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Katherine F. Garland, Stephanie E. Burnett, Michael E. Day, and Marc W. van Iersel

Two investigations were conducted to determine the morphological and physiological impacts of varying light and substrate water levels on Heuchera americana ‘Dale's Strain’ (american alumroot). Both investigations used a capacitance sensor automated irrigation system to maintain constant substrate volumetric water contents (θ = volume of water/volume of substrate). In the first study, the substrate was maintained at one of eight θ ranging from 0.15 to 0.50 L·L−1. Leaf area of plants grown at the highest θ was more than twice that of plants grown at the lowest θ. Shoot dry weight also responded positively to θ increasing from 0.15 to 0.35 L·L−1, but plants did not have greater dry weights when maintained at θ higher than 0.35 L·L−1. The second experiment assessed american alumroot's performance under four daily light integrals (DLIs) (7.5, 10.8, 14.9, and 21.8 mol·m−2·d−1) with θ maintained at 0.35 L·L−1. Increasing DLI from 7.5 to 21.8 mol·m−2·d−1 caused shoot dry weight, leaf area, maximum width, and leaf count to change quadratically. Dry weight and leaf area reached their maximum at 10.8 mol·m−2·d−1, whereas leaf count was greatest at 14.9 mol·m−2·d−1. Increasing DLI to 21.8 mol·m−2·d−1 negatively impacted leaf area and leaf count but did not lower shoot dry weight. Leaf area ratio and petiole length of the uppermost fully expanded leaf decreased with increasing DLI. Measures of leaf-level net photosynthesis, light response curves, and CO2 response curves indicated no physiological differences among plants grown under different water or light levels. In both studies, long-term, whole crop measures of water use efficiency based on shoot dry weight and water applied (WUEc) did not reflect the same water use trends as instantaneous, leaf-level measures of WUE based on leaf gas exchange (WUEl). WUEc decreased with increasing θ and DLI, whereas WUEl was not influenced by θ and increased with increasing DLI. WUEl is often used to provide insight as to how various abiotic and biotic factors influence how efficiently water is used to produce biomass. However, these findings demonstrate that there are limitations associated with making such extrapolations.

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Shuyang Zhen, Stephanie E. Burnett, Michael E. Day, and Marc W. van Iersel

Two experiments were conducted to determine how different substrate volumetric water contents (θ equals volume of water per volume of substrate) affected morphology and physiology of three popular perennials using a capacitance sensor-automated irrigation system. In the first study, rosemary (Rosmarinus officinalis) was grown at one of eight θ set points ranging from 0.05 to 0.40 L·L−1. In the second study, Canadian columbine (Aquilegia canadensis ‘Pink Lanterns’) and cheddar pink (Dianthus gratianopolitanus ‘Bath’s Pink’) were grown at one of nine θ set points ranging from 0.05 to 0.45 L·L−1. Total leaf number and area as well as shoot fresh and dry weight of rosemary plants grown at θ of 0.20 L·L−1 or greater were approximately twice that of those grown at lower θ. Canadian columbine height increased as θ increased. Leaf area of cheddar pink grown at θ of 0.35 L·L−1 or higher was twice that of plants grown at the lowest θ. Shoot dry weight of Canadian columbine was not significantly affected by θ. Shoot dry weight of cheddar pink responded quadratically to increasing θ and peaked at θ of 0.35 L·L−1. θ also significantly influenced photosynthetic activities; net photosynthetic rate (AN) and stomatal conductance (g s) of Canadian columbine increased with increasing θ. AN of cheddar pink also increased as θ increased. Greater water volumes were applied to maintain higher θ set points. Irrigation water use efficiency (IWUE = shoot dry weight ÷ total amount of water applied per plant) of Canadian columbine and cheddar pink was not influenced by θ. Growth of all three plants was reduced when grown at lower θ; in the case of cheddar pink and Canadian columbine, this was attributable at least in part to reduced AN.

