Effects of Seeding Pattern and Cultivar on Productivity of Baby Spinach (Spinacia oleracea) Grown Hydroponically in Deep-Water Culture - MDPI
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horticulturae Article Effects of Seeding Pattern and Cultivar on Productivity of Baby Spinach (Spinacia oleracea) Grown Hydroponically in Deep-Water Culture Daniel B. Janeczko and Michael B. Timmons * Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA; dbj42@cornell.edu * Correspondence: mbt3@cornell.edu Received: 1 December 2018; Accepted: 21 February 2019; Published: 28 February 2019 Abstract: Baby spinach (Spinacia oleracea) was grown in a bench-scale deep-water culture (DWC) system in expanded polystyrene (EPS) plug trays. Two experiments were performed. In the first, different seeding patterns, [1-2-1-2 . . . ] or [3-0-3-0 . . . ] seeds per sequential cell, at the same overall density per tray, were compared to evaluate the potential of an EPS tray designed with fewer cells, but sown with more seeds per cell (to preserve canopy density). Using such a flat would lower growing substrate requirements. Seeding in the [3-0-3-0 . . . ] pattern reduced seed germination, but only by 5%. Harvested fresh weight was also less numerically in the [3-0-3-0 . . . ] pattern but not statistically. The second experiment observed cultivars Carmel, Seaside and Space grown concurrently. Carmel had the highest germination, nearly 100%, which was significantly greater than Seaside but not Space. Germination for Space was not significantly different from that of Seaside. Carmel also had the highest harvested fresh weight but was not significantly different from Space; both Carmel and Space produced significantly more harvested fresh weight than Seaside. Keywords: hydroponics; deep-water culture; baby spinach; Carmel; Seaside; pericarp; dibbler; density; Spinacia oleracea 1. Introduction Hydroponics is the soilless culture of plants grown in a nutrient solution. Within hydroponics, several different techniques are employed to expose plant roots to a nutrient solution. The Deep-Water Culture (DWC) method is characterized by use of a trough or tub filled with aerated nutrient solution to which a floating raft is added that holds the plants with their roots suspended vertically in the water column [1]. Hydroponic greenhouse crop production has the potential to reduce dependence on imported crops and to increase food security and safety while simultaneously enhancing environmental quality and energy/resource efficiency of food production systems [2,3]. Baby spinach is an attractive choice for hydroponic production since it is popular as a fresh vegetable and has relatively high concentrations of bioactive compounds including ascorbic acid, carotenoids, and flavonoids that contribute to its high nutritional value, despite its low caloric content [4]. Growing this crop in the northeast is particularly attractive since 86% of US fresh spinach production in 2017 (53,030 acres total) occurred in Arizona or California [5]. Expanded polystyrene (EPS) flats with varying cell volume and spacing are commonly used for baby leaf production in DWC systems. Typical cell volumes can range from 2 or 3 cm3 to 10 cm3 . Cell volume impacted tomato, eggplant, and pepper seedling fresh weight when grown from seed [6]. Planting (canopy) density was less predictive of seedling fresh weight (within the range considered: Horticulturae 2019, 5, 20; doi:10.3390/horticulturae5010020 www.mdpi.com/journal/horticulturae
Horticulturae 2019, 5, 20 2 of 12 Horticulturae 2019, 5, x FOR PEER REVIEW 2 of 13 had greater 94–1125 yield per plants/m 2 ) [6]. unitBaby area romaine and resulted in plants lettuce (Lactucawith longer sativa) but fewer seeded leavesdensities at higher compared in to EPSlower- flats density arrangements had greater yield per unit [7]. This area demonstrated and resulted in that planting plants withdensity longer butand fewer available cellcompared leaves volume can to impact the morphological development of baby leaf crops. However, the lower-density arrangements [7]. This demonstrated that planting density and available cell volume effect of proportional cell volume can impactavailable to spinach seedlings the morphological development and the higher of baby leafdensity of plants in crops. However, thethe early effect of stages of growth proportional cell associated with sowing volume available to spinachmultiple seeds and seedlings per cell the have higher not been thoroughly density of plants ininvestigated. the early stages of growth The soilless associated medium with sowing in which multiple seeds seeds perarecellsown have is notoften beenathoroughly large fixed investigated. material cost of each crop. WhenThesowing an off-the-shelf soilless medium in which EPS flat (generally seeds re-used are sown for 30 is often successive a large fixedplantings), material cost withofsphagnum each crop. peat moss germination mix and spinach seed at a density of 1.5 seeds per When sowing an off-the-shelf EPS flat (generally re-used for 30 successive plantings), with sphagnum cell, the proportional cost of growing peat medium can moss germination mixbeand estimated spinachtoseed be over 50% of of at a density the1.5total seedsmaterial per cell,input costs (Figurecost the proportional 1). Employing of growing amedium flat withcan fewer cells, but to be estimated sowing be overmore50% seeds of the in total each material cell (thusinput preserving the same costs (Figure 1). canopy density), could meaningfully reduce material expenses. While Employing a flat with fewer cells, but sowing more seeds in each cell (thus preserving the same the density of the canopy would canopytheoretically density), could remain