4.1 Higher vulnerability of roots compared to branches
We observed a notable increase in xylem resistance to cavitation (P 50), but a decline in hydraulic efficiency (K S) from roots to branches. Roots were more vulnerable to cavitation than branches for a given xylem diameter, and roots had 62% higher (less negative) P 50 value than branches (Figure 1). These results implied that roots with lower hydraulic safety, might be more susceptible to drought stress than branches along plant water transport pathway from soil to the atmosphere (Jackson et al ., 2000). However, roots with higher hydraulic efficiency than branches ensuring the water supply of the whole plant and prevent water deficit caused by high transpiration (Rodriguez-Dominguez & Brodribb, 2020). This difference in xylem cavitation vulnerability between branches and roots is matched by variability in wood anatomical characteristics, in that we found a lower vessel wall reinforcement (t 2/b 2) and a wider hydraulically weighted mean diameter (D h) in root xylem compared to branch xylem (Figure 1), and these anatomical traits showed significant correlations with P 50 both in roots and branches (Figure 2). Not only to ensure anchorage, woody species commonly require roots to absorb and transport water with mineral nutrients, wider conduits are therefore needed to improve efficiency of transportation in roots. However, branches have to resist stronger mechanical stress induced by gravity, wind, and ice loading etc., thus they need a relatively more robust woody structure than roots (MacFarlane & Kane, 2017). It has been shown at the cellular level, branches tended to have thicker cell walls and conduits with highert 2/b 2 than roots, which are often associated with lower conducting efficiency but greater resistance to cavitation (Hacke & Jansen, 2009; Schulte et al ., 2015).
4.2 Relationships between xylem hydraulic and anatomical traits
Our results suggest a generally positive relationship betweenD h and P 50 (Barottoet al ., 2018; Jacobsen et al ., 2019). However, cavitation formation may not be directly related to conduit size, but pit properties (Lemaire et al ., 2021; Levionnois et al ., 2021). The pit area hypothesis (also called ‘rare pit’ hypothesis) proposed that the larger conduits tend to have a greater pitted wall area, which is more vulnerable to cavitation (Mrad et al ., 2018). Moreover, thicker cell walls may also influence pit morphology, including the pit membrane, thereby increasing xylem cavitation resistance (Li et al ., 2016). Obviously, the pit membrane area is at least partly affected by conduit dimensions (Hacke et al ., 2017), suggesting that root xylem with larger conduits and thinner cell walls than those of branches, may be more vulnerable to drought-induced collapsing stress or cavitation.
The vulnerability segmentation between branches and roots may be an adaptive strategy for woody plants to cope with drought stress. Localized failure in relatively cheap and replaceable units (e.g., roots and leaves) would avoid disruption of major arteries of axial transport (Klepsch et al ., 2018; Aritsara et al ., 2022). In this study, we found a positive correlation between P 50 root - branch and P 50 branch (Figure 3a), but no correlation was found between P 50 root - branchand P 50 root (Figure 3b). These results implied that the degree of vulnerability segmentation between roots and branches is likely to be dominantly affected by the changes in cavitation vulnerability of branches. By contrast, roots generally maintain a relatively lower vulnerability to cavitation (Figure 3b). This may due to the fact that coarse roots (root diameter > 5 mm) are not easily exposed to very negative water potential. During drought exposure, cortical lacunae formation caused the outside xylem hydraulic conductivity within the cortical cells declined earlier before cavitation formed in coarse roots (Cuneo et al ., 2021). Cavitation formation in fine roots (root diameter < 2 mm) would interrupt the soil-root-connectivity during drought, thereby restricting hydraulic failure to replaceable units. (Carminati & Javaux, 2020; Aritsara et al ., 2022). Moreover, coarse roots may also act as hydraulic fuses like fine roots, which form a multi-stage circuit breaker with fine roots together to protect the hydraulic safety of the organs with high carbon investment (e.g., main root and trunk) against potentially extreme drought. We found woody species with branches that are more resistant to cavitation also have a greater degree of vulnerability segmentation (Figure 3a). However, this may be at the cost of partial loss of xylem hydraulic conductivity. We also found that K S in both roots and branches were markedly lower in woody species with greater degree of vulnerable segmentation (Figure 3c, 3d), highlighting a trade-off between hydraulic safety (capacity to resist cavitation) and efficiency (capacity to transport water) in the xylem of branches and roots (Gleason et al ., 2016).