Apoplastic water transport in roots

Results obtained from combined measurements at the cell and root levels (cell and root pressure probes) indicate an important role of apoplastic water transport in roots, even in the presence of apoplastic barriers (Casparian bands and suberin lamellae in the endo- and exodermis). The composite transport model (CTM) of the root explains the variable root hydraulic conductivity (Lpr) and its physiological benefits, as well as low root reflection coefficients, and the switching between pathways depending on the water demand from the shoot (apoplastic vs. cell-to-cell transport). Switching between pathways provides a coarse, and changes in aquaporin activity a fine regulation of root Lpr. In experiments with excised corn roots (cell and root levels), the extent of suberisation of roots was varied but successfully tested the CTM. Recent measurements of the hydraulics of rice roots also supported the CTM, and the view that apoplastic barriers exhibit water permeabilities of greater than usually assumed. In the rice experiments, the hydraulic conductivity through the apoplastic passage could be modified by blocking apoplastic pores with ink particles or insoluble precipitates of copper ferrocyanide, analogous to a Pfeffer cell.