Root pressure

Abstract: Xylem vessels in grapevines Vitis labrusca L. and Vitis riparia Michx. growing in New England contained air over winter and yet filled with xylem sap and recovered their maximum hydraulic conductance during the month before leaf expansion in late May. During this period root pressures between 10 and 100 kilopascals were measured. Although some air in vessels apparently dissolved in ascending xylem sap, results indicated that some is pushed out of vessels and then out of the vine. Air in the vessel network distal to advancing xylem sap was compressed at about 3 kilopascals; independent measurements indicated this was sufficient to push air across vessel ends, and from vessels to the exterior through dead vine tips, inflorescence scars, and points on the bark. Once wetted, vessel ends previously air-permeable at 3 kilopascals remained sealed against air at pressures up to 2 and 3 megapascals. Permeability at 3 kilopascals was restored by dehydrating vines below −2.4 megapascals. We suggest that the decrease in permeability with hydration is due to formation of water films across pores in intervascular pit membranes; this water seal can maintain a pressure difference of roughly 2 megapascals, and prevents cavitation by aspirated air at xylem pressures less negative than −2.4 megapascals.

Abstract: A root pressure probe was employed to measure hydraulic properties of primary roots of maize ( Zea mays L) The hydraulic conductivity ( Lp r ) of intact root segments was determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder In hydrostatic experiments, Lp r was constant along the segment except for an apical zone of approximately 20 millimeters in length which was hydraulically isolated due to a high axial resistance In osmotic experiments, Lp r decreased toward the base of the roots Lp r (osmotic) was significantly smaller than Lp r (hydrostatic) At various distances from the root tip, the axial hydraulic resistance per unit root length ( R x ) was measured either by perfusing excised root segments or was estimated according to Poiseuille9s law from cross-sections The calculated R x was smaller than the measured R x by a factor of 2 to 5 Axial resistance varied with the distance from the apex due to the differentiation of early metaxylem vessels Except for the apical 20 millimeters, radial water movement was limiting water uptake into the root This is important for the evaluation of Lp r of roots from root pressure relaxations Stationary water uptake into the roots was modeled using measured values of axial and radial hydraulic resistances in order to work out profiles of axial water flow and xylem water potentials

Abstract: Using root- and cell-pressure probes, the effects of the stress hormone abscisic acid (ABA) on the water-transport properties of maize roots (Zea mays L.) were examined in order to work out dose and time responses for root hydraulic conductivity. Abscisic acid applied at concentrations of 100–1,000 nM increased the hydraulic conductivity of excised maize roots both at the organ (root Lpr: factor of 3–4) and the root cell level (cell Lp: factor of 7–27). Effects on the root cortical cells were more pronounced than at the organ level. From the results it was concluded that ABA acts at the plasmalemma, presumably by an interaction with water channels. Abscisic acid therefore facilitated the cell-to-cell component of transport of water across the root cylinder. Effects on cell Lp were transient and highly specific for the undissociated (+)-cis-trans-ABA. The stress hormone ABA facilitates water uptake into roots as soils start drying, especially under non-transpiring conditions, when the apoplastic path of water transport is largely excluded.

Abstract: A pressure chamber and a root pressure probe technique have been used to measure hydraulic conductivities of rice roots (root Lpr per m 2 of root surface area). Young plants of two rice (Oryza sativa L.) varieties (an upland variety, cv. Azucena and a lowland variety, cv. IR64) were grown for 31–40 d in 12 h days with 500 mmol m � 2 s � 1 PAR and dayunight temperatures of 27 8C and 22 8C. Root Lpr was measured under conditions of steady-state and transient water flow. Different growth conditions (hydroponic and aeroponic culture) did not cause visible differences in root anatomy in either variety. Values of root Lpr obtained from hydraulic (hydrostatic) and osmotic water flow were of the order of 10 � 8 ms � 1 MPa � 1 and were similar when using the different techniques. In comparison with other herbaceous species, rice roots tended to have a higher hydraulic resistance of the roots per unit root surface area. The data suggest that the low overall hydraulic conductivity of rice roots is caused by the existence of apoplastic barriers in the outer root parts (exodermis and sclerenchymatous (fibre) tissue) and by a strongly developed endodermis rather than by the existence of aerenchyma. According to the composite transport model of the root, the ability to adapt to higher transpirational demands from the shoot should be limited for rice because there were minimal changes in root Lpr depending on whether hydrostatic or osmotic forces were acting. It is concluded that this may be one of the reasons why rice suffers from water shortage in the shoot even in flooded fields.