Peer Reviewed Publications
The influence of risk perceptions on consumer willingness-to-pay for nutrient enhanced microgreens
Rihn, A., K.J. Walters, N. Bumgarner. 2024 HortScience.
Microgreens are young plants used as ingredients and flavoring in various dishes. Their production time is short, and production methods can be altered to enhance nutritional content. To date, consumer preferences for microgreens displaying different aesthetic and non-aesthetic traits has not been addressed. Nor have consumers’ perceived risk of production methods used to enhance nutrition been investigated. An online survey and choice experiment were used to investigate these topics using a sample of 821 Tennessee consumers. A mixed logit model was used to analyze the data. Both aesthetic and non-aesthetic traits influenced consumer preferences. Green microgreens were preferred and valued more than light or dark purple microgreens. When fertilizer was used during production to enhance nutrients, consumers perceived the microgreens as risker to the environment and for personal consumption relative to microgreens with light induced nutrient enhancement. Using lighting during production to enhance nutrients generated a $0.47 to $0.79 premium relative to no nutrient enhancement. In general, if participants’ perceived risk aligned with the nutrient enhancement attribute (i.e., light, fertilizer), their willingness to pay for the microgreens decreased. This was amplified for the fertilizer nutrient enhancement attribute more than the light enhancement attribute. In-state production and microgreen height also impacted consumer preferences for microgreens.
Light intensity and photoperiod: Tools for improving the phytonutrient profile of Brassica rappa ssp. nipposinica for supplementing the space diet
Darby, E., S. Armstrong, G. Massa, and K. Walters. 2024. Scientific Reports Preprint
Beyond mere caloric intake, the nutritional quality of food will be key to maintaining astronaut health during exploration-length missions. The production of leafy greens aboard spacecraft can help to provide consistent daily nutrition; however, maximizing the nutritional yield of each plant will be key to sustainable and efficient dietary supplementation. Brasssica rapa ssp. nipposinica ‘Red Hybrid’ was grown under environmental conditions similar to those of the International Space Station and the effect of light intensity and photoperiod on nutritional and biomass yields were evaluated. Four light intensities (200, 400, 600, and 800 µmol·m− 2·s− 1) applied over a 16- or 24-hr photoperiod were implemented and the resulting concentrations of ascorbic acid, thiamine, phylloquinone, β-carotene, lutein, zeaxanthin, total anthocyanins, calcium, potassium, magnesium, and iron were quantified. Providing 800 µmol·m− 2·s− 1 over a 16-hr photoperiod produced the best nutritional profile for supplementing the astronaut diet, offering the following percentages of recommended daily intake per 75 g serving: 100% phylloquinone, 63% ascorbic acid, 2.7% thiamine, and 97% retinol (from precursor β-carotene). However, when evaluated for light use efficiency (yield·MJ − 1), the 200 µmol·m− 2·s− 1 treatments (16- and 24-hr) performed better, yielding more mass and phytonutrients per MJ of energy utilized.
Propagation light intensity influences yield, morphology, and phytochemistry of purple-leaf butterhead lettuce (Lactuca sativa)
Das, P., D. Del Moro, S. Givens, S. Armstrong, and K. Walters. 2024. Journal of Agriculture and Food Research 16, 101210.
Lettuce (Lactuca sativa) is an economically important leafy green produced in controlled environment agriculture (CEA). Optimization of lighting conditions is a crucial factor for sustainable production of high-quality lettuce in CEA systems. This study aimed to determine the effects of propagation light intensity (60, 100, 200, 400, and 600 μmol m−2 s−1, 24-h photoperiod) on harvestable yield, morphology, carotenoids, chlorophylls, and anthocyanins of purple-leaf butterhead 'Teodore' lettuce at transplant (14 days after showing) and harvest (21 days after transplanting in a greenhouse environment). We observed a 275 % increase in shoot fresh mass in seedlings grown under 600 μmol m−2 s−1 compared to those under 60 μmol m−2 s−1, and a 163 % improvement persisted through harvest. The HPLC quantification demonstrated that all carotenoids, including violaxanthin, neoxanthin, zeaxanthin, antheraxanthin, lutein, α-carotene, and total carotenoid content and/or concentration increased in lettuce seedlings as light intensities increased from 60 to 400 or 600 μmol m−2 s−1 (5.4–35.9 or 52.7 mol m−2 d−1 DLI). A similar increasing trend was also observed for chlorophyll accumulation. Anthocyanin concentration and content increased with increasing propagation light intensity, but no differences persisted through harvest. Overall, differences in most phytonutrient concentrations and contents persisted through harvest; carotenoid content trends were similar to those at transplant, but carotenoid and chlorophyll concentration trends were somewhat opposite with quadratic decreases as light intensity increased. The results indicate that propagation light intensity could be used as an effective and sustainable strategy to improve the harvestable yield and nutritional quality of lettuce grown in CEA.
Optimizing sowing density for parsley, cilantro, and sage in controlled environment production: Balancing Productivity and Plant Quality
Li, J., A. Martin, L. Carver, S. Armstrong, S. Givens, and K. Walters. 2024. HortTechnology 34(3), 305-312.
