During propagation of plant cuttings, high humidity and frequent mist irrigation are provided to hydrate unrooted cuttings, encourage callus development, and stimulate adventitious root formation (Santos et al., 2011); however, overwatering can potentially delay rooting and increase disease risk (Chérif et al., 1997; Heiskanen, 1995; Leakey, 2004). An appropriate combination of substrate selection and irrigation practices is therefore needed to balance adequate supply of water for propagule hydration and oxygen supply for root respiration, both of which are requirements for rapid root growth, development, and subsequent plant health (Bilderback and Lorscheider, 1995; Reisch, 1967).
The combination of short container height and fine substrate particles in propagation increases risk of inadequate gas exchange in the substrate. A wide range of substrates and amendments is used during young plant production, including peat (sphagnum), bark, coir, wood fiber, vermiculite, perlite, phenolic foam, and rockwool (Fonteno and Nelson, 1990; Handreck and Black, 2002; Milks et al., 1989a). Fine particle sizes of these components are often required to evenly fill small container cells, resulting in decreased pore size and higher water retention (da Silva et al., 1993). Container size modifies the ratio of water and air within the substrate, because as column height decreases, there is an increased proportion of water and corresponding reduction in air (Argo et al., 1996; Handreck and Black, 2002; Milks et al., 1989b, 1989c; Rivière and Caron, 2001). Physical properties of propagation substrates are further modified by the use of “stabilized” substrates, which include phenolic foam, peat-polymer blends, fabric-wrapped cells, and other materials that hold the substrate together negating the need for a complete root ball and allowing for a shorter crop cycle and a reduction in transplanting stress (Huang and Fisher, 2014).
For gravimetric analysis of physical properties, substrates at a standardized volume and level of compaction are weighed at full saturation, after drainage, and after drying to quantify VWC, VAC, volumetric solid content (VSC), and dry bulk density (Fonteno, 1993). The porometer method (Fonteno, 1993) uses a 348 cm3 standard volume that can be modified for propagation plug cells with a shorter column height (Milks et al., 1989c). The maximum and minimum ratio of water to air at container capacity (which is the field capacity within a particular container type) describes the substrate water-air relations, with typical levels of 45% to 65% water and 10% to 30% air space with a variety of substrates (Bilderback et al., 2005) using the volume pressure plate extractor (Milks et al., 1989a) in 348-cm2 containers (7.3-cm diameter × 7.6-cm height). However, water and air balance is highly dependent on container size and shape, with air-filled porosity decreasing from 19% to less than 1% as substrate height decreased from a 15-cm-tall pot to a 1.3-cm-tall seedling plug tray based on a modeled peat-vermiculite substrate (Caron and Nkongolo, 1999). Gravimetric measurements also can be made directly in propagation trays, or with individual stabilized cells that do not easily conform to a standard porometer shape and volume (Huang and Fisher, 2014). A survey of commercial propagation substrates found widely differing physical properties, with loose-filled products having VWC from 57% to 86% and VAC ranging from 4.8% to 9.7% in 25 cm3 cells. In contrast, stabilized substrates had a VWC from 37% to 91% and VAC between 1.9% and 5.9% in 10- to 28-cm3 cells (Huang and Fisher, 2014). A limitation of gravimetric analysis at one moisture level is that it ignores the dynamic change in air and water as moisture level changes during crop production (Caron and Nkongolo, 1999).
Water potential of a substrate, and the relationship between VWC and plant available water, are usually measured as a substrate dries from saturation over time by measuring the tension of water using a ceramic-tip tensiometer (Wallach, 2008). Moisture retention curves (MRCs) describe water availability for uptake by plant roots. Water held in substrate below 50 cm of tension has been defined as easily plant available water, 50 to 100 cm describes water-buffering capacity that is available to plants during periods of rapid transpiration, whereas tension above 100 cm may not be available for plant root uptake (DeBoodt and Verdonck, 1972; Naasz et al., 2005). In horticultural production in containers, the tension at which wilting occurs depends on the plant species and growing conditions (DeBoodt and Verdonck, 1972). For example, Kiehl et al. (1992) found that potted chrysanthemum grown under moist conditions wilted above 10 kPa (102 cm), and recommended automatic irrigation triggered at a tension of 5 kPa (51 cm). Because plant cuttings initially have limited or no roots, the moisture level is typically maintained close to container capacity during callus formation (Gislerød, 1983; Healy, 2008), and gradually changes to wet-dry cycles following the emergence of adventitious roots to provide aeration (Loach, 1988). In propagation, many cells are also shorter than 5 cm (Huang and Fisher, 2014; Milks et al., 1989c), and differences in moisture and air level at low tensions are therefore of great importance.
An alternative method to generate an MRC is the use of the frozen column method, whereby the substrate is brought to field capacity, frozen, and then sectioned to quantify VWC and VAC within each vertical section (Altland et al., 2010; Dane and Hopmans, 2002; Owen and Altland, 2008). The VWC using the frozen column method allows a comparison between the water potential from the column height to the water potential measured using a tensiometer. In this study, Altland et al. (2010) found similar, but statistically different, MRCs for bark-based substrates tested with either the pressure plate or frozen column (core) methods, with the pressure plate method estimating a higher water content at saturation, slightly lower moisture levels at tensions below 10 cm, and similar moisture levels at higher tensions compared with the frozen column method.
Through a more recently developed method, X-ray CT, the root zone microenvironment of water and air relations can be quantified and visualized (Daly et al., 2015; Nimmo, 2004) in addition to root morphology (Tracy et al., 2013, 2015a, 2015b). Daly et al. (2015) CT-scanned clay and sand substrates at different moisture levels and found that estimating or modeling VWC and VAC and other physical properties by CT provided a complementary method to using a ceramic plate or gravimetric measurement.
Because of the small container size, specialized materials, and the high moisture conditions in propagation, standard testing protocols need to be modified for quality control testing in propagation substrates (Huang and Fisher, 2014). The objective of this study was to quantify and compare substrate water and air relations of three propagation substrates (peat, rockwool, and phenolic foam) that varied widely in physical characteristics by using four methods: 1) MRCs by evaporation, 2) frozen column, 3) gravimetric analysis, and 4) CT analysis. The goal was to identify strengths and weaknesses of each method for quality control testing for propagation substrates and to inform substrate selection and irrigation management for plant propagation.
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