven by physiological tolerances. Protea is a speciesrich genus in the Cape Floristic Region of South Africa and provides the opportunity to examine whether drought response traits may have contributed to differentiation in this stress-tolerant genus. Commonly utilized drought responses might be indicative of traits important to species diversification in stressful environments.Methodology. We studied how greenhouse-grown plants of six white Protea species physiologically responded to drought stress. We measured leaf-level physiological traits such as stomatal conductance, temperature, pubescence, chlorophyll, and abscisic acid (ABA) content.Pivotal results. Most traits showed similar drought responses across all six species and 29 populations we examined. Only for foliar ABA content, leaf hair density, and foliar chlorophyll content did species respond to drought in different ways, indicating that some differences in plasticity might be important to this evolutionary radiation.Conclusions. Our data support stomatal conductance as a trait important to stress tolerance across a range of environmental conditions. Moreover, population variation in the plasticity of physiological traits might be important to evolutionary trajectories in this system.

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