unchanged, meaningfully reduceit ismaterial unclearexpenses. how packing Whilemore seeds into the density of theeach cell affects canopy would germination and growth theoretically remain throughout unchanged, the short it is unclear howcrop cycle.more packing It is hypothesized seeds into eachthat cellseeding more seeds affects germination per andcell will negatively growth throughoutaffect germination the short crop cycle. andItgrowth as the seed-substrate is hypothesized that seedingcontact needed more seeds perfor seeds, cell will especially negativelylarge affectones, to imbibe germination andand germinate growth as thewill be disruptedcontact seed-substrate in cellsneeded with multiple for seeds,seeds by the especially earliest germinating large ones, to imbibeseeds and [8]. One of our germinate willobjectives be disrupted was into evaluate cells withthis hypothesis. multiple seeds by the earliest germinating seeds [8]. One of our objectives was to evaluate this hypothesis. Figure 1. Flat material input costs. Figure 1. Flat material input costs. Spinach is not considered to be a high-value crop by producers and generally not tailored to hydroponic Spinachproduction. A main is not considered todriving force behind be a high-value cropcultivar selection by producers andduring seednot generally production is tailored to resistance toproduction. hydroponic blue mold aimedA main at driving field-grown forceapplications, behind cultivarbut selection this is notduring of critical seedimportance productionfor is hydroponictobaby resistance blueleaf moldproduction aimed atwhen compared field-grown to other cultivar applications, characteristics but this such importance is not of critical as simultaneity for of germination, hydroponic baby germination rate, and leaf production whenplantcompared morphology to during the shortcharacteristics other cultivar crop cycle [9].suchRecent as research on hydroponic simultaneity baby germination of germination, leaf spinach rate, production in DWC and plant has focused morphology on the during twoshort round-leaf, downy crop cycle [9]. mildew-resistant Recent research on cultivars: hydroponicSpace andleaf baby Carmel. spinachSeaside is a smooth-leaf, production in DWC has slow-bolting focused oncultivar that has two round-leaf, not beenmildew-resistant downy thoroughly tested in a DWC cultivars: system Space [2,10]. Comparative and Carmel. testing of these Seaside is a smooth-leaf, cultivars is slow-bolting a next cultivar step has that in determining which will not been thoroughly growinvigorously tested a DWC system in a DWC [2,10].environment. Comparative testing of these cultivars is a nextThe stepobjectives of this study in determining whichwere: a) to vigorously will grow determine effects in a DWCof seeding methods on performance of environment. babyThe spinach planted objectives of at high this densities study were: and a) tob)determine to compare performance effects of seedingcharacteristics methods on from seeding of performance to harvest baby for three spinach cultivars planted (Carmel, at high Seaside, densities andandb) toSpace) using compare one of the high performance density seeding characteristics frommethods. seeding to harvest for three cultivars (Carmel, Seaside, and Space) using one of the high density seeding 2. Materials and Methods methods. Experiments were conducted to test seeding patterns (Experiment 1) and performance 2. Materials and characteristics Methods 2). Germination, fresh weight production, and observations of canopy (Experiment characteristics Experiments were considered were conductedthe to mosttestrelevant seedingmetrics to evaluate patterns (Experimenttreatments. 1) and Experiment performance1 compared seeding characteristics patterns with (Experiment the Carmel variety 2). Germination, only and fresh weight consisted and production, of either seeding identically observations of canopy sized cells with a one-two-one alternating pattern [1-2-1-2] of seeds per sequential characteristics were considered the most relevant metrics to evaluate treatments. Experiment cell or using three 1 seeds in a cell and then an empty cell and then three seeds in a cell etc. [3-0-3-0], giving compared seeding patterns with the Carmel variety only and consisted of either seeding identically the same sized cells with a one-two-one alternating pattern [1-2-1-2] of seeds per sequential cell or using three
Horticulturae 2019, 5, 20 3 of 12 average density of 1.5 seeds per cell in both treatments. Experiment 2 observed three cultivars that were grown concurrently side by side, using the cultivars Carmel, Seaside, and Space seeded at a fixed density of 1.5 seeds per cell in the [1-2-1-2] pattern. Data on both germination and fresh weight was collected by counting seedlings in each treatment mid-way through the crop cycle and weighing hand-harvested leaves after they reached marketable size. Speedling (Ruskin, FL) 338 cell flats were used for all experiments (see Figures 3 and 9a for a representative flat). The flats had dimensions of 34.3 cm by 57.2 cm by 4.4 cm. Each flat was divided into two blocks by using two center rows (no seeds) to separate blocks. Thus, each block had 130 cells for planting. Both seeding patterns resulted in 195 seeds being planted per block with a potential to have 195 seedlings. Treatments were assigned randomly to the blocks. Cells are square on the top, 4.4 cm deep, spaced on 2.54 cm centers, and tapered to a square open bottom, giving the cells a pyramid-like shape. The blocks described above are replications of the treatments imposed in both experiments. An advantage of this approach was that time or season was eliminated as a variable that could impact response in an