Steady-state water relations of soybean seedling roots

A previous four-compartment steady-state model of the root is extended to include the following: gradients of pressure (P) and osmotic pressure (Π) in the rooting medium, cortical apoplast and xylem; unloading of solutes to the xylem from internal stores; and apoplastic water flow across the endodermis. The model is tested by determining the steady-state relationship between applied pressure (P^o) and volume flow rate (Q_v) for excised roots of 4-day-old soybean seedlings under various conditions (vermiculite- versus water-cultured plants, different salt concentrations, permeant versus impermeant solutes). It is concluded that apoplastic volume flow across the endodermis is negligible. It is also argued that external or apoplastic gradients of P and Π tend to reduce the magnitude of the anomalous offset often observed in P^o-Q_v curves. The trans-root difference in water potential (ΔΨ) is shown to be a linear function of Q_v at low Q_v , but both ΔΨ(Q_v) and P^o(Q_v) become nonlinear at high Q_v . This unusual behavior may result from a nonlinear dependence of apoplastic pressure gradients and / or symplastic solute-concentration gradients on Q_v . Alternatively, it might reflect saturation of water transport in plasmodesmata or aquaporins. The ΔΨ associated with growth-induced water uptake is shown to be negligible compared with the growth-induced water potentials measured in the stem elongation zone.