Article Influence of Atomization Nozzles and Spraying Intervals on Growth, Yield, and Uptake of Butter-Head under Aeroponics System

Mazhar H. Tunio , Jianmin Gao *, Imran A. Lakhiar , Kashif A. Solangi, Waqar A. Qureshi , Sher A. Shaikh and Jiedong Chen

School of Agricultural Equipment Engineering, Jiangsu University, Zhenjiang 212013, ; [email protected] (M.H.T.); [email protected] (I.A.L.); [email protected] (K.A.S.); [email protected] (W.A.Q.); [email protected] (S.A.S.); [email protected] (J.C.) * Correspondence: [email protected]; Tel.: +86-136-5528-2069

Abstract: The atomized nutrient solution droplet sizes and spraying intervals can impact the chem- ical properties of the nutrient solution, biomass yield, root-to-shoot ratio and nutrient uptake of aeroponically cultivated . In this study, four different nozzles having droplet sizes N1 = 11.24, N2 = 26.35, N3 = 17.38 and N4 = 4.89 µm were selected and misted at three nutrient solution spraying intervals of 30, 45 and 60 min, with a 5 min spraying time. The measured parameters were power of hydrogen (pH) and electrical conductivity (EC) values of the nutrient solution, shoot and root growth, ratio of roots to shoots (fresh and dry), biomass yield and nutrient uptake. The results indicated that the N1 presented significantly lower changes in chemical properties than those of N2, N3 and

 N4, resulting in stable lateral root growth and increased biomass yield. Also, the root-to-shoot ratio  significantly increased with increasing spraying interval using N1 and N4 nozzles. The N1 nozzle also revealed a significant effect on the phosphorous, and uptake by the plants Citation: Tunio, M.H.; Gao, J.; Lakhiar, I.A.; Solangi, K.A.; Qureshi, misted at proposed nutrient solution spraying intervals. However, the ultrasonic nozzle showed W.A.; Shaikh, S.A.; Chen, J. Influence a nonsignificant effect on all measured parameters with respect to spraying intervals. In the last, of Atomization Nozzles and Spraying this research experiment validates the applicability of air-assisted nozzle (N1) misting at a 30-min Intervals on Growth, Biomass Yield, spraying interval and 5 min of spraying time for the cultivation of butter-head lettuce in aeroponic and Nutrient Uptake of Butter-Head systems. Lettuce under Aeroponics System. Agronomy 2021, 11, 97. https:// Keywords: soilless cultivation; food security; climate change; aeroponics; management; doi.org/10.3390/agronomy11010097 air-assisted atomizer; root characteristics

Received: 7 November 2020 Accepted: 3 January 2021 Published: 6 January 2021 1. Introduction

Publisher’s Note: MDPI stays neu- Presently, the global population continues to grow and has crossed a milestone in tral with regard to jurisdictional clai- human history. A study reported that there have been significant global changes observed ms in published maps and institutio- over the last decade, and food security remains a major concern. Besides, the unpredictable nal affiliations. climate changes have negatively affected the availability of natural resources, such as water, agricultural land and food production [1,2]. Through flooding, hurricane, storms and droughts have drastically reduced land [3,4]. Scientists predicted that adverse weather conditions and climate change will result in the deprivation of the large Copyright: © 2021 by the authors. Li- areas of arable land, rendering them unstable for farming [5,6]. Accordingly, climate censee MDPI, Basel, Switzerland. change and water scarcity are considered among the most significant problems and are This article is an open access article expected to worsen in the future and could create serious food security issues for feeding distributed under the terms and con- large populations [7–10]. Recently, increased interest has been directed towards ditions of the Creative Commons At- production in closed facilities such as plant factories, vertical farms and indoor-growing tribution (CC BY) license (https:// modules [11–14]. Indoor planting can provide a healthy environment for growing food creativecommons.org/licenses/by/ and is not affected by climatic conditions [14]. Indoor agriculture also uses advanced 4.0/).