individual experiment. Experiments took place in Ithaca, NY (42◦26056.2” N 76◦28008.3” W). Flats were filled and seeded, then placed in a dark, climate-controlled growth chamber where germination occurred for two and a half days after seeding. Subsequently, flats were moved to a conventional glass greenhouse and transferred into tubs using a bench-scale deep-water culture (DWC) system, where they remained until harvest. A summary of seeding, floating, and harvest dates are given in Table 1. Table 1. Experiment descriptions and dates. Experiment Number Experiment Description Seeding Date Float Date Harvest Date 1 Seeding Pattern 10/11/2017 10/13/2017 10/28/2017 2 Cultivar Comparison 10/28/2017 10/30/2017 11/17/2017 2.1. Germination Chamber and Greenhouse Description A climate controlled chamber (0.93 m2 ) with no light and constant temperature of 24 ◦ C was used for germination. The DWC system was located on a waist-high bench in the NE corner of a conventional glass greenhouse. The greenhouse thermal characteristics are well-described by Vandam et al. [10] who raised spinach in a greenhouse section within the same complex under similar operational protocols. No supplemental light was provided. 2.2. Greenhouse Environmental Conditions Greenhouse ambient temperature was maintained at 24 ◦ C from 8 am–8 pm and 19 ◦ C at night by an automated Argus system (Argus Control Systems Ltd., Surrey, BC, Canada). The relatively new glass structure was estimated to have a solar light transmissivity of 65% based upon previous measurements by our research group. Light values were calculated using W/m2 readings gathered from an outdoor sensor connected to the greenhouse automation system that controlled the shade curtain and took readings at ten minute intervals. The quantum units were converted to PAR and multiplied by the transmissivity of the structure (65%). When the curtain was drawn over the bench on any time step, the PAR value was multiplied by the transmissivity of the shade curtain (40%) to estimate PAR reaching the canopy. These PAR values were integrated over 24 hour periods, yielding daily light integral (DLI) values. For Experiment 1 (seeding pattern) in which the flats were in the DWC tubs for 16 days, the average cumulative light received was 5.5 mol/m2 /day (Std. Dev. = 1.46) for a total of 88.4 mol per m2 over the entire crop cycle. For Experiment 2 (cultivar comparison), the average light received was 3.0 mol/m2 /day (Std. Dev. = 1.72) for a total of 56.2 mol per m2 received over the 19-day crop cycle. In both experiments the light levels were below the 17 mol/m2 /day considered ideal for continuous production [11].
Horticulturae 2019, 5, x FOR PEER REVIEW 4 of 13 both experiments the light levels were below the 17 mol/m2/day considered ideal for continuous Horticulturae 2019, 5, 20 4 of 12 production [11]. 2.3 2.3.Deep-Water Deep-WaterCulture CultureSystem SystemDescription Description The production productionperiod periodfor forthe thespinach spinachplants plantswaswas conducted conducted using using twotwo steel steel tubs tubs (61(61cm cm by 122 by 122 cm cm by 30.5 by 30.5 cm deep) cm deep) which which allowedallowed five five cut cut(described flats flats (described below)below) to be floated to be floated when completely when completely utilized. utilized. Nutrient Nutrient solution solution (described (described in Sectionin2.3) Section flowed2.3)freely flowed freely between between the two channels the two channels through through a 1.9 cm aID1.9 cm Tubs hose. ID hose. wereTubs were with insulated insulated 1.9 cmwith 1.9 cm polystyrene polystyrene boards on theirboards on their exterior exterior heat to minimize to minimize transfer heat transfercooling and reduce and reduce load.cooling A 1/6 HP load. A 1/6 HP submersible submersible pump was pump used towas pumpusednutrient to pump nutrient water water through a through a venturi injector (for aeration), 1 followed by a ¼ HP inline chiller that kept the water venturi injector (for aeration), followed by a 4 HP inline chiller that kept the water temperature at 19 ◦ C. Water at temperature was19 returned °C. Watertowas thereturned to thea tubs tubs through 2.5 cmthrough ID PVC a 2.5 pipecmwith ID PVC pipe outlets with into outlets each tub. into The each tub.water nutrient The nutrient and rootwater and root zone were zone were maintained below 20 ◦ C andbelow maintained highly20aerated, °C andsince highlythisaerated, since temperature this temperature has been has been to be effective to be effective for mitigating spreadforofmitigating spread ofespecially Pythium infection, Pythium P. infection, especiallyand aphanadermatum, P. aphanadermatum, the high degree ofand the high aeration alsodegree of aeration contributes alsoresistance to greater contributes to greater to infection resistance [12]. Oxygen to infection levels were [12]. Oxygen levels were also enhanced by using two air stones in each also enhanced by using two air stones in each tub. Dissolved oxygen saturation was verified using a tub. Dissolved oxygen saturation calibrated YSIwasPro20 verified using (Yellow a calibrated Springs, OH, USA)YSI Pro20 meter (Yellow when theSprings, system wasOH,operating. USA) meter when the system was operating. Sections of the tubs that were not occupied by flats were covered with StyrofoamTM to prevent growthSections of the of algae andtubs that were entering contaminants not occupied by flats the water. were covered Between with Styrofoam experiments, TM to prevent the tubs were emptied, growth cleaned,ofandalgae and contaminants sanitized with Greenshieldentering the water. (BASF, Between experiments, Ludwigshafen, Germany) tothe tubs were minimize emptied, risk of root cleaned, infection and and sanitized with Greenshield ensure consistent (BASF,conditions nutrient solution Ludwigshafen,between Germany) to minimize experiments. Figure 2risk of root shows the infection and ensure consistent nutrient solution experimental setup and placement of tubs, chiller and air pump. conditions between experiments. Figure 2 shows the experimental setup and placement of tubs, chiller and air pump. Figure 2. Figure Bench-scale deep-water 2. Bench-scale deep-water culture culture (DWC) (DWC) system system at at half-capacity. half-capacity. 