Sowing density is a key management practice influencing productivity and quality of leafy greens and culinary herbs grown in controlled environments. However, research-based information on optimal density is limited for many culinary herbs. This greenhouse study aimed to quantify sowing density impacts on biomass output, individual plant growth, and morphological traits in hydroponically produced ‘Giant of Italy’ parsley (Petroselinum crispum), ‘Santo’ cilantro (Coriandrum sativum), and sage (Salvia officinalis). Seedlings were grown in phenolic foam cubes with 1, 5, 10, 15, or 20 seeds per cell, transplanted into an ebb-and-flow hydroponic system in a glass-glazed greenhouse with 23 °C target average daily temperature, 16-hour photoperiod, a target daily light integral of 13 mol·m−2·d−1, and harvested at 16 to 28 d after transplanting depending on species. ‘Giant of Italy’ parsley and ‘Santo’ cilantro fresh weight per cell increased quadratically by 274% (57.3 g) and 305% (19 g), respectively, as sowing density increased from 1 to 15 seeds per cell, then plateaued as density further increased. Sage fresh weight plateaued at 10 seeds per cell with an increase of 225% (29.2 g) compared with 1 seed per cell. Cilantro and sage dry weight per cell plateaued at 14 and 8 seeds per cell, respectively, and parsley dry weight quadratically increased as sowing density rose up to 20 seeds per cell. Although fresh and dry weight increased, individual plant height, stem diameter, and individual plant dry weight exhibited linear or quadratic declines as sowing density increased, indicating higher sowing densities restricted individual plant growth. In summary, as sowing density increased, fresh and dry weight per cell generally increased but individual plant quality decreased. For the greatest fresh and dry weight, 20, 18, and 10 seeds per cell should be sown for parsley, cilantro, and sage, respectively. However, to balance fresh weight and crop quality, our results suggest sowing density (seeds per cell) targets of 16 seeds for parsley, 18 seeds for cilantro, and 10 seeds for sage.
Bioregenerative dietary supplementation in space: Brassica rapa var. nipposinica and other Brassica cultivars
Darby, E.W., S.P. Armstrong, K.J. Walters. 2023. Life Sciences in Space Research
Despite the precise environmental manipulation enabled by controlled environment agriculture (CEA), plant genotype remains a key factor in producing desirable traits. Brassica rapa var. nipposinica (mizuna) is a leading candidate for supplementing deficiencies in the space diet, however, which cultivar of mizuna will respond best to the environment of the International Space Station (ISS) is unknown. It is also unclear if there are more inter-varietal (mizuna - mustards) or intra-varietal (mizuna - mizuna) differences in response to the ISS environment. Twenty-two cultivars of mustard greens, including 13 cultivars of mizuna, were grown under ISS-like conditions to determine which would provide the greatest yield and highest concentrations of carotenoids, anthocyanins, calcium, potassium, iron, magnesium, ascorbic acid, thiamine, and phylloquinone. The experiment was conducted thrice, and data were analyzed to determine which cultivar is most suited for further optimization of space-based cultivation. It was found that phylloquinone and β-carotene concentrations did not vary between cultivars, while all other metrics of interest showed some variation. ‘Amara’ mustard (B. carinata) provided the best overall nutritional profile, despite its low biomass yield of 36.8 g, producing concentrations of 27.85, 0.40, and 0.65 mg∙g−1 of ascorbic acid, thiamine, and lutein, respectively. Of the mizuna cultivars evaluated, open pollinated mibuna provided the best profile, while 'Red Hybrid’ mizuna provided a complimentary profile to that of ‘Amara’, minimally increasing dietary iron while providing beneficial anthocyanins lacking in ‘Amara’.
Light Intensity during Green-Leaf Butterhead Lettuce Propagation Influences Yield and Carotenoids at Harvest
Givens, S.R., D.S. Del Moro, S.E. Parker, A.G. Renny, C.E. Sams, and K.J. Walters. 2023. Horticulturae. 9(2), 223
Controlled environment agriculture (CEA) operations must increase resource-use efficiency, yield, and phytonutrient concentrations to remain competitive. Carotenoids are phytonutrients of interest due to their purported health promoting effects. Their content is impacted by environmental controls, including lighting. Light-use efficiency increases with greater planting density, which is highest during seedling production. This creates the opportunity to raise light intensity during seedling production to improve growth characteristics and phytonutrient concentrations at harvest. Therefore, the objective of this research was to quantify the extent to which light intensity influences carotenoid accumulation in green butterhead lettuce seedlings, and if differences remain at harvest. Lettuce ‘Rex’ (Lactuca sativa L.) seedlings were grown under fluorescent lighting with intensities of 60, 100, 200, 400, or 600 µmol·m−2·s−1 with a 24-h photoperiod. After 14 days, seedlings were transplanted into raft hydroponic systems in a common greenhouse environment and grown for 21 days. At transplant and final harvest, tissue samples were collected and stored at −80 °C for phytonutrient analysis. Carotenoids, β-carotene, lutein, neoxanthin, zeaxanthin, and violaxanthin, and chlorophylls a and b were quantified using high-performance liquid chromatography (HPLC). We observed a 475% fresh mass enhancement in seedlings grown under 400 versus 60 µmol·m−2·s−1, with a 174% improvement persisting to final harvest. Higher seedling light intensities also generally increased leaf numbers in seedlings and at final harvest, as well as seedling carotenoid concentrations. Final harvest carotenoid concentrations generally decreased with increasing light intensity. Thus, producers should be cognizant that seedling light intensity strongly influences seedling and finished production yield, morphology, and carotenoid content.
