How does water get through roots?

Abstract

uptake of water. On the contrary, at low rates of transpiration such as during the night or during stress conOn the basis of recent results with young primary ditions (drought, high salinity, nutrient deprivation), maize roots, a model is proposed for the movement of the apoplastic path will be less used and the hydraulic water across roots. It is shown how the complex, ‘com- resistance will be high. The role of water channels posite anatomical structure’ of roots results in a ‘com- (aquaporins) in the transcellular path is in the fine posite transport’ of both water and solutes. Parallel adjustment of water flow or in the regulation of uptake apoplastic, symplastic and transcellular pathways play in older, suberized parts of plant roots lacking a suban important role during the passage of water across stantial apoplastic component. The composite transthe different tissues. These are arranged in series port model explains how plants are designed to within the root cylinder (epidermis, exodermis, central optimize water uptake according to demands from the cortex, endodermis, pericycle stelar parenchyma, and shoot and how external factors may influence water tracheary elements). The contribution of these struc- passage across roots. tures to the root’s overall radial hydraulic resistance is examined. It is shown that as soon as early metaxy- Key words: Composite transport model, endodermis, exolem vessels mature, the axial (longitudinal) hydraulic dermis, hydraulic conductivity, reflection coefficient, root, resistance within the xylem is usually not rate-limiting. water, water channels. According to the model, there is a rapid exchange of water between parallel radial pathways because, in contrast to solutes such as nutrient ions, water per- Introduction meates cell membranes readily. The roles of apoplastic One of the essential functions of roots is to supply the barriers (Casparian bands and suberin lamellae) in the shoot with water from the soil. The process of water root’s endo- and exodermis are discussed. The model