2.4. Nutrient Solution Conditions 2.4 Nutrient Solution Conditions The stock solutions recommended by Sonneveld and Straver [13] were prepared per the recipe The stock solutions recommended by Sonneveld and Straver [13] were prepared per the recipe (Table 2). The day before the flats were removed from the germination chamber for floating, stock (Table 2). The day before the flats were removed from the germination chamber for floating, stock solutions were added to reverse osmosis (RO) water in the tubs in a 1:1 ratio (A:B) to achieve an solutions were added to reverse osmosis (RO) water in the tubs in a 1:1 ratio (A:B) to achieve an electrical conductivity (EC) of 1200–1400 µS/cm. This corresponds to a half strength solution as electrical conductivity (EC) of 1200–1400 μS/cm. This corresponds to a half strength solution as recommended by Sonneveld and Straver [13]. The half strength solution has been shown to be effective recommended by Sonneveld and Straver [13]. The half strength solution has been shown to be for lettuce and spinach [10,14]. The nutrient solution resulted in a pH between 5.7 and 6.3 without the effective for lettuce and spinach [10,14]. The nutrient solution resulted in a pH between 5.7 and 6.3 addition of any additional acid or base over the course of each experiment. without the addition of any additional acid or base over the course of each experiment. Throughout each experiment’s growth period, RO water and stock solutions were added to Throughout each experiment’s growth period, RO water and stock solutions were added to maintain the EC and pH within the desired range as checked by handheld digital meters, calibrated maintain the EC and pH within the desired range as checked by handheld digital meters, calibrated at the beginning of every experiment and every two days thereafter. Water level in the tubs was at the beginning of every experiment and every two days thereafter. Water level in the tubs was maintained high enough so that growing flats were not shaded by the tub walls. maintained high enough so that growing flats were not shaded by the tub walls.
Horticulturae 2019, 5, x FOR PEER REVIEW 5 of 13 Table 2. Stock solution recipes. Table 2. Stock solution recipes. Stock A – Chemicals/salts added to 30 L of RO Water Stock A – Chemicals/salts added to 30 L of RO Water Calcium Nitrate 2916.0 g Horticulturae 2019, 5, 20 5 of 12 Calcium Nitrate 2916.0 g Potassium Nitrate 613.2 g Potassium Nitrate 613.2 g Ammonium Nitrate 84.0 g Ammonium Nitrate Table 2. Stock solution recipes. 84.0 g Sprint 330 Iron – DTPA (10% Iron) 67.0 g Sprint Stock 330 Iron – DTPA A—Chemicals/salts (10%toIron) added 67.0Water 30 L of RO g Calcium Nitrate 2916.0 g Stock BNitrate Potassium – Chemicals/salts added to 30 L Water 613.2 g Stock Ammonium B – Chemicals/salts added to 30 L Water PotassiumNitrate Nitrate 2037.8 g 84.0 g Sprint 330 Iron—DTPA Potassium Nitrate (10% Iron) 2037.8 g 67.0 g Monopotassium Phosphate 816.0 g Stock B—Chemicals/salts Monopotassium added to 30 L Water Phosphate 816.0 g Potassium Sulfate 65.6 g Potassium PotassiumNitrate Sulfate 65.6 g 2037.8 g Magnesium Sulfate Monopotassium Phosphate 1478.0 g 816.0 g Magnesium Potassium Sulfate Sulfate 1478.0 g 65.6 g Manganese Sulfate* H2O (25%Mn) 2.56 g Magnesium Manganese Sulfate Sulfate* H2O (25%Mn) 2.56 g 1478.0 g Zinc Sulfate* Manganese H 2O Sulfate* H2(35%Zn) O (25%Mn) 3.44 g 2.56 g Zinc Zinc Sulfate* Sulfate* H 2O (35%Zn) 3.44 g Boric AcidH2 O (35%Zn) 5.58 g 3.44 g Boric Acid Boric Acid 5.58 g 5.58 g Copper Copper Sulfate* Sulfate* 5H2 O 2O (25%Cu) 5H(25%Cu) 0.56 g 0.56 g Copper Ammonium Sulfate* 5H Molybdate 2O (25%Cu) 0.56 g 0.26 g Ammonium Molybdate 0.26 g Ammonium Molybdate 0.26 g 2.5 2.5.Dibbling DibblingTools Tools 2.5 Dibbling Tools A combination of conventionally built and 3D-printed dibbling dibbling tools was used for these A combination experiments (Figures of (Figures3 andconventionally 3 and built 4). Dibblers 4). Dibblers and 3D-printed developed developed dibbling by Cornell by Cornell CEAtools CEA specifically forwas used specifically spinach forfor these spinach production experiments production create a divot (Figures create in which 3 and a divotseeds 4). in which Dibblers developed seeds atop are sown, are sown, whichatop moreby whichCornell CEA more can substrate specifically substrate can be be added for so added spinach that thesolarge that production the create largeseeds spinach spinach were a adequately divot seeds in which were deepseeds adequately are sown, and deep atop which andincovered covered the EPS more inflats. the EPS substrate flats. can be added so that the large spinach seeds were adequately deep and covered in the EPS flats. Figure 3. Conventionally-built plate dibbler. Figure 3. Figure Conventionally-built plate 3. Conventionally-built plate dibbler. dibbler. (a) (b) (a) (b) Figure Figure 4. (a) (a) Single Single rolling rolling dibbler; dibbler; (b) Whole flat dibble roller. Figure 4. (a) Single rolling dibbler; (b) Whole flat dibble roller. 2.6. Seeds Seeds were purchased from Jonny’s Seeds Inc. (Winslow, ME) in September 2017. When not in use, they were stored in a cool, dark, dry environment. Germination percentages reported by the