Walters, K.J. and R. G. Lopez. 2022. Plos one. 17(8) e0273562
Radiation intensity and carbon dioxide (CO2) concentration can be precisely controlled to manipulate plant yield and quality. Due to increased plant densities during seedling production, fewer inputs per plant are required, creating the potential to increase production efficiency. Therefore, the objectives of this research were to: 1) quantify the extent radiation intensity and CO2 concentration under sole-source lighting influence morphology and yield of sweet basil (Ocimum basilicum) seedlings, and 2) determine if differences in morphology, yield, and volatile organic compound (VOC) concentration persist after transplant in a common environment. Sweet basil ‘Nufar’ seedlings were grown in growth chambers with target CO2 concentrations of 500 or 1,000 μmol·mol‒1 under light-emitting diodes (LEDs) providing target photosynthetic photon flux densities (PPFD) of 100, 200, 400, or 600 μmol·m‒2·s‒1 for 16 h per day. After two weeks, seedlings were transplanted into a common greenhouse environment and grown until harvest. At transplant and three weeks after transplant (harvest), growth and developmental differences were quantified along with key terpenoid and phenylpropanoid concentrations at harvest. Radiation intensity and CO2 interacted influencing many aspects of plant morphology, though CO2 concentration effects were less pronounced than those of radiation intensity. As radiation intensity during seedling production increased from 100 to 600 μmol·m‒2·s‒1, basil seedlings were 38% taller, had a 713% larger leaf area, and had 65% thicker stems; at harvest, plants were 24% taller, had 56% more branches, 28% more nodes, 22% thicker stems, and weighed 80% more when fresh and dry. Additionally, after growing in a common environment for three weeks, eugenol concentration was greater in plants grown under a PPFD of 600 μmol·m‒2·s‒1 as seedlings compared to lower intensities. Therefore, increasing radiation intensity during seedling production under sole-source lighting can carry over to increase subsequent yield and eugenol concentration during finished production.
Wedegaertner, K, A. Shekoofa, S. Purdom, K. Walters, L. Duncan, and T. Raper. 2022. Journal of Cotton Research. 5(1), 1-11.
Cotton (Gossypium hirsutum L.) is often grown in locations characterized by high atmospheric evaporative demand. It has been hypothesized that plants which resist hydraulic flow under this condition will limit water use and conserve soil water. Therefore, in a series of controlled environment experiments ten cotton cultivars were exposed to two different temperature and vapor pressure deficit (VPD) conditions (i.e., 38 °C, > 3 kPa and 32 °C, 1∼1.5 kPa) as well as a progressive soil drying. Then, individual differences in shoot hydraulic conductance (Kshoot) was measured using a hydraulic conductance flow meter (HCFM). Physiological parameters were reported included leaf area, dry leaf weight, stomatal conductance (gs), and water use efficiency coefficient (WUEk). Differences were observed in Kshoot among cultivars under the 38 °C, > 3 kPa but not the 32 °C, 1∼1.5 kPa environment. Under the 38 °C, > 3 kPa environment, correlations were found between Kshoot, stomatal conductance (gs), VPD breakpoint, WUEk, total leaf area, dry leaf weight, fraction transpirable soil water (FTSW) threshold, and slope of TR decline after FTSW threshold. Results show that the ability of some cotton cultivars to restrict water loss under high evaporative demand through early stomatal closure is associated with the cultivars’ Kshoot. The Kshoot is influential in the limitation of TR trait under high temperature and VPD.
Walters, K.J. and R.G. Lopez. 2022. Horticulturae. 8(1),76
Altering the growing temperature during controlled-environment production not only influences crop growth and development, but can also influence volatile organic compound (VOC) production and, subsequently, sensory attributes of culinary herbs. Therefore, the objectives of this study were to (1) quantify the influence of mean daily temperature (MDT) and daily light integral (DLI) on key basil phenylpropanoid and terpenoid concentrations, (2) determine if differences in sensory characteristics due to MDT and DLI influence consumer preference, and (3) identify the sweet basil attributes consumers prefer. Thus, 2-week-old sweet basil ‘Nufar’ seedlings were transplanted into deep-flow hydroponic systems in greenhouses with target MDTs of 23, 26, 29, 32, or 35 °C and DLIs of 7, 9, or 12 mol·m−2·d−1. After three weeks, the two most recently mature leaves were harvested for gas chromatography–mass spectrometry (GC–MS) and consumer sensory analysis. Panel evaluations were conducted through a sliding door with samples served individually while panelists answered Likert scale and open-ended quality attribute and sensory questions. The DLI did not influence VOC concentrations. Increasing MDT from 23 to 36 °C during production increased 1,8 cineole, eugenol, and methyl chavicol concentrations linearly and did not affect linalool concentration. The increases in phenylpropanoid (eugenol and methyl chavicol) were greater than increases in terpenoid (1,8 cineole) concentrations. However, these increases did not impact overall consumer or flavor preference. The MDT during basil production influenced appearance, texture, and color preference of panelists. Taken together, MDT during production influenced both VOC concentrations and textural and visual attribute preference of basil but did not influence overall consumer preference. Therefore, changing the MDT during production can be used to alter plant growth and development without significantly affecting consumer preference.
