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.