Policy analysis of water management for the Netherlands. Vol XI: Water distribution model

This volume describes the Water Distribution Model (DM), the central model in PAWN's analysis methodology. The DM is a computer simulation of the water distribution system in the Netherlands. It determines water demands over time, allocates water resources to help meet the demands, calculates the concentrations of several water pollutants throughout the distribution system, calculates losses to agriculture and shipping due to water shortages and water salinity, and provides output from which the costs of meeting water thermal standards can be determined for the electrical power industry. The DM was used in the screening stage of the PAWN analysis to estimate the expected monetary benefits (or disbenefits) to agriculture and shipping from implementation of the technical and managerial tactics being evaluated. In the impact assessment stage of the analysis, the DM was used to determine the monetary benefits (or disbenefits) of promising water management policies to agriculture, shipping, and the electrical power generating industry, and to determine any effects of these policies on pollutant concentrations throughout the country.PAW

Activation kinetics of the K+ outward rectifying conductance (KORC) in xylem parenchyma cells from barely roots.

The activation kinetics of outward currents in protoplasts from barley root xylem parenchyma was investigated using the patch-clamp technique. The

Evidence for discontinuous water columns in the xylem conduit of tall birch trees

The continuity of the xylem water columns was studied on 17- to 23-m tall birch trees (trunk diameter about 23 cm; first branching above 10 m) all year round. Fifty-one trees were felled, and 5-cm thick slices or 2-m long boles were taken at regular, relatively short intervals over the entire height of the trees. The filling status of the vessels was determined by (i) xylem sap extraction from trunk and branch pieces (using the gas bubble-based jet-discharge method and centrifugation) and from trunk boles (using gravity discharge); (ii) (1)H nuclear magnetic resonance imaging of slice pieces; (iii) infusion experiments (dye, (86)Rb(+), D(2)O) on intact trees and cut branches; and (iv) xylem pressure measurements. This broad array of techniques disclosed no evidence for continuous water-filled columns, as postulated by the Cohesion-Tension theory, for root to apex directed mass transport. Except in early spring (during the xylem refilling phase) and after extremely heavy rainfall during the vegetation period, cohesive/mobile water was found predominantly at intermediate heights of the trunks but not at the base or towards the top of the tree. Similar results were obtained for branches. Furthermore, upper branches generally contained more cohesive/mobile water than lower branches. The results suggest that water lifting occurs by short-distance (capillary, osmotic and/or transpiration-bound) tension gradients as well as by mobilisation of water in the parenchymatic tissues and the heartwood, and by moisture uptake through lenticels

Differential regulation of K+ channels in Arabidopsis epidermal and stelar root cells.

The patch clamp technique was applied to protoplasts isolated from the epidermis and pericycle of Arabidopsis roots and their plasma membrane currents investigated. In the whole cell configuration, all protoplasts from the epidermis exhibited depolarization-activated time-dependent outwardly rectifying (OR) currents whereas OR currents were present in only 50% of cells from the pericycle. The properties of the OR currents in the epidermis and pericycle were compared with respect to their selectivity, pharmacology and gating. The time-dependent activation kinetics, selectivity and sensitivity to extracellular tetraethyl ammonium of the OR current in each cell type were not significantly different. The reversal potential (Erev) of the OR currents indicated that they were primarily due to the movement of K+. However, the gating properties of the OR currents from the epidermis differed markedly from those exhibited in the pericycle. Although both cell types displayed OR currents with voltage-dependent gating modulated in a potassium-dependent fashion [i.e. the activation threshold (V0.5) was displaced to more positive voltages as extracellular K+ increased], the OR currents in the epidermis also displayed voltage-independent gating by extracellular K+ which dramatically regulated current density. In the present study, reducing extracellular K+ activity from 40 to 0.87 mm reduced the OR current density in epidermal cells by approximately 80%. The chord conductance of the OR current saturated as a function of extracellular K+ and could be fitted with a Michaelis–Menten function to yield a binding constant (Km) of 10.5 mm. The ability of other monovalent cations to substitute for K+-gating of the OR currents was also investigated and shown to exhibit a relative sequence of K+ ≥ Rb+ > Cs+ > Na+ ≥ Li+ (Eisenmann sequence IV) with respect to efficacy of gating. Furthermore, single channel recordings demonstrated that channel activity rather than the single channel conductance was modulated by extracellular K+. In contrast, OR current density in the pericycle was largely independent of extracellular K+. It is suggested that the contrasting gating properties of the K+ channels in the epidermis and pericycle reflect their different physiological roles, particularly with respect to their role in K+ (nutrient) transport from the soil solution to the shoot