Walters, K.J. and R.G. Lopez. 2021. PLOS One. 16(3), e0248662
In controlled environments, crop models that incorporate environmental factors can be developed to optimize growth and development as well as conduct cost and/or resource use benefit analyses. The overall objective of this study was to model growth and development of dill ‘Bouquet’ (Anethum graveolens), parsley ‘Giant of Italy’ (Petroselinum crispum), and watercress (Nasturtium officinale) in response to photosynthetic daily light integral (DLI) and mean daily temperature (MDT). Plants were grown hydroponically in five greenhouse compartments with MDTs ranging from 9.7 to 27.2 °C under 0%, 30%, or 50% shade cloth to create DLIs ranging from 6.2 to 16.9 mol·m‒2·d‒1. MDT and DLI interacted to influence dill fresh mass and height, and watercress maximum quantum yield of dark adapted leaves (Fv/Fm), height, and branch number while only MDT affected dill leaf number and watercress fresh mass and branch length. Besides dry matter concentration (DMC), parsley was influenced by MDT and not DLI. Increasing MDT from ≈10 to 22.4 °C (parsley) or 27.2 °C (dill and watercress), linearly or near-linearly increased fresh mass. For dill, increasing DLI decreased fresh mass when MDT was low (9.7 to 13.9 °C) and increased fresh mass when MDT was high (18.4 to 27.2 °C). DMC of dill, parsley, and watercress increased as MDT decreased or DLI increased, indicating a higher proportion of plant fresh mass is water at higher MDTs or lower DLIs. With these data we have created growth and development models for culinary herbs to aid in predicting responses to DLI and MDT.
Leveraging controlled-environment agriculture to increase key basil terpenoid and phenylpropanoid concentrations: The effects of radiation intensity and CO2 concentration on consumer preference.
Walters, K.J., R.G. Lopez, and B.K. Behe. 2021. Frontiers in Plant Science. 11, 598519
Altering the radiation intensity in controlled environments (CE) can influence volatile organic compound (VOC) biosynthetic pathways, including those of terpenoids and phenylpropanoids. In turn, the concentrations of these compounds can have a profound effect on flavor and sensory attributes. Since sweet basil (Ocimum basilicum) is a popular culinary herb, our objectives were to 1) determine the extent radiation intensity and carbon dioxide (CO2) concentration influence seedling terpenoid and phenylpropanoid concentrations; 2) determine if differences in phenylpropanoid and terpenoid concentrations influence consumer preference; and 3) characterize consumer preferences to better inform production and marketing strategies. Seeds of basil ‘Nufar’ were sown and placed in a growth chamber with CO2 concentrations of 500 or 1,000 µmol·mol‒1 under radiation intensities of 100, 200, 400, or 600 µmol·m–2·s–1 with a 16-h photoperiod to create daily light integrals (DLIs) of 6, 12, 23, and 35 mol·m–2·d–1. After two weeks, leaves were harvested for evaluation. To determine the concentrations of the terpenoids 1,8 cineole and linalool, and the phenylpropanoids eugenol and methyl chavicol, gas chromatography mass spectrometry (GCMS) analysis was conducted. Consumer sensory panel evaluations were conducted to quantify preferences through Likert scale, open-ended quality attribute, and sensory questions. Overall, increasing radiation intensity from 100 to 600 µmol·m‒2·s‒1 increased 1,8 cineole, linalool, and eugenol concentrations 2.4-, 8.8-, and 3.3-fold, respectively, while CO2 concentration did not influence VOCs. Contrary to our hypothesis, increased VOC concentrations were not correlated with consumer preference. However, overall liking was correlated with aftertaste and flavor. The conclusion that consumer preference is dependent upon flavor can be drawn. However, increasing VOC concentrations to increase flavor did not improve flavor preference. Many consumer sensory preference characteristics (favorable preference for aftertaste, bitterness/sweetness, color, flavor, overall liking, and texture) were correlated with basil grown under a radiation intensity of 200 µmol·m‒2·s‒1. This led us to determine that consumers prefer to detect the characteristic basil flavor made up of 1,8 cineole, eugenol, and linalool, that was not as prevalent in basil grown under 100 µmol·m‒2·s‒1, but too high in basil grown under 400 and 600 µmol·m‒2·s‒1, which led to lower consumer preference.
