The total path length hydraulic resistance according to known anatomical patterns: What is the shape of the root-to-leaf tension gradient along the plant longitudinal axis?

Xylem conduit diameter widens from leaf tip to stem base and how this widening affects the total hydraulic resistance (R_TOT) and the gradient of water potential (Ψxyl) has never been thoroughly investigated. Data of conduit diameter of Acer pseudoplatanus,Fagus sylvatica and Picea abies were used to model the axial variation of R_TOT and Ψxyl. The majority of R_TOT (from 79 to 98%) was predicted to be confined within the leaf/needle. This means that the xylem conduits of stem and roots, accounting for nearly the total length of the hydraulic path, theoretically provide a nearly negligible contribution to R_TOT. Consequently, a steep gradient of water potentials was predicted to develop within the leaf/needle base, whereas lower in the stem water potentials approximate those of rootlets. Our results would suggest that the strong partitioning of R_TOT between leaves/needles coupled with basal conduit widening is of key importance for both hydraulic safety against drought-induced embolism formation and efficiency, as it minimizes the exposure of stem xylem to high tensions and makes the total plant's conductance substantially independent of body size.

Keywords: Conduit widening; Hydraulic resistance; Leaf; Stem; Tension gradient; Water potential; Xylem.