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Katherine F. Garland, Stephanie E. Burnett, Lois B. Stack, and Donglin Zhang

Coleus (Solenostemon scutellarioides) traditionally has been recommended as a shade plant, but many cultivars are also suitable for full sun. In regions of the country where light limits growth and photosynthesis, supplemental lights are used to increase daily light integral (DLI). Understanding the minimum DLI necessary to produce coleus would minimize supplemental lighting use, reducing costs and improving production sustainability. ‘Kong Red’ and ‘Wizard Coral Sunrise’ coleus were grown in a greenhouse under a 12-hour photoperiod and a mean DLI of 2.9, 3.8, 5.8, or 10.0 mol·m−2·d−1 to determine the lowest light level needed to produce high-quality plants. After 8 weeks, both cultivars had a 4.2-fold increase in shoot dry weight as DLI increased from 2.9 to 10.0 mol·m−2·d−1. Plants grown under 10.0 mol·m−2·d−1 were 22% to 25% taller and 18% to 21% wider compared with those grown under 2.9 mol·m−2·d−1. ‘Kong Red’ had 3.6 times as many branches and ‘Wizard Coral Sunrise’ had over twice as many branches when grown under 10.0 mol·m−2·d−1 compared with those grown under the lowest DLI. Leaf counts for both cultivars were 64% greater when grown under the highest DLI compared with those produced under the lowest DLI; leaf area for both cultivars was also positively correlated with DLI. Leaves of both cultivars had significantly more green area (i.e., less variegation) when grown under lower DLIs. Overall, both cultivars exhibited a more dense growth habit and greater degree of variegation when grown under the highest DLI. Therefore, we recommend growing ‘Kong Red’ and ‘Wizard Coral Sunrise’ coleus under a minimum DLI of 10.0 mol·m−2·d−1.

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Stephanie E. Burnett, Svoboda V. Pennisi, Paul A. Thomas, and Marc W. van Iersel

Polyethylene glycol 8000 (PEG-8000) was applied to a soilless growing medium at the concentrations of 0, 15, 20, 30, 42, or 50 g·L-1 to impose controlled drought. Salvia (Salvia splendens F. Sellow. ex Roem & Shult.) seeds were planted in the growing medium to determine if controlled drought affects morphology and anatomy of salvia. Polyethylene glycol decreased emergence percentage and delayed emergence up to 5 days. Stem elongation of salvia treated with the five lowest concentrations was reduced up to 35% (21 days after seeding), and salvia were a maximum of 53% shorter and the canopy was 20% more narrow compared to nontreated seedlings 70 days after seeding. These morphological changes were attributed to PEG-8000 mediated reduction in leaf water potential (Ψw). The growing medium Ψw ranged from -0.29 to -0.85 MPa in PEG-8000 treated plants, and plant height was positively correlated with Ψw 21 days after seeding. Stem diameter of PEG-treated seedlings was reduced up to 0.4 mm mainly due to reductions in vascular cross-sectional area. Xylem cross-sectional area decreased more than stem and phloem cross-sectional area. Polyethylene glycol 8000 reduced vessel element number, but not diameter.

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Bryan J. Peterson, Olivia Sanchez, Stephanie E. Burnett, and Darren J. Hayes

Overhead mist (OM) facilitates the propagation of stem cuttings by preventing transpirational water loss. However, drawbacks to OM include the application of large volumes of water, potentially unsanitary conditions, irregular misting coverage, and leaching of foliar nutrients. We explored three alternatives to OM that might avoid these problems by applying moisture below, rather than overhead. These included 1) a submist (SM) aeroponic system configured to provide intermittent mist only to the rooting zone, 2) a subirrigation (SI) system that provided water via capillary action through perlite from a reservoir maintained below the base of each cutting, and 3) a subfog (SF) aeroponic system that was configured to provide constant fog only to the rooting zone. To initiate each system, we wetted perlite or filled reservoirs using either water or quarter-strength Hoagland solution. Stem cuttings of ‘Wizard Mix’ coleus (Plectranthus scutellarioides) were propagated in the systems for 21 days. Cuttings in the SM system produced more than three times as many roots as cuttings in the OM system, with roots more than six times the length. Root dry weights averaged 28 mg for cuttings in the SM system, compared with only 3.5 mg among cuttings receiving OM. The SF and SI systems produced results broadly comparable to the OM. Fertilizer did not consistently improve rooting measures across the systems. Although we observed few fine roots on cuttings rooted using SM, they transplanted well into a soilless substrate and quickly produced new root growth. The SM system used less than 1/5 the water used by the SI system, and less than 1/50 the water used by the SF system. In comparison, a single OM nozzle operating for 10 seconds released about one-third of the total water lost through transpiration from each SM system over the entire experiment. Our results show that SM systems merit further evaluation for propagation of plants by stem cuttings.