Slack, S.M., K.J. Walters, C.A. Outwater, and G.W. Sundin. 2021. Plant Disease. 105 (6) 1843-1850
We assessed the effect of three antibiotics (streptomycin, oxytetracycline, and kasugamycin) on populations of the fire blight pathogen Erwinia amylovora on apple flower stigmas during three field seasons. Application timing relative to E. amylovora presence on flower stigmas had little impact on population dynamics and subsequent disease incidence. Although E. amylovora populations on water-treated flowers increased to 106–7 cfu flower−1 after 4 to 5 days during each experiment, the antibiotics streptomycin and kasugamycin caused statistically significant reductions in stigma populations by as many as 4 to 5 logs over a 4- to 5-day period during two of the three experiments. In contrast, the effect of oxytetracycline on E. amylovora populations on stigmas was more variable, with reductions in E. amylovora populations only observed during one of the three experiments. In agreement with the population data, the disease incidence was significantly higher for oxytetracycline-treated flowers compared with the other antibiotic treatments during 2 of 3 years. Statistical analyses of the effects of weather parameters on antibiotic activity revealed that solar radiation and temperature negatively impacted the activity of both kasugamycin and oxytetracycline. We further assessed the potential for photodegradation of formulated kasugamycin (Kasumin 2L) and found that Kasumin 2L was susceptible to degradation in vitro after exposure to a 16-h photoperiod of daily light integrals (DLIs) varying from 6 to 35 mol⋅m−2⋅d−1. We further determined that exposure to three consecutive 16-h photoperiods of DLIs of 23 or 35 mol⋅m−2⋅d−1 reduced the available concentration of Kasumin 2L (assessed using a bioassay) from 100 μg⋅ml−1 to 10 to 20 μg⋅ml−1. Our results correlate the superior blossom blight control efficacy of kasugamycin and streptomycin with significant population reductions in E. amylovora on apple flower stigmas but indicate that, similar to oxytetracycline, kasugamycin is vulnerable to photodegradation, which would suggest that further considerations are necessary when applying this antibiotic.
Historical, current, and future perspectives for controlled environment hydroponic food crop production in the United States.
Walters, K.J., B.K. Behe, C.J. Currey, and R.G. Lopez. 2020. HortScience. 55(6): 758-767.
Controlled environment (CE) food crop production has existed in the United States for many years, but recent improvements in technology and increasing production warranted a closer examination of the industry. Therefore, our objectives were to characterize historical trends in CE production, understand the current state of the U.S. hydroponics industry, and use historical and current trends to inform future perspectives. In the 1800s, CE food production emerged and increased in popularity until 1929. After 1929, when adjusted for inflation (AFI), CE food production stagnated and decreased until 1988. From 1988 to 2014, the wholesale value of CE food production increased from $64.2 million to $796.7 million AFI. With the recent increase in demand for locally grown food spurring an increase in CE production, both growers and researchers have been interested in using hydroponic CE technologies to improve production and quality. Therefore, we surveyed U.S. hydroponic food crop producers to identify current hydroponic production technology adoption and potential areas for research needs. Producers cited a wide range of technology utilization; more than half employed solely hydroponic production techniques, 56% monitored light intensity, and more than 80% monitored air temperature and nutrient solution pH and electrical conductivity. Additionally, the growing environments varied from greenhouses (64%), indoors in multilayer (31%) or single-layer (7%) facilities, to hoop houses or high tunnels (29%). Overall, producers reported managing the growing environment to improve crop flavor and the development of production strategies as the most beneficial research areas, with 90% stating their customers would pay more for crops with increased flavor. Lastly, taking historical data and current practices into account, perspectives on future hydroponic CE production are discussed. These include the importance of research on multiple environmental parameters instead of single parameters in isolation and the emphasis on not only increasing productivity but improving crop quality including flavor, sensory attributes, and postharvest longevity.
Modeling temperature effects on the growth and development of basil species.
Walters, K.J. and C.J. Currey. 2019. HortScience. 54(11): 1915-1920.
Basil (Ocimum sp.) is the most popular fresh culinary herb, but the effects of air temperature on growth and development of basil have not been well characterized. Our objective was to quantify the effects of air temperature on growth and development of three basil species. Seedlings of sweet basil (Ocimum basilicum ‘Nufar’), holy basil (O. tenuiflorum), and lemon basil (O. ×citriodorum ‘Lime’ and O. basilicum ‘Sweet Dani’) were placed in five different growth chambers with target air temperatures of 11, 17, 23, 29, or 35 °C. After 3 weeks, chlorophyll fluorescence (Fv/Fm), plant height, node and branch number, fresh and dry weight, and flowering data were recorded. For all species, Fv/Fm increased as temperature increased to 17 or 23 °C, then plateaued, whereas height increased with temperature to 23 or 29 °C. Also, the percentage of plants with flowers or flower buds increased with temperature to 17 or 23 °C for all species, with the exception of sweet basil, of which all plants were vegetative and node appearance rate was calculated. Sweet basil node appearance increased from 0.03 to 0.30 node/day as the temperature increased from 11 to 29 °C. Fresh weight gain increased with increasing temperature to 29 °C, but then decreased at 35 °C. Data from plants grown within the linear air temperature range were used to develop models for calculating the base temperature (Tb) and predicting growth in response to air temperature. These models can be applied by commercial producers to schedule crops and predict yields.
