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Root hydraulic conductance and conductivity are physiological traits describing the ease with which water can move through the belowground vascular system of a plant, and are used as indicators of plant performance and adaptability to a given environment. The ability to measure hydraulic conductance of container-grown herbaceous and semiwoody plants with soft conductive tissue was tested using a hydraulic conductance flowmeter (HCFM). Although the HCFM is a hydraulic apparatus that has been used on woody plants to measure hydraulic conductance of intact roots, it has never been reportedly used on container-grown horticultural plants. Two herbaceous species, Chrysanthemum L. and Solenstemon scutellarioides Thonn., were grown in containers and hydraulic parameters were measured, including root conductance and root conductivity, as well as physical traits such as stem diameter and dry root mass. The HCFM was easily connected to intact roots even on herbaceous stems and was used to determine hydraulic conductance and conductivity directly on container-grown plants with minimal disturbance on the root system. Chrysanthemums, Buddleja davidii Franch., and Hibiscus moscheutos L. were grown in three different substrates, and both root mass and root hydraulic parameters were determined. Chrysanthemums showed a positive response with increasing root hydraulic conductance with increasing root mass. The substrates used in these studies only had an effect on root biomass of chrysanthemums, but substrates had no differential effect on root hydraulic conductivity. Water transport in plants is influenced by factors from the surrounding environment, as well as characteristics from the plant structure, root morphology, and anatomy. Over several decades, efforts have been made to better understand water uptake and transport in plants. The flow of water through plants has been commonly analyzed using the parameters of flow rate and the difference in water potentials between the rhizosphere, roots, and leaves. Alternative measures are hydraulic conductivity and hydraulic conductance (reciprocal of hydraulic resistance) (Passioura and Munns, 1984; Ramos and Kaufmann, 1979; Rieger and Litvin, 1999). Hydraulic conductance (Kr) is a measure of the efficiency of bulk flow through plant shoots or roots and defined as the flow rate per unit pressure driving force. Hydraulic conductivity (krdm) in a plant describes the ease with which water can move through a given section of the vascular system but is defined as the hydraulic conductance per unit dry mass or conductive area of the root segment transporting water (Tyree, 2003). Plant hydraulic conductivity can be influenced by physical traits, such as size and density of vessels in the xylem and phloem (Tataranni et al., 2012). However, most resistances to hydraulic movement in vascular plants occur in the roots (Tataranni et al., 2012). Root resistance to water flow determines the water status of the shoot, even under well-watered conditions (Markhart and Smit, 1990), and imposes the greatest resistance to liquid water flow in the soil– plant–atmosphere continuum (Rieger and Litvin, 1999). Root architecture, morphology, and anatomy affect the degree of hydraulic resistance; radial and axial pathways, apoplastic barriers, aquaporin activity, exoand endodermis layers (presence and/or thickness), cortex presence, and/or thickness are all possible anatomical characteristics affecting plant hydraulic resistance (Miyamoto et al., 2001; Rieger and Litvin, 1999; Steudle, 2000; Tataranni et al., 2012). It is well known that the hydraulic properties of roots vary with species and environmental conditions (Domec et al., 2004; Miyamoto et al., 2001), and these conditions can strongly influence root morphology and anatomy (Domec et al., 2010; Steudle and Peterson, 1998). Pine wood–based substrates generally involves the use of entire pine trees (i.e., bark, wood, cambium, and needles), which is a departure from using just pine bark (PB) in the greenhouse and nursery industry, and these wood-based substrates have been shown to have suitable physical and chemical properties (compared with PB or peat) while enhancing/increasing root growth (Gruda and Schnitzler, 2004a, 2004b; Jackson et al., 2010; Wright and Browder, 2005). With the reported increase in root growth of plants grown in wood substrates, measuring the hydraulic conductivity of root systems grown in substrates containing wood components may be insightful and help explain the growth increase/ substrate effect. New instruments have been developed to measure hydraulic conductance/resistance with rapid water-flow measurements. One such device is the commercially available hydraulic conductance flow meter (Dynamax, Inc., Houston, TX). The HCFM can measure the hydraulic conductance of shoots and root systems by connecting the device to a branch, stem, or rootstock. To measure entire root systems, shoots are excised from the root system a few centimeters above substrate level and