Fig. 1.
Schematic representation of the plant and of equations. Purple, environmental variables at the plant boundaries; red, water potentials; blue, fluxes; green, conductances. Numbers after equations refer to the numbers in the text, except Eqn 13. The water flows from the bulk soil (with water potential Ψ bulk) to the root (Ψ r) via the rhizosphere (Ψ soil), then in the root (Ψ r), the xylem (Ψ xyl), and bundle sheaths (Ψ b). Mature tissues (Ψ cell) either divert part of the xylem flow or participate in transpiration, depending on the sign of (Ψ cell – Ψ b). Jin, Jxyl, Jxc, Jout: water flux entering into roots, in the xylem, from bundle sheaths to mature cells, and transpiration flux, respectively. Gr, Gxl, Gc, g s: conductances for water in the roots, from the xylem to bundle sheaths, from bundle sheaths to mature cells, and stomatal conductance, respectively. Eqn 13 refers to all transport in the plant from point i to point j, with water potentials Ψ i and Ψ j, with a conductance for water Gij from i to j.