Fig. 2.
Testing hypotheses for the drivers of the correlations among the drought tolerance traits. Most of the trait correlations are predicted to be driven by concerted convergence, wherein the selective pressure of water stress (Ψmin, MD or Ψmin, PD) acts independently on each trait to optimize overall plant function during drought (10, 17, 28). These hypotheses are indicated with dashed lines. Additionally, πtlp was hypothesized to influence Kleaf Ψ50 mechanistically (20). Kleaf Ψ50, in turn, would influence gS Ψ50 and Ψ95 and the threshold Ψleaf for leaf death (leaf Ψlethal) (30, 31), and the stem and root hydraulic traits would influence the plant mortality threshold (plant Ψlethal) (19). These hypotheses are indicated with solid lines. As predicted, πtlp and Kleaf Ψ50 were more correlated than expected from water stress and relatedness alone. Functionally coordinated traits are indicated with blue lines. Other correlations were best explained by the independent relationship of each trait with water stress. Concerted convergence is indicated with black lines. Conversely, Kstem Ψ50 was also more strongly correlated with Kleaf Ψ50 and, when characterizing water stress with Ψmin, MD, with πtlp than expected from concerted convergence, consistent with strong functional coordination within the hydraulic system across organs (SI Appendix, Tables S5 and S6). The remaining hypotheses had insufficient data to test (indicated with gray lines).