Nutrient solution strength does not interact with the daily light integral to affect hydroponic cilantro, dill, and parsley growth and tissue mineral nutrient concentrations.
Currey, C.J., K.J. Walters, N.J. Flax. 2019. Agronomy. 9(7): 389-402.
Our objectives were to quantify the growth and tissue mineral nutrient concentrations of cilantro (Coriandrum sativum ‘Santo’), dill (Anethum graveolens ‘Fernleaf’), and parsley (Petroselinum crispum ‘Giant of Italy’) in response to nutrient solution electrical conductivity (EC) under low and high photosynthetic daily light integrals (DLI). Three-week old seedlings of cilantro, dill, and parsley were transplanted into nutrient-film technique hydroponic systems with one of five nutrient solution EC treatments (0.5, 1.0, 2.0, 3.0, or 4.0 dS·m−1) in greenhouses under a low (~7.0 mol·m−2·d−1) or high (~18.0 mol·m−2·d−1) DLI. The DLI, but not nutrient solution EC, affected culinary herb growth. For example, fresh mass increased by 21.0 (154%), 17.1 (241%), or 13.3 g (120%) for cilantro, dill, and parsley, respectively, for plants grown under high DLI compared to those grown under a low DLI; dry mass followed a similar trend. Tissue nutrient concentrations were generally affected by either DLI or EC. For those nutrients affected by DLI, concentrations increased with increasing DLI, except for potassium (K; all species) and manganese (Mn; dill). For those nutrients affected by EC, Ca and Mg decreased with increasing EC, while the remaining increased with increasing EC. When our tissue nutrient data are compared to recommended tissue concentrations, the vast majority of elements were either within or above recommended tissue ranges for cilantro, dill, and parsley. Our results demonstrate cilantro, dill, and parsley can be successfully grown across a range of EC, regardless of the light intensity of the growing environment.
Flowering, stem extension growth, and cutting yield of foliage annuals in response to photoperiod
Walters, K.J., A.A. Hurt, and R.G. Lopez. 2019. HortScience. 54(4): 661-666.
Foliage annuals are primarily grown for the aesthetic appeal of their brightly colored, variegated, or patterned leaves rather than for their flowers. Once foliage annuals become reproductive, vegetative growth of many species diminishes or completely ceases and plants can become unappealing. Therefore, the objectives of this study were to quantify how growth and development during production and stock plant cutting yield of bloodleaf (Iresine herbstii), Joseph’s coat (Alternanthera sp.) ‘Brazilian Red Hots’ and ‘Red Threads’, Persian shield (Strobilanthes dyerianus), and variegated potato vine (Solanum jasminoides) are influenced by photoperiod and night interruption (NI) lighting with or without far-red (FR) radiation. Photoperiods consisted of a 9-hour short day (SD) or a 9-hour SD extended to 10, 12, 13, 14, or 16 hours with red (R):white (W):FR light-emitting diode (LED) lamps (R:FR = 0.8) providing a total photon flux density (TPFD) of ≈2 µmol·m−2·s–1 of radiation. In addition, two treatments consisted of a 9-hour SD with a 4-hour NI from lamps containing the same R:W:FR or R:W LEDs (R:FR = 37.4). Bloodleaf plant and Joseph’s coat ‘Brazilian Red Hots’ and ‘Red Threads’ developed inflorescences or flowers under photoperiods ≤12 to 13 hours and were classified as obligate SD plants. Under LEDs providing R:W:FR radiation, stem elongation of reproductive bloodleaf and Joseph’s coat ‘Brazilian Red Hots’ and ‘Red Threads’ increased as photoperiod increased from 9 to 12 hours. In addition, stem elongation of bloodleaf, Joseph’s coat ‘Brazilian Red Hots’ and ‘Red Threads’, and Persian shield and growth index (GI = {plant height + [(diameter 1 + diameter 2)/2]}/2) of bloodleaf and Persian shield was significantly greater under NI with FR radiation than without FR radiation. Fewer or no cuttings were harvested from Joseph’s coat ‘Brazilian Red Hots’ and ‘Red Threads’ under photoperiods ≤12 or ≤13 hours, respectively. To prevent unwanted flowering of bloodleaf plant and Joseph’s coat, a photoperiod ≥14 hours or 4-hour NI must be maintained with LEDs providing either R:W or R:W:FR radiation, however; stem elongation is significantly reduced under R:W LEDs.
Ethephon foliar sprays are influenced by carrier water alkalinity and ambient air temperature at application
Walters, K.J. and R.G. Lopez. 2019. HortScience. 53(12): 1835-1841.