Free access

Marc W. Van Iersel, Sue Dove, Jong-Goo Kang, and Stephanie E. Burnett

More efficient irrigation practices are needed in ornamental plant production to reduce the amount of water used for production as well as runoff of fertilizers and pesticides. The objective of this study was to determine how different substrate volumetric water contents (θ) affected petunia (Petunia ×hybrida) growth and to quantify the daily water use of the plants. A soil moisture sensor-controlled irrigation system was used to maintain θ within ≈0.02 m3·m−3 of the θ threshold values for irrigation, which ranged from 0.05 to 0.40 m3·m−3. Shoot dry weight increased as the θ threshold increased from 0.05 to 0.25 m3·m−3 and was correlated with the total amount of irrigation water applied over the 3-week course of the experiment. The daily water use of the petunias grown with a θ threshold of 0.40 m3·m−3 was 12 to 44 mL/plant and was positively correlated with both plant age and daily light integral. Lower θ thresholds resulted in a decrease in both leaf water (ψ) and osmotic potential (ψS). A decrease in turgor pressure (P) at lower θ was seen at 11, but not 20 days after the start of the treatments. There were no significant effects of θ on ψ, ψS, or P on fully rehydrated plants at the end of the study. Plants were able to survive and grow at all θs, although water at a θ less than 0.20 m3·m−3 is generally considered to be unavailable to the plants. Results show that it is possible to automatically irrigate plants with the use of soil moisture sensors, and this approach to irrigation may have applications in controlling the growth of ornamental plants.

Open access

Stephanie E. Burnett, Bryan J. Peterson, and Marjorie Peronto

The novel propagation system submist, which applies water to the bases of cuttings rather than overhead, is a promising alternative. We developed and tested a commercial-scale submist system to make this propagation system more accessible to commercial propagators. Five species, including blue star flower (Amsonia tabernaemontana), faassen nepeta (Nepeta ×faassenii ‘Six Hills Giant’), panicle hydrangea (Hydrangea paniculata ‘Grandiflora’), sweetgale (Myrica gale), and sweetfern (Comptonia peregrina), were propagated from cuttings in commercial-scale submist and overhead mist systems. Blue star flower and faassen nepeta cuttings had greater root length, root rating, and root number with the submist system. Panicle hydrangea cuttings had more roots in submist, but longer roots in overhead mist. There were no differences in rooting between the systems for sweetgale and sweetfern cuttings. The comparable or superior rooting of these five species in a submist system compared with traditional overhead mist systems is evidence that submist is a viable alternative propagation system. Water use in submist systems was 98% less than that for overhead mist systems.

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Celina Gómez, Christopher J. Currey, Ryan W. Dickson, Hye-Ji Kim, Ricardo Hernández, Nadia C. Sabeh, Rosa E. Raudales, Robin G. Brumfield, Angela Laury-Shaw, Adam K. Wilke, Roberto G. Lopez, and Stephanie E. Burnett

The recent increased market demand for locally grown produce is generating interest in the application of techniques developed for controlled environment agriculture (CEA) to urban agriculture (UA). Controlled environments have great potential to revolutionize urban food systems, as they offer unique opportunities for year-round production, optimizing resource-use efficiency, and for helping to overcome significant challenges associated with the high costs of production in urban settings. For urban growers to benefit from CEA, results from studies evaluating the application of controlled environments for commercial food production should be considered. This review includes a discussion of current and potential applications of CEA for UA, references discussing appropriate methods for selecting and controlling the physical plant production environment, resource management strategies, considerations to improve economic viability, opportunities to address food safety concerns, and the potential social benefits from applying CEA techniques to UA. Author’s viewpoints about the future of CEA for urban food production are presented at the end of this review.