Horticulturae 2019, 5, x FOR PEER REVIEW 6 of 13 2.6 Seeds Seeds2019, Horticulturae were 5, purchased 20 from Jonny’s Seeds Inc. (Winslow, ME) in September 2017. When not 6 of in 12 use, they were stored in a cool, dark, dry environment. Germination percentages reported by the supplier were: Carmel—84%, Seaside— 97%, Space—99%. Carmel and Space are both semi-savoy, supplier meaning were: Carmel—84%, the true leaves have aSeaside— 97%, Space—99%. slightly crinkled appearance.Carmel Seasideand Space are both is a smooth-leaf semi-savoy, variety. meaning the true leaves have a slightly crinkled appearance. Seaside is a smooth-leaf variety. 2.7 Sowing, Germination, and Floating Procedure 2.7. Sowing, Germination, and Floating Procedure Before seeding, flats were cleaned, disinfected with a Greenshield solution, thoroughly rinsed, Before seeding, flats were cleaned, disinfected with a Greenshield solution, thoroughly rinsed, and dried. Sungro (Vancouver, BC, Canada) Propagation mix, composed of sphagnum peat moss, and dried. Sungro (Vancouver, BC, Canada) Propagation mix, composed of sphagnum peat moss, horticultural vermiculite, pH adjustment lime, wetting agent, and starter nutrients was used to fill horticultural vermiculite, pH adjustment lime, wetting agent, and starter nutrients was used to fill flats. Reverse Osmosis (RO) water was added to the dry medium at a ratio of 0.6 kg RO water per 1 flats. Reverse Osmosis (RO) water was added to the dry medium at a ratio of 0.6 kg RO water per kg dry medium and mixed in 20 L buckets. This moisture content was chosen from a trial run that 1 kg dry medium and mixed in 20 L buckets. This moisture content was chosen from a trial run that showed comparable performance between flats germinated with either 60% or 100% medium wet showed comparable performance between flats germinated with either 60% or 100% medium wet weight to medium dry weight. The medium and water were well-mixed, and the lid was kept on the weight to medium dry weight. The medium and water were well-mixed, and the lid was kept on the buckets except when mixing or filling flats to minimize moisture loss. buckets except when mixing or filling flats to minimize moisture loss. When filling the flats for the first pass, a 0.6 cm metal mesh screen (Figure 5) was used as a sieve When filling the flats for the first pass, a 0.6 cm metal mesh screen (Figure 5) was used as a sieve to to remove any clumps from the medium and ensure homogenous, loosely-packed filling of cells. remove any clumps from the medium and ensure homogenous, loosely-packed filling of cells. Excess Excess medium was scraped off the top of the flat with a straight edge. The plate dibbler in Figure 3 medium was scraped off the top of the flat with a straight edge. The plate dibbler in Figure 3 was used was used to create consistent dibbles in each cell and then flats were seeded at the appropriate density to create consistent dibbles in each cell and then flats were seeded at the appropriate density using the using the designated cultivar. designated cultivar. Figure 5. Figure Metal mesh 5. Metal mesh sieve sieve used used to to remove remove clumps clumps and and ensure ensure loose loose filling. filling. After initial seeding, another layer of medium was added on top of the seeds, following the same After initial seeding, another layer of medium was added on top of the seeds, following the same sieving procedure described for the first pass. Finally, the rolling dibbler pictured in Figure 4 was used sieving procedure described for the first pass. Finally, the rolling dibbler pictured in Figure 4 was to compress the medium on top of the seeds in the second pass by rolling the dibbler across the flat used to compress the medium on top of the seeds in the second pass by rolling the dibbler across the surface back and forth several times. flat surface back and forth several times. All flats were stacked randomly after they had been seeded, filled, and dibbled. Non-seeded All flats were stacked randomly after they had been seeded, filled, and dibbled. Non-seeded guard flats were placed on the top and bottom of the stack. The entire stack of flats was wrapped in guard flats were placed on the top and bottom of the stack. The entire stack of flats was wrapped in plastic to prevent moisture loss. The stack was moved into a larger, rigid plastic bin for transportation plastic to prevent moisture loss. The stack was moved into a larger, rigid plastic bin for transportation to the germination chamber, where the flats were stored for 60 hours in the dark at 24 ◦ C before to the germination chamber, where the flats were stored for 60 hours in the dark at 24 °C before removal for inspection and floating. removal for inspection and floating. Upon removal from the germination chamber, flats were inspected for pests and abnormalities. Upon removal from the germination chamber, flats were inspected for pests and abnormalities. Throughout the experiments, no pests were found in any seedlings. Germination appeared successful Throughout the experiments, no pests were found in any seedlings. Germination appeared successful upon removal from the chamber, meaning there were some visible seedlings protruding from the upon removal from the chamber, meaning there were some visible seedlings protruding from the medium, as seen in Figure 6, and many more young roots protruding out of the bottoms of flats. After medium, as seen in Figure 6, and many more young roots protruding out of the bottoms of flats. After this step, the flats were floated in the tubs, where they remained until removal for harvest. this step, the flats were floated in the tubs, where they remained until removal for harvest.