The plant growth regulator (PGR) ethephon [(2-chloroethyl) phosphonic acid; ETH] can be sprayed on floriculture crops to inhibit internode elongation, hinder apical dominance, increase lateral branching, and abort flower buds and flowers. However, the efficacy of ETH can be reduced as the pH of the carrier water used to mix the spray solution or temperature increase. Therefore, our objective was to quantify how the efficacy of ethephon sprays is influenced by carrier water alkalinity (CaCO3; ALK) and the air temperature at application (TEMP). Young plants of verbena (Verbena peruviana) ‘Aztec Blue Velvet’, ivy geranium (Pelargonium×peltatum) ‘Precision Pink’, and petunia (Petunia ×hybrida) ‘Easy Wave Neon Rose’ were transplanted into 11-cm-diameter containers and grown in a greenhouse with an average daily air temperature (ADT) set point of 21 °C. Before the ETH spray application(s), the ADT in each greenhouse compartment was changed from a set point of 21 °C to 14, 17, 20, 23, or 26 °C for ≈24 hours. Plants were sprayed with 0, 250, 500, or 750 mg·L−1ETH mixed with carrier water containing ≈50, 150, or 300 mg·L−1 CaCO3 2 and 3 weeks (Expt. 1) or 1 or 2 weeks (Expt. 2) after transplant. Generally, high ALK had a negative effect on spray efficacy. For example, an increase in ALK from 50 to 300 mg·L−1 CaCO3 resulted in one and five fewer ivy geranium and verbena branches, respectively. In addition, as application TEMP increased above 23 °C, chemical efficacy generally decreased in all species. For instance, as ETH increased from 0 to 750 mg·L−1 across ALKs, inflorescence number of ivy geraniums increased from 7 to 18 at a TEMP of 23 °C, but was unaffected at 26 °C. Based on our results, we can conclude that both ALK and TEMP influence ETH efficacy and are additional factors for greenhouse growers to consider when making applications.
Effects of nutrient solution concentration and daily light integral on growth and nutrient concentration of several basil species in hydroponic production
Walters, K.J. and C.J. Currey. 2018. HortScience 53(9): 1319-1325.
Our objective was to quantify the effect of mineral nutrient concentration of a nutrient solution on the growth of basil species and cultivars grown under high and low photosynthetic daily light integrals (DLIs). Sweet basil (Ocimum basilicum ‘Nufar’), lemon basil (O. ×citriodorum ‘Lime’), and holy basil (O. tenuiflorum ‘Holy’) seedlings were transplanted into nutrient-film technique (NFT) systems with different nutrient solution electrical conductivities (EC; 0.5, 1.0, 2.0, 3.0, or 4.0 dS·m–1) in greenhouses with a low (≈7 mol·m–2·d–1) or high (≈15 mol·m–2·d–1) DLI. Although nutrient solution EC did not affect growth and morphology, increasing DLI did. For example, when sweet basil was grown under a high DLI, the fresh and dry weight, height, and node number increased by 144%, 178%, 20%, and 18%, respectively, compared with plants grown under the low DLI, and branching was also stimulated. In contrast, DLI had little effect on tissue nutrient concentration, although nutrient solution did. Most tissue nutrient concentrations increased with increasing EC, with the exception of Mg and Ca. For example, N in sweet basil increased by 0.6% to 0.7% whereas Mg decreased by 0.2% as EC increased from 0.5 to 4.0 dS·m–1. Across treatments and basil species, tissue nutrient concentrations were generally within recommended ranges with no visible deficiencies. Based on our results, nutrient solution concentrations for hydroponic basil production can be selected based on factors such as other species grown in the same solution or by reducing fertilizer inputs.
Concentration and timing of ethephon drench applications interact to affect growth and flowering of containerized angelonia and geranium
Currey, C.J., K.G. McCabe, and K.J. Walters. 2016. HortScience 51(12): 1542-1546.
Ethephon drenches have been reported to effectively control growth of containerized bedding plants. However, previous researchers have indicated that the effects of ethephon drenches on growth and flowering may differ depending on the timing of applications. Our objectives were to quantify the effects of ethephon concentration, timing of substrate drench application, and their interaction on the growth, size, and flowering of two annual bedding plants. Angelonia (Angelonia angustifolia) and geranium (Pelargonium ×hortorum), seedlings were planted in 10.2-cm-diameter containers filled with a commercial, soilless growing substrate composed of (by vol.) 75% sphagnum peatmoss, and 25% perlite. Five, 10, 15, or 20 days after transplanting seedlings, 70-mL aliquots containing 0, 50, 100, or 200 mg·L−1 ethephon were applied as substrate drenches. Species varied in their growth and flowering responses to ethephon concentration, drench application timing, and their interaction. For angelonia, flowering was delayed most with early applications and high concentrations, and delay was diminished with later applications. Angelonia height was unaffected by late applications, though lateral growth was suppressed 20 days after transplant with 200 mg·L−1 ethephon. Flowering of geranium was only delayed when ethephon was applied 5 days after transplanting, whereas flowering, vegetative height, and shoot dry weight were affected more by earlier applications and higher concentrations. Width and root weight were only affected by ethephon concentration, with growth suppression increasing as concentrations increased. Ethephon is an effective growth regulator when applied as a substrate drench. However, the degree of activity and resulting impact on flowering, size, and growth is influenced by the interaction between ethephon concentrations and the timing of drench applications after transplanting.
