Water uptake by roots: effects of water deficit

The variable hydraulic conductivity of roots (Lp(r)) is explained in terms of a composite transport model. It is shown how the complex, composite anat omical structure of roots results in a composite transport of both water an d solutes. In the model, the parallel apoplastic and cell-to-cell (symplast ic and transcellular) pathways play an important role as well as the differ ent tissues and structures arranged in series within the root cylinder (epi dermis, exodermis, cortex, endodermis, stelar parenchyma). The roles of Cas parian bands and suberin lamellae in the root's endo- and exodermis are dis cussed. Depending on the developmental state of these apoplastic barriers, the overall hydraulic resistance of roots is either more evenly distributed across the root cylinder (young unstressed roots) or is concentrated in ce rtain layers (exo- and endodermis in older stressed roots). The reason for the variability of root Lp(r), is that hydraulic forces cause a dominating apoplastic flow of water around protoplasts, even in the endodermis and exo dermis. In the absence of transpiration, water flow is osmotic in nature wh ich causes a high resistance as water passes across many membranes on its p assage across the root cylinder. The model allows for a high capability of roots to take up water in the presence of high rates of transpiration (high demands for water from the shoot). By contrast, the hydraulic conductance is low, when transpiration is switched off. Overall, this results in a non- linear relationship between water flow and forces (gradients of hydrostatic and osmotic pressure) which is otherwise hard to explain. The model allows for special root characteristics such as a high hydraulic conductivity (wa ter permeability) in the presence of a low permeability of nutrient ions on ce taken up into the stele by active processes. Low root reflection coeffic ients are in line with the idea of some apoplastic bypasses for water withi n the root cylinder. According to the composite transport model, the switch from the hydraulic to the osmotic mode is purely physical. In the presence of heavily suberized roots, the apoplastic component of water flow may be too small. Under these conditions, a regulation of radial water flow by wat er channels dominates. Since water channels are under metabolic control, th is component represents an 'active' element of regulation. Composite transp ort allows for an optimization of the water balance of the shoot in additio n to the well-known phenomena involved in the regulation of water flow (gas exchange) across stomata. The model is employed to explain the responses o f plants to water deficit and other stresses. During water deficit, the coh esion-tension mechanism of the ascent of sap in the xylem plays an importan t role. Results are summarized which prove the validity of the coehesion/te nsion theory. Effects of the stress hormone abscisic acid (ABA) are present ed. They show that there is an apoplastic component of the flow of ABA in t he root which contributes to the ABA signal in the xylem. On the other hand , (+)-cis-trans-ABA specifically affects both the cell level (water channel activity) and water flow driven by gradients in osmotic pressure at the ro ot level which is in agreement with the composite transport model. Hydrauli c water flow in the presence of gradients in hydrostatic pressure remains u nchanged. The results agree with the composite transport model and resemble earlier findings of high salinity obtained for the cell (Lp) and root (Lp, ) level. They are in line with known effects of nutrient deprivation on roo t Lp, and the diurnal rhythm of root Lp, recently found in roots of Lotus.