Horticulturae 2019, 5, x FOR PEER REVIEW 7 of 13 Horticulturae 2019, 5, 20 7 of 12 Flats just Figure 6. Flats Figure just after after floating floating with seedlings breaching the medium surface. 2.8. Seedling Count 2.8 Seedling Count On the sixth day after floating in the tubs, the visible seedlings were counted for each treatment. On the sixth day after floating in the tubs, the visible seedlings were counted for each treatment. This count included “popups” (seedlings that withered because their roots did not reach the water), This count included “popups” (seedlings that withered because their roots did not reach the water), which were uncommon [9]. Counting seedlings earlier than six days was determined to be a poor which were uncommon [9]. Counting seedlings earlier than six days was determined to be a poor indication of germination and initial growth success. indication of germination and initial growth success. 2.9. Harvest Protocol 2.9 Harvest Protocol Flats were harvested when the fastest growing leaves of a treatment reached marketable size, Flats were harvested when the fastest growing leaves of a treatment reached marketable size, which occurred between 14 and 18 days after transfer to the tubs. This harvest date initiation was which occurred between 14 and 18 days after transfer to the tubs. This harvest date initiation was chosen so that no leaves exceeded 7.5 cm in length (when measured from the base of the leaf). chosen so that no leaves exceeded 7.5 cm in length (when measured from the base of the leaf). Harvest was performed by hand with scissors. A handful of leaves would be carefully grasped. Harvest was performed by hand with scissors. A handful of leaves would be carefully grasped. The stems would then be cut about halfway down the stems of the longest leaves. This meant some The stems would then be cut about halfway down the stems of the longest leaves. This meant some smaller leaves sitting lower in the canopy were harvested as well, but with shorter stems. A large smaller leaves sitting lower in the canopy were harvested as well, but with shorter stems. A large variance in leaf size was found for all treatments, which was due both to the non-simultaneous variance in leaf size was found for all treatments, which was due both to the non-simultaneous germination observed, and the presence of young, second true leaves in addition to the larger first true germination observed, and the presence of young, second true leaves in addition to the larger first leaves. Harvest weight was measured immediately after leaves were harvested using a digital scale true leaves. Harvest weight was measured immediately after leaves were harvested using a digital accurate to one gram. As can be seen in Figure 7, many small leaves sitting low in the canopy were scale accurate to one gram. As can be seen in Figure 7, many small leaves sitting low in the canopy not harvested. were not harvested. 2.10. Statistical Analysis In Experiment 1 (seeding pattern with single cultivar) there were four replications of each pattern and in Experiment 2 (cultivar response to fixed seeding method), there were four replications of the Carmel and Seaside cultivars and two replications of the Space cultivar. For both experiments, a one-way analysis of variance using Tukey–Kramer paired t-tests was performed using JMP PRO 11 statistical analysis software [15]. The average and standard deviation of each treatment (pattern or cultivar) except for the cultivar Space was calculated for both final seedling counts and fresh weight. Data from a typical high-germinating flat is presented to show the intra-flat variation observed, even when germination was high.
Horticulturae 2019, 5, 20 8 of 12 Horticulturae 2019, 5, x FOR PEER REVIEW 8 of 13 Figure 7. Before and mid-harvest of Carmel; some small second true leaves are left behind low in Figure 7. Before and mid-harvest of Carmel; some small second true leaves are left behind low in the the canopy. canopy. 3. Results 2.10 Statistical Analysis 3.1. Experiment 1: Seeding In Experiment Pattern pattern with single cultivar) there were four replications of each 1 (seeding pattern and in Experiment 2 (cultivar response to fixed seeding method), there were four replications Results of fromand the Carmel theSeaside seeding patternand cultivars experiment showed two replications that of the seeding Space pattern cultivar. [1-2-1-2] For both or [3-0-3-0] experiments, negatively a one-way analysis of variance using Tukey–Kramer paired t-tests was performed using JMP PROblock affected germination. There were 157 (Std. Dev. = 4.2) seedlings per replicate 11 in the [1-2-1-2] statisticalpattern analysisversus software148[15]. (Std. Dev. The = 4.5) average seedlings and standardper replicate deviation block of each in the [3-0-3-0] treatment (pattern pattern or (Tablecultivar) 3) fromexcept a totalforofthe 195cultivar seeds Space planted wasper replicateforblock. calculated The seedling both final Carmel cultivar counts andhad a manufacturer fresh weight. Datasprouting reported from a typical high-germinating percentage of 83%. Ourflat is presented results to show for both the intra-flat treatments variation achieved highobserved, evenof the percentages when