4.4 AI drives hydraulic vulnerability segmentation by affecting xylem structure
Increasing vulnerability to cavitation along the hydraulic pathway from roots to branches is likely affected by the diameter and the physical properties of the xylem conduits. Our results identified the decoupling responses of root and branch structural traits to environmental aridity (i.e., AI), which may be an important greason for determining the vulnerability segmentation. We found an apparent decline ofD h in branches with increasing AI, suggesting conduit diameter in branch xylem be constrained by environmental aridity (Rodríguez-Ramírez et al ., 2020, Fontes et al ., 2022). By contrast, the relationship between D h and AI in roots was non-significant. Previous studies had reported that root cortical cells can reduce hydraulic conductivity by forming cortical lacunae before embolism (Cuneo et al ., 2016; Carminati & Javaux, 2020). This may help roots to avoid exposure to negative water potential in face of drought, and root hydraulic conductivity is likely to be restored rapidly after re-wetting (Bourbia et al ., 2021). Under water-limited conditions, tolerating drought stress while maintaining hydraulic function requires modification of hydraulic architecture, including increased the relative number of narrow conduits (Brodribbet al ., 2012; Yao et al ., 2021). However, plants in humid habitats tend to grow taller, which requires more water transporting to the top of crowns to sustain transpiration and photosynthesis (Kochet al ., 2004; Chin et al ., 2022). A network composed of wide conduits over a large distance of the vertical transport pathway would theoretically meet these requirements. This is consistent with our current observations that as the available water increases, plants choose to widen the diameter of vessels or tracheids downstream (i.e., in branches) of the water transport pathway, therefore, matches the usually wider conduits upstream (i.e., in roots) of the pathway, to enable their branches to get stronger competitiveness for light and carbon uptake (Pfautsch et al ., 2016). The safety versus efficiency trade-off theory in the xylem claimed that when the tension to pull sufficient water and transport through the stem exceeds the physical limits, the risk of cavitations in the xylem of branches will inevitably increase (Tyree & Zimmermann, 2002; Aritsara et al ., 2021; Wang et al ., 2022). This will gradually reduce the gap with the cavitation vulnerability in roots, resulting in the decrease ofP 50 root - branch along environmental aridity gradients.
Conclusions
Our results demonstrates that hydraulic vulnerability segmentation between branches and roots (P 50 root - branch> 0) is common for species from arid environments. Such a segmentation has been proved important for plant drought adaptation, with roots functioning as “hydraulic fuses” in the water transport pathway. However, species from humid habits display a lower degree or even a lack of the segmentation. These species may opt for more radical hydraulic strategies (e.g., lower cavitation vulnerability and higher hydraulic conductivity) to adapt to humid conditions. Along the environmental aridity gradients, xylem structure in branches changes from “more efficient” (e.g., wider conduit, higher hydraulic conductivity) to “safer” (e.g., narrower conduit, more negative P 50). By contrast, root xylem tends to maintain hydraulic efficiency. Therefore, by identifying both environmental drivers and structural basis of vulnerability segmentation between branches and roots, our results provide a novel ecological perspective to better understand hydraulic vulnerability segmentation in woody plants.