The Influence of Substrate Hydraulic Conductivity on Plant Water Status of an Ornamental Container Crop Grown in Suboptimal Substrate Water Potentials

in HortScience

Many soilless substrates are inefficient with regard to water (i.e., high porosity and low water holding capacity), which provides an excellent opportunity to increase water efficiency in containerized production. We suggest that increasing hydraulic conductivity in the dry range of substrate moisture content occurring during production can increase water availability, reduce irrigation volume, and produce high quality, marketable crops. Three substrates were engineered using screened pine bark (PB) and amending with either Sphagnum peatmoss or coir to have higher unsaturated hydraulic conductivity between water potentials of −100 and −300 hPa. There was no correlation between substrate unsaturated hydraulic conductivity and saturated hydraulic conductivity (r = 0.04, P = 0.8985). Established Hydrangea arborescens (L.) ‘Annabelle’ plants were grown in the three engineered and a conventional (control) PB substrates exposed to suboptimal irrigation levels (i.e., held at substrate water potentials between −100 and −300 hPa) for 32 days. The plants in the engineered substrates outperformed the control in every growth and morphological metric measured, as well as exhibiting fewer (or no) physiological drought stress indicators (i.e., vigor, growth, plant development, etc.) compared with the control. We observed increased vigor measures in plants grown in substrates with higher unsaturated hydraulic conductivity, as well as greater plant water uptake. The coir increased unsaturated hydraulic conductivity and provided an increased air space when incorporated into coarse bark vs. if peat was incorporated into bark at the same ratio by volume. Increasing PB hydraulic conductivity, through screening bark or amending bark with fibrous materials, in concert with low irrigations can produce marketable, vigorous crops while reducing water consumed and minimizing water wasted in ornamental container production.

Contributor Notes

Funding for this work was provided in part, by the Virginia Agricultural Experiment Station and the Hatch Program of the National Institute of Food and Agriculture including the Specialty Crop Research Initiative Project Clean WateR3 (2014-51181-22372), the U.S. Department of Agriculture.

This article is a portion of a dissertation submitted by Jeb S. Fields to fulfill requirements of PhD degree.

We thank James Altland, Marc van Iersel, and Joshua Heitman for their comprehensive reviews. We would also like to thank Pacific Organics and FibreDust, LLC for contributing substrate supplies to this study. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by Virginia Tech and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

Corresponding author. E-mail: jsowen@vt.edu.

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    Hydraulic conductivity models for substrate water potential between −100 and −300 hPa, based on data from evaporative moisture tension and hydraulic conductivity measures of four experimental bark-based substrates. Substrates include a control (unprocessed bark, UB), bark particles < 4 mm (fine bark, FB), bark > 4 mm with 35% by vol. Sphagnum peatmoss (bark-peat, BP), and bark > 4 mm with 35% by vol. coir (bark-coir, BC).

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    Moisture characteristic data (points) fit to a Brooks and Corey (1964) model (line) for four experimental bark-based substrates. Data measured via evaporative method, porometer, and dewpoint potentiametry. Substrates include a control unprocessed bark (A), bark particles < 4 mm (B), bark > 4 mm with 35% by vol. Sphagnum peatmoss (C), and bark > 4 mm with 35% by vol. coir (D).

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    Growth index of containerized plants grown in four experimental substrates at substrate water potentials between −100 and −300 hPa for 32 d. Plant growth index was normalized to at the initiation of the research to demonstrate changes over the experimental production period. Substrates include a control (unprocessed bark, UB), bark particles < 4 mm (fine bark, FB), bark > 4 mm with 35% by vol. Sphagnum peatmoss (bark-peat, BP), and bark > 4 mm with 35% by vol. coir (bark-coir, BC).

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    A digital image of a representative plant from each of the four experimental substrate treatments collected 32 d after initiation of the low substrate water potential irrigation management. Substrates include a control (unprocessed bark, UB), bark particles < 4 mm (fine bark, FB), bark > 4 mm with 35% by vol. Sphagnum peatmoss (bark-peat, BP), and bark > 4 mm with 35% by vol. coir (bark-coir, BC).

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    The reduction in volumetric water content of four experimental pine bark-based substrates used to produce Hydrangea arborescens plants. Substrates included conventional pine bark (unprocessed bark, UB), bark particles that pass through a 4.0-mm screen (fine bark, FB), bark particles that do not pass through a 4.0-mm screen while at 65% moisture content amended with fibrous materials including 35% Sphagnum peat (bark-peat, BP) and 35% coir (bark-coir, BC) by volume. Substrates with fully rooted plants were watered to effective container capacity (maximum water holding capacity via spray stake irrigation) before allowing to dry past permanent wilt until the plant ceased withdrawing water from the substrate. Daily reduction in substrate volumetric water contents were plotted against volumetric water content for each substrate to illustrate at what volumetric water content evapotranspiration shifts to primarily evaporation due to plant water uptake diminishing.

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