germination was high. manufacturer’s reported germination percentage although numerically the [1-2-1-2] pattern was 97% of the reported value while the [3-0-3-0] pattern was 91% of the reported value. Fresh weight harvest 3. Results yield was lower numerically but not statistically with the [3-0-3-0] sowing pattern as well. Since the reductions in germination 3.1 Experiment 1: Seeding and yield were less than 10%, growers should consider and evaluate the Pattern cost of additional substrate versus reductions in performance based upon these results. Results from the seeding pattern experiment showed that seeding pattern [1-2-1-2] or [3-0-3-0] negatively affected germination. There were 157 (Std. Dev. = 4.2) seedlings per replicate block in the Table 3. Effects of seeding pattern on seed germination from 195 seeds per replicate block and harvested [1-2-1-2] pattern versus 148 (Std. Dev. = 4.5) seedlings per replicate block in the [3-0-3-0] pattern (Table fresh weight yield of Carmel spinach. Cells were seeded according in [1-2-1-2] or [3-0-3-0] patterns 3) from a total of 195 seeds planted per replicate block. The Carmel cultivar had a manufacturer giving average density of 1.5 seeds per cell. Different superscripts indicate significant differences reported sprouting percentage of 83%. Our results for both treatments achieved high percentages of between the treatmentsreported manufacturer’s by Tukey–Kramer germinationpaired t-tests atalthough percentage α = 5%. numerically the [1-2-1-2] pattern was 97% of the reported Treatmentvalue while Replication the [3-0-3-0] pattern was 91% Seedling of the reported Count value. (g) Fresh Weight Fresh weight harvest yield was lower numerically but not statistically with the [3-0-3-0] sowing pattern as well. [1-2-1-2] 1 150 144 Since the reductions in germination and 2 yield were less159 than 10%, growers should 157 consider and evaluate the cost of additional substrate3 versus reductions158 in performance based148 upon these results. 4 161 156 Mean 157 a 151.2 a Std. Dev. 4.2 5.4 [3-0-3-0] 1 144 134 2 155 152 3 144 146 4 149 145 Mean 148 b 144.2 a Std. Dev. 4.5 6.5
Horticulturae 2019, 5, 20 9 of 12 3.2. Cultivar Comparison As shown in Table 4, Carmel and Space cultivars had a significantly higher germination percentage than Seaside (α = 5%) and fresh weight yield production (α = 5%). Carmel had the highest germination (81%) of the three cultivars tested. Carmel averaged 98% of its reported 83% germination rate. Final germination percentage was stable for Carmel across both experiments; Experiment 1 was also 81%. Space and Seaside cultivars germinated and produced seedlings at a relatively high rate, as well, but at significantly lower rates than reported by the supplier. Table 4. Effects of Carmel, Seaside, and Space cultivars on germination and harvested fresh weight yield with all cells seeded in a [1, 2, 1, 2] seeding pattern (195 seeds planted). Different superscripts indicate significant differences between treatments by Tukey–Kramer paired t-tests at α = 5%. Reported Actual Seedling Harvested Fresh Cultivar Germination % Germination % Count Weight (g) Carmel 83 81 157 a 150 a Seaside 97 74 144 b 94 b Space 99 76 147 a 146 a Seaside baby spinach grown in the low-light conditions of November in Ithaca produced a short, bushy canopy compared to that of Carmel and Space as can be seen in Figure 8. The latter two cultivars Horticulturaelonger produced 2019, 5, xstemmed FOR PEERleaves REVIEWthat sat higher above the cotyledons and were easier to harvest. 10 of 13 Figure 8. Carmel (left) and Seaside (right) at harvest illustrating the contrast between the long stems Figure 8. Carmel (left) and Seaside (right) at harvest illustrating the contrast between the long stems of Carmel and the short, bushy canopy of Seaside. of Carmel and the short, bushy canopy of Seaside. Pericarps are the hard outer coat of spinach seeds that are sometimes stuck to the tips of emerging Pericarps are the hard outer coat of spinach seeds that are sometimes stuck to the tips of cotyledons (Figure 9a). At time of harvest, they were infrequent and hard to spot as cotyledons become emerging cotyledons (Figure 9a). At time of harvest, they were infrequent and hard to spot as tangled in the lower leaves. However, it was rare to spot a pericarp on cotyledons after a week in the cotyledons become tangled in the lower leaves. However, it was rare to spot a pericarp on cotyledons tubs as most were pushed off by the growing cotyledons and true leaves. (Figure 9b). after a week in the tubs as most were pushed off by the growing cotyledons and true leaves. (Figure In the Carmel and Space treatments, fewer cotyledons ended up in the harvested spinach, in 9b). general, due to the longer stems of the true leaves and the nature of the cotyledons to fold over In the Carmel and Space treatments, fewer cotyledons ended up in the harvested spinach, in under the true leaves compared to Seaside. Overall, this resulted in almost no pericarps in the general, due to the longer stems of the true leaves and the nature of the cotyledons to fold over under harvested leaves. the true leaves compared to Seaside. Overall, this resulted in almost no pericarps in the harvested leaves.