Quantifying growth control of lantana cultivars varying in vigor with ancymidol, flurprimidol, paclobutrazol, and uniconazole substrate drenches
Currey, C.J., K.J. Walters, and K.G. McCabe. 2016. HortTechnology 26(3): 320-326.
Our objective was to quantify the efficacy of different plant growth regulator (PGR) substrate drenches on growth of lantana (Lantana camara) cultivars varying in growth habit. Rooted ‘Little Lucky Peach Glow’, ‘Lucky Peach’, and ‘Landmark Peach Sunrise’ lantana cuttings were individually planted into 4-inch-diameter containers filled with a commercial, soilless growing substrate. Fourteen days after planting, solutions containing 0 (control), 0.5, 1, 2, or 4 mg·L−1 ancymidol, flurprimidol, paclobutrazol, or uniconazole were applied to the surface of the growing substrate. Six weeks after applying PGR drenches, data were collected. The growth index (GI), an integrated measurement of plant size incorporating the height and widths of plants, was calculated. There was variation in the GI among the control plants, reflecting variation among cultivars within the species. In addition, we measured variation in activity among the different PGRs applied. Across the concentrations applied, ancymidol generally had the lowest activity across the four PGRs. For example, drenches containing 4 mg·L−1 ancymidol resulted in plants that were similar to plants treated with 0.5 to 1 mg·L−1 flurprimidol or uniconazole or 2 mg·L−1 paclobutrazol for ‘Lucky Peach’ lantana. Across all cultivars, flurprimidol and uniconazole had the greatest activity in suppressing plant height, width, and GI. Substrate drenches containing flurprimidol, paclobutrazol, or uniconazole are useful to control size of lantana produced in containers, though the recommended concentration depends on the active ingredient and the growth habit of cultivars being treated.
Foliar sprays of flurprimidol, paclobutrazol, and uniconazole suppress height of seed-propagated New Guinea impatiens
Currey, C.J., N.J. Flax, and K.J. Walters. 2016. HortTechnology 26(1): 20-25.
Our objective was to quantify the efficacy of foliar plant growth retardant applications on plant height and time to flower of seed-propagated new guinea impatiens (Impatiens hawkeri) produced in packs and flats. ‘Divine Cherry Red’, ‘Divine Scarlet Bronze Leaf’, and ‘Divine White Blush’ seedlings were planted in 1801-cell packs. Seven days after planting, deionized water (control) or solutions containing ancymidol (15 to 60 mg·L−1), chlormequat chloride (750 to 3000 mg·L−1), daminozide (1250 to 5000 mg·L−1), ethephon (250 to 1000 mg·L−1), flurprimidol (10 to 40 mg·L−1), paclobutrazol (10 to 40 mg·L−1), or uniconazole (5 to 20 mg·L−1) were applied to seedlings. A second experiment was performed with the same cultivars quantifying the growth and development in response to a broader range of flurprimidol or paclobutrazol (5 to 40 mg·L−1) or uniconazole (2.5 to 20 mg·L−1) sprays. Plant height was measured 7 weeks after planting. For Expt. 1, ancymidol, chlormequat chloride, and daminozide had little to no impact on stem elongation. However, flurprimidol, paclobutrazol, and uniconazole suppressed height at flowering of all three cultivars. In Expt. 2, plant height with concentrations flurprimidol, paclobutrazol, or uniconazole up to 27 to 30, 20 to 30, or 4 to 5 mg·L−1, respectively, depending on the cultivar. Five to 20 mg·L−1flurprimidol or paclobutrazol, or < 2.5 mg·L−1 uniconazole may be used to control stem elongation of seed-propagated new guinea impatiens for production in flats.
Hydroponic greenhouse basil production: Comparing systems and cultivars
Walters, K.J. and C.J. Currey. 2015. HortTechnology 25(5): 645-650.
Basil (Ocimum sp.) is the most popular fresh culinary herb. However, there is a lack of data characterizing the effect of hydroponic production systems and cultivars on the yield of hydroponically produced basil. Our objectives were to quantify productivity and characterize growth of basil cultivars grown in two hydroponic production systems. Thirty-five basil cultivars, including selections of sweet basil (O. basilicum), holy basil (O. tenuiflorum), and lemon basil (O. ×citriodorum and O. basilicum) were chosen. Seedlings were transplanted into nutrient film technique (NFT) or deep flow technique (DFT) systems and grown for 3 weeks. There was no interaction between basil cultivars and hydroponic production system. Fresh weight of plants grown in DFT systems was 2.6 g greater compared with plants grown in NFT systems. Basil cultivars differed greatly in fresh weight. In general, holy, lemon, and sweet basil cultivars produce moderate to high fresh weight, but vary greatly. Dissimilarly, bush (O. basilicum var. minimum), cinnamon (O. basilicum), large-leaf (O. basilicum), and thai basils (O. basilicum var. thyrisiflorum) produce moderate fresh weight and purple basil (O. basilicum) cultivars produce the least fresh weight. The yield of basil seems to be affected more by cultivar selection than hydroponic production system. Therefore, hydroponic basil producers should select basil cultivars based on flavor and yield, while hydroponic systems should be selected based on operational preferences.