Precision irrigation of ornamental plants can be a difficult task for nursery growers as a result of the lack of quantitative information regarding the specific water needs of different plant species. To prevent drought stress and ultimately crop losses and/or reductions in growth rate resulting from dehydration, many growers apply excessive amounts of irrigation (Kim et al., 2011; Mathers et al., 2005). This strategy can lead to leaching and runoff of fertilizer and pesticides from the substrate. Majsztrik et al. (2011) and Million et al. (2007) have shown that by reducing leaching of fertilizer from container-grown ornamentals, nurseries can reduce production costs and increase profits. Previous research has also shown that efficient irrigation systems and proper scheduling can save significant amounts of irrigation water without adversely affecting crop yield or quality in ornamental production (Bacci et al., 2008; Beeson, 2012; Fereres et al., 2003). By using soil moisture sensor-based, automated irrigation systems, precision irrigation can be effectively implemented in ornamental plant production (Chappell et al., 2013). One critical piece of information for the implementation of soil moisture sensor-based irrigation is the substrate water content at which plants need to be irrigated. Because not all water in substrates is available to plants, it is important to know how much of the water in the substrate can be used by plants. It is generally proposed that plants can no longer take up water from a soilless substrate at a VWC less than 0.20 m3·m−3 (Drzal et al., 1999; Milks et al., 1989b). However, previous studies have shown that various species can grow below this proposed threshold for water availability (e.g., Burnett and van Iersel, 2008; van Iersel et al., 2010). Thus, the effect of VWC on the ability of different species to take up water from soilless substrates needs further research.
When plants are exposed to decreasing water availability, they respond by progressively closing their stomates to reduce transpiration and prevent dehydration (Sperry et al., 2002; Tezara et al., 1999), although the severity of the drought response is species-specific (Niu et al., 2006). Some of the more drought-tolerant plants can undergo osmotic adjustment in the root and leaf tissues that enable the plant to preserve the water potential gradient necessary to facilitate water uptake under drought conditions (Hsiao and Xu, 2000). As the water content within the plant decreases, cells will begin to lose turgor, resulting in decreased leaf expansion and root elongation. When cell turgor approaches zero, leaves wilt (Taiz and Zeiger, 2002). When the substrate becomes too dry for a plant to extract water from the substrate, the plant will no longer be able to maintain transpiration, ultimately causing the death of the plant. The soil/substrate water potential at which plants can no longer take up enough water to recover, even if transpiration is negligible, has been termed the permanent wilting point (PWP) (Tolk, 2003). A common assumption for field crops is that there is no plant-available water at a soil water potential less than –1.5 MPa based on research conducted by Furr and Reeve (1945) on the PWP of sunflowers. As a consequence, a pressure plate apparatus is commonly used to determine the VWC at which the matric potential is –1.5 MPa to determine the PWP in different soils/substrates (Tolk, 2003). However, the actual water potential threshold for the PWP of different plants is dependent on the relationship among plant species, soil type, and weather (Taiz and Zeiger, 2002; Tolk, 2003). In addition, the ability of plants to take up water from soils or soilless substrates not only depends on the matric potential, but also on the hydraulic conductivity of that soil or substrate. The hydraulic conductivity of soils and substrates decreases as the soil or substrate dries out (Campbell and Campbell, 1982). As a consequence, low hydraulic conductivity may slow water movement in drying substrates and thus limit plant water uptake (van Iersel et al., 2013).
When comparing soilless substrates and soil, VWC at a matric potential of –1.5 MPa differs significantly between a mineral soil (0.162 m3·m−3; Cecil clay loam) and a bark-based substrate [0.215 m3·m−3; 3 bark:1 sand:1 peat (v:v:v)] (Milks et al., 1989b). Drzal et al. (1999) suggested that water present in a soilless substrate at a water potential below –1.5 MPa is bound within ultramicropores and is unavailable to plants based on the pressure/tension required to extract such water in a laboratory setting. However, when applied to actual plant material grown in soilless substrates, moisture release theory may not accurately reflect the ability of plants to take up water from soilless substrates. Lobet et al. (2014) recently emphasized the importance of combining soil and plant hydraulic properties for predicting plant water uptake and the same likely holds true for soilless substrates.
In studies conducted on the water requirements of bedding plants in peat-based substrates, a VWC of 0.15 m3·m−3 was not low enough to cause a severe inhibition of growth in vinca (Catharanthus roseus), petunia (Petunia ×hybrida) (Nemali and van Iersel, 2005), or chrysanthemum (Chrysanthemum ×morifolium) (Olson et al., 2002). van Iersel and Dove (2005) also concluded that there was no effect of VWC on whole-plant photosynthesis of abelia (Abelia ×grandiflora) or hydrangea (Hydrangea macrophylla) at a VWC greater than 0.15 m3·m−3 in a bark-based substrate and that wilting did not occur until VWC reached 0.06 m3·m−3 for abelia and 0.08 m3·m−3 for hydrangea. Thus, plants can grow at VWC levels well below the commonly used 0.20 m3·m−3 threshold for plant-available water. Plant water use at different VWCs may be more indicative of the actual availability of substrate water to plants than measuring substrate hydraulic properties. Therefore, we conducted this study to quantify the hydraulic properties of a bark-based substrate and to determine the relationship between substrate VWC and plant water uptake and conductance in Hydrangea macrophylla and Gardenia jasminoides. Our objectives were to determine how much of the water present in a pine bark-based substrate is actually plant-available and to test whether this is species-dependent, as suggested by van Iersel and Dove (2005).
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