after a week in the tubs as most were pushed off by the growing cotyledons and true leaves. (Figure 9b). In the Carmel and Space treatments, fewer cotyledons ended up in the harvested spinach, in general, due to the longer stems of the true leaves and the nature of the cotyledons to fold over under the true leaves compared to Seaside. Overall, this resulted in almost no pericarps in the harvested Horticulturae 2019, 5, 20 10 of 12 leaves. Horticulturae 2019, 5, x FOR PEER REVIEW 11 of 13 Figure 9. (top) Pericarp stuck on cotyledon late in the crop cycle; (bottom) A flat one week after floating Figure 9. (top) Pericarp stuck on cotyledon late in the crop cycle; (bottom) A flat one week after with few pericarps. floating with few pericarps. 3.3. Intra-Flat Variation 3.3 Intra-Flat Variation One flat of Carmel was evaluated to look at row to row variation in germination. Even for a One flat of Carmel high-germinating flat of was evaluated Carmel, to lookin the variation atseeds row to row that variation in germinated germination. between rows ofEven a flatfor wasa high-germinating flat of Carmel, the variation in seeds that germinated between rows of surprising: across 22 rows, germination was 80.4% (Std. Dev. = 2.32), and ranged from 56% to 97%. As a flat was surprising: the seedlingsacross grew,22they rows, germination took advantagewas 80.4% of all (Std. space available Dev. =to2.32), and create ranged from a relatively 56% to uniform 97%. canopy As the seedlings which seemed togrew, theyeffects mitigate took advantage caused by of all available intra-row spacein variation toseed create a relatively uniform canopy germination. which seemed to mitigate effects caused by intra-row variation in seed germination. 4. Discussion 4. Discussion These experiments correspond to first steps in a comprehensive comparison of the cultivars represented in these experiments. These experiments Ideally, correspond the steps to first studyin would have been conducted a comprehensive in a growth comparison chamber of the cultivars with consistent represented DLI,experiments. in these temperature,Ideally, and relative humidity the study wouldashave in [2]. beenHowever, conducted thein germination results a growth chamber were consistent and largely independent of fluctuations in greenhouse light conditions. It with consistent DLI, temperature, and relative humidity as in [2]. However, the germination results is peculiar that the were cultivarand consistent withlargely the lowest reportedof independent germination, fluctuationsCarmel, had thelight in greenhouse highest overall It conditions. germination is peculiar that the cultivar with the lowest reported germination, Carmel, had the highest overall germination percentage (nearly 100% of that reported by the supplier) and the most vigorous growth during the short 15–18 day in-tub cycles used in our experiments. Space also performed relatively well when compared to Seaside. Seaside was the least suitable for DWC hydroponic production given its lower germination, short stems, and reduced growth.
Horticulturae 2019, 5, 20 11 of 12 percentage (nearly 100% of that reported by the supplier) and the most vigorous growth during the short 15–18 day in-tub cycles used in our experiments. Space also performed relatively well when compared to Seaside. Seaside was the least suitable for DWC hydroponic production given its lower germination, short stems, and reduced growth. It appears the current sowing and germination procedure is sub-optimal for Space and Seaside, which germinated less frequently than their reported rate. In future tests, cells will be taken apart and studied where no seedlings emerged to observe the seed condition. If seeds had germinated in the cell but then had not developed properly, this would suggest altering the planting procedures. For instance, if the seed germinated but the seedling withered and died before it could breach the medium surface or its roots reached the nutrient solution, different cell dimensions, seed positions in cells, or compaction/dibbling schemes should be tested. On the other hand, if cells were found to have non-germinated seeds, then different germination temperatures, a wider range of medium moisture contents, or duration spent in the germination chamber should be investigated, among other germination procedure alterations. As mentioned previously, seedling numbers earlier than six days after floating started were not necessarily indicative of the final numbers. This suggests methods to imbibe or prime seeds could be applied to synchronize germination timing and speed up the germination of the slowest seeds. If the last seedlings are emerging several days after the first, there is a clear loss of productivity and the maximum potential of the seeds is not being realized. Cornell CEA has done extensive work on imbibing procedures [9,16,17]. The pattern experiment results showed that there was a small reduction (10%) in final yield of Carmel when fewer cells were seeded but at a higher density per cell. This implies that proportional cell volume available to each seedling is not predictive of the growth within the range considered, which could be due to the significantly shorter cycle observed than considered by Romano et al. [6]. Plus, in a DWC system the large majority of the root mass is dangling in the nutrient solution below the flat, so the cell volume available does not necessarily restrict root growth or nutrient uptake. A custom-designed flat with half as many cells as the Speedling 338, but spaced twice as far apart would cut medium costs roughly in half. The relative costs of medium and seeds are such that compensating for the reduced germination associated with sowing more seeds in each cell (as was observed in Experiment 1) with 10% more seeds while using half the medium would result in a large proportional decrease in material input costs per flat. These preliminary tests corroborate similar results observed with the spinach cultivar Alrite and highlight the potential for a flat custom designed for baby spinach production [9,16,17]. Another promising observation from Experiment 1 was that the canopies of blocks in both treatments were full and relatively uniform. This validates the experimental design assumption that the canopy density effects between the treatments would be the same. It also suggests that seedlings will not be shaded out by neighbor plants when more seeds are planted in each cell if the overall canopy density is not too high and there is adequate room adjacent to the cell for seedlings to expand into during growth. Author Contributions: D.B.J. and M.B.T. conceived and designed the experiment; D.B.J. performed the experiment; D.B.J. and M.B.T. analyzed the data; D.B.J. wrote the paper with significant contributions from M.B.T. Acknowledgments: This research was supported by the Cornell University Department of Biological and Environmental Engineering as well as Element Farms, Inc. who supplied the seeds. We would like to thank Francoise Vermeylen from the Cornell Statistical Consulting Unit for her guidance on experimental design and statistical analysis. We express our gratitude to Olav Imsdahl (Cornell M.Eng 2017) for his work on 3D printed dibbling tools. Additional thanks to Alan Taylor and David deVilliers for advice and guidance and Neil Mattson for equipment and greenhouse space. Conflicts of Interest: The authors declare no conflict of interest.
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