the rootstock or shoot stem is fitted with water-filled tubing of the HCFM. The HCFM uses constantly increasing pressure to cause water to flow into the attached root or shoot system, and this water flows in the opposite direction of normal transpiration (Tsuda and Tyree, 2000; Tyree et al., 1995). The HCFM has mainly been used to measure stem, leaf, and root hydraulic conductivities on woody plants: Vitis vines (Vandeleur, 2007; Vandeleur et al., 2009, 2014), Prunus plants (Tataranni et al., 2012), and Tsuga trees (Domec et al., 2013). Using a device called the high pressure flow meter, root hydraulic conductance has been positively correlated to stem diameter (Tyree et al., 1995), root system surface area (Muhsin and Zwiazek, 2002; Tyree et al., 1995), and root length (Smith and Roberts, 2003). Rieger and Litvin (1999), using another method, measured Kr of several plants with differing root anatomical features and found correlation was inversely related to both root diameter and cortex width. The question remains if there are possible correlations between increasing hydraulic conductance and increasing whole root system mass. Received for publication 5 Oct. 2015. Accepted for publication 31 Dec. 2015. This article is a portion of a thesis submitted by Lesley A. Judd as required to fulfill degree requirements. Graduate Research Assistant. Associate Professor. Professor. Corresponding author. E-mail: brian_ jackson@ ncsu.edu. 192 HORTSCIENCE VOL. 51(2) FEBRUARY 2016 The objectives of this study were to 1) explore the feasibility of measuring root hydraulic conductance and hydraulic conductivity of container-grown herbaceous and semiwoody plants using the HCFM; 2) investigate possible correlations between hydraulic conductivity and anatomical features, including whole root mass and plant stem diameter, and 3) investigate the potential influence of substrates containing wood components on plant root mass as measured by root hydraulic conductivity. Materials and Methods Operation of the HCFM. The HCFM is an apparatus designed to inject water into the base of either a root system, shoot system, or leaf stem while changing the delivery pressure and simultaneously measuring the flow of water. The operation of the HCFM is fully described in detail by Tyree et al. (1995). For all studies conducted (plants tested) in the series of experiments that are outlined and described in this article, the following protocol was used; container-grown plants were removed from the greenhouse and placed in a temperature-controlled room (21 C; Cochard et al., 2000) due to the apparatus having a calibration factor, which is a function of the viscosity of water passing through the capillary tubes, as well as a function of the temperature dependence of the electronics in the system (Tyree et al., 1995). The plants were then placed into a plastic tub that was filled with water until level with the top of the substrate in the containers, and then allowed to sit/saturate for 30 min. This process was done to saturate the substrate (root ball) and pores around the roots, thus removing air from all pores and minimizing moisture content and pressure potential differences in the containers/root balls. After 30 min and while still saturated, plant stems were severed 3 cm above the substrate surface. The HCFM connection was immediately attached on the severed stem of the root ball (root system) and the HCFM began measuring root hydraulic conductance. Water was then systematically forced into the root system so as to not disrupt the roots but actually push the water throughout the root system, thereby obtaining a measure (conductivity) of the entire root mass at once. Root conductance, Kr, was computed from the slope of the linear regression between water flux (in kilograms per second) and pressure (range 0 to 0.55 MPa) supplied from the HCFM. The Kr measurement was used in testing for possible correlations between root hydraulic conductance and root dry mass or stem diameter, respectively, to observe any effects of these covariables. Chrysanthemum. Greenhouse experiments were conducted on Chrysanthemum ‘Garden Alcala Red’ grown in four different substrates: peat amended with 20%, 30%, or 40% (v/v) shredded pine wood (SPW) and a traditional 80 peat : 20 perlite (PL; by volume) substrate, which was used as a control. On 19 Dec. 2011, 8-year-old loblolly pine trees (Pinus taeda L.) were harvested in Chatham County, NC, at ground level, delimbed, and stored under shelter from weather. On 2 Jan. 2012, the delimbed pine logs were shredded in a wood shredder (Morbark WoodHogmodel 3800;Morbark, Winn, MI). The SPW was then processed in a hammer-mill through a 6.35-mm screen (Meadows Mills, North Wilkesboro, NC) to produce SPW. The SPW had an initial moisture content after milling of 45%. The material was then air-dried to a moisture content of 35%, after which it was stored under shelter in unsealed 1.5 m bags until t

Measuring Root Hydraulic Parameters of Container-grown Herbaceous and Woody Plants Using the Hydraulic Conductance Flow Meter