Non-Vessel Water Flow in Angiosperm Trees Enables Bypassing of Partially Blocked Xylem

Blockage of xylem vessels can compromise water flow in trees, eventually leading to reduced gas exchange and productivity. The extent of these impairments also depends on how effectively blocked vessels can be bypassed through lateral pathways. We hypothesize that the ability to bypass can vary crucially between different species of the same clade, leading to differences in the hydraulic limitations after a defined loss of conducting vessels. Here, we test this hypothesis on 1-year-old seedlings of two Mediterranean angiosperm tree species, carob (Ceratonia siliqua) and oak (Quercus calliprinos). We consecutively notched stems to artificially block water flow through vessels in one half of the cross-section. We measured the effect of notching on leaf gas exchange and visualized altered water flow pathways using microscopy and μCT imaging. In carobs, stomatal conductance (g_s) of leaves on the notched side decreased by more than 90%. Water transport in the notched side of the stem had ceased. In oaks, leaves on the notched side maintained more than 50% of their g_s with no signs of dehydration. Microscopy and μCT imaging revealed that water supply to these leaves occurred through lateral pathways outside vessels. This can be explained by the presence of tangentially oriented arrays of tracheids with bordered pits, which we found in oaks but not carobs. Our study emphasizes the importance of non-vessel water flow in angiosperm trees when the xylem becomes partially blocked.