Fresh water is a limited resource that is necessary for the production of all plants. Forty percent of freshwater withdrawn in the United States is used for irrigation of crops (Kenney et al., 2009). Furthermore, plants in intensively controlled container production systems must receive quality fresh water daily or multiple times per day, in the absence of precipitation, to prevent actual or perceived plant water stress. Because of this, growers often apply excess water to container crops to alleviate concerns of underwatering that could render the plant unmarketable or delay time to sale (Mathers et al., 2005). This has led to container nurseries applying upward of 180 m3 of irrigation per hectare per day during the warm season (Fulcher and Fernandez, 2013). With potential future water restrictions, growers will have to adopt more sustainable cultural practices to thrive.
Most container nurseries use overhead irrigation on all or a portion of their operation and may not have the infrastructure to switch to more sustainable irrigation systems (Beeson et al., 2004). Therefore, growers must expand their efforts beyond irrigation technology to increase water sustainability (Fulcher et al., 2016). One area where growers can make modifications that provide potential water savings, without additional infrastructure, is selecting more sustainable soilless substrates (Barrett et al., 2016). Substrates with increased sustainability would include those that increase water storage capacity or more effectively deliver stored water to the plant. Conventional soilless substrates were initially developed to provide growers with increased control over the container system. Substrates are highly porous so that they drain rapidly, prevent salt stress, and are initially pathogen-free (Raviv and Lieth, 2008). Furthermore, these substrates were developed to allow containers to receive excess water from precipitation without concerns of flood stress as observed in some mineral soils. As a result, the current best management practices (BMPs) for container nursery production recommend maximum water holding capacity or container capacities (CCs) >45% and air spaces (ASs) <30% of the container volume (45% to 65% by vol. and 10% to 30% by vol., respectively; Bilderback et al., 2013). These increased AS values, compared with a field soil, allude to the primary focus of substrate design being able to release water as opposed to water retention or storage. Furthermore, conventional wisdom based on past research infers that the degree of water availability has strict cutoffs of easily available water [between −10 and −50 hPa substrate water potential (Ψ)] and water buffering capacity (Ψ between −50 and −100 hPa), with all water held at Ψ < −100 hPa not readily accessible to plants (de Boodt and Verdonck, 1972; Pustjarvi and Robertson, 1975). We believe substrates should provide a better balance of sufficient drainage and water retention. Such substrates should retain water during and after irrigation events to reduce the volume of water required to grow containerized crops.
As substrate science develops, understanding more about using dynamic hydraulic properties as measures of substrate productivity as it relates to resource (i.e., water and mineral nutrient) sustainability is becoming imperative (Caron et al., 2014). For example, moisture characteristic curves (MCC) provide information on dynamic hydraulic properties that depict the relationship between volumetric water contents (VWCs) and Ψ (Bunt, 1961). Better defining the relationship between hydraulic conductivity (K) and MCC provides information on substrate environmental sustainability (through increased resource retention; Naasz et al., 2005) and water availability (O’Meara et al., 2014).
While the relationships between substrate K and Ψ or VWC are not commonly measured, saturated hydraulic conductivity (Ks) is increasingly used. Concurrent work by the authors demonstrated little correlation (r = −0.32, P = 0.1536) between optimal production K (Ψ = −75 hPa) and measured Ks for bark-based substrates (Fields, 2017). This is due to K being a limiting factor for water uptake by roots in soilless substrates (Raviv et al., 1999) and field soils (Campbell and Campbell, 1982). Historically, unlike Ks, unsaturated K has been difficult to accurately measure (Raviv et al., 1999). However, measuring substrate K can aid in irrigation decisions and help reduce water stress in container production (da Silva et al., 1993). Therefore, using more recently developed substrate hydraulic property measurements, which allow for accurate measures of K, we may be able to maximize water distribution and use in container substrates, thereby reducing water consumption by container nurseries.
One metric used to measure plant response to modified substrates in regard to water dynamics is water use efficiency (WUE). WUE has been described in numerous ways, from intrinsic (rate of carbon assimilation: rate of transpiration) to integrated (biomass produced: total transpiration), all of which provide useful information regarding plant-water interactions (Bacon, 2004). However, these measures may not be as important for ornamental growers, as growth alone may not be the most influential factor in sales. Another metric to measure plant-water response to modified substrates is water availability, which affects crop stress, time to market, and corresponding nutrient availability. Water availability is a measure of percentage of water held by a substrate that a plant can use to sustain life. This metric may be beneficial to producers attempting to grow with reduced water. Other metrics that should be considered are drought stress indicators, many of which are measurable. Each metric has a value to researchers and when used in concert it can provide information to the water dynamics of the substrate-plant system holistically.
The goal of this research was to determine if substrates engineered to have optimal hydraulic properties could continue to produce a quality, salable Hydrangea arborescens crop grown at Ψ, which was previously considered unfavorable for container production. Furthermore, we wanted to determine how suboptimal Ψ influences crop physiology and morphology. Finally, we wanted to determine differences in plant water availability between the substrates engineered to have increased K vs. an unaltered bark substrate conventionally used in the mid-Atlantic and southeast nursery industry. We hypothesized that these engineered substrates will provide the plant with access to higher proportions of water and increase the WUE, while reducing indicators of drought stress, which are common to plants grown at low Ψ in traditional bark substrate. Moreover, the readily-available commercial materials used for engineering substrates (i.e., peat and coir fibrous materials added to bark) will affect the plant-substrate water dynamics measured through subsequent plant physiology and morphology.
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