Agronomy 2021, 11, 97. https://doi.org/10.3390/agronomy11010097 https://www.mdpi.com/journal/agronomy Agronomy 2021, 11, 97 2 of 17

agricultural technologies to reduce water consumption by providing precise irrigation and effective scheduling [15–17]. Innovative agricultural techniques are advisable to adopt soilless cultivation systems that are independent of climate conditions, fertility and soil-borne diseases and do not require large spaces and intensive work labor. These plant cultivation techniques are the most intensive and uses all available resources efficiently and maximize the yield com- pared to that of the traditional agriculture [15]. The use of soilless cultivation for improved production and aeroponic cultivation is the most appropriate modern soilless practice [15]. This system can be defined as a closed air and water/nutrient ecosystem that promotes swift growth of plants with virtually no water, soil or medium [18–24]. Aeroponics uses mist or nutrient solution as an alternative to water, which is an effective and competent method for plant growth. Aeroponics can save 95% of the water used in conventional agricultural practices and requires minimal space. This technique has been applied successfully for cultivation of leafy vegetables, root vegetables, aromatic herbs and medicinal plants, because the nutritional quality and properties, such as phenolic compound, flavonoid, antioxidant and vitamin contents, were higher in aeroponic-grown plants than in other soilless plants and soil-grown plants [25]. Moreover, the aeroponics system is still scientifically unclear, and several aspects of the system, such as the proper selection of atomizers (droplets), spraying time, spraying interval, root-zone temperature, humidity and best nutrient solution for different varieties of vegetables, have yet to be investigated [26–28]. In aeroponics systems, the nutrient solution is misted through at- omizers, such as mechanical atomizers and piezoelectric ultrasonic foggers. Additionally, the mechanical atomizers are further categorized as air-based and airless (high and low pressures) atomizers. However, the ultrasonic foggers are categorized as high, medium and low frequency [29–31]. In aeroponics systems, plants receive nutrient droplets directly, which can affect the plant root environment, therefore, the main problem of the system is the size of the nutrient droplets, because relatively large droplets could reduce the available to the root system [22,27]. Moreover, a relatively low oxygen concentration could affect root respiration and nutrient absorption, and the amount of oxygen available to the root environment is a highly significant factor for plant growth [22]. Besides, the small water droplets could produce many root hairs, and roots may not form a lateral root system and thus cannot continue to grow. Furthermore, choosing an appropriate nutrient solution misting method is very important for plant growth, because the frequency, atomization time and atomization interval could affect the physical and chemical properties and quality of the nutrient solution. Electrical conductivity (EC) and pH are the most important parameters of the nutrient solution, and thus far, studies have reported that the atomizer (droplet size) can change the EC and pH values of the nutrient solution after dissolution, resulting in a reduction in yield biomass [21]. Lettuce is one of the most widely cultivated vegetable in the world and is eaten as a salad green. Lettuce has become the focus of many studies due to its high nutritional value, and contents of minerals, vitamins and folic acid, which play an important role in the human diet and nutrition [32–36]. Hence, for this study, we selected the lettuce as the test crop. The objective of this study was to determine the proper droplet size (atomizer) and nutrient solution spraying interval of the aeroponic system and the effects on the chemical properties of the nutrient solution, root-to-shoot ratio and nutrient uptake of aeroponically cultivated lettuce.

2. Materials and Methods 2.1. Experimental Site and Climate Conditions The experiment was conducted in the semi-controlled Venlo-type greenhouse of Jiangsu University P.R. China (33.57◦ N, 118.16◦ E), during November–December 2019. For the determination of climate conditions, an automatic weather station (Hobo U12-012, Onset Computer Corp.) was placed in the center of the aeroponics systems. The measured Agronomy 2021, 11, x FOR PEER REVIEW 3 of 18

2. Materials and Methods 2.1. Experimental Site and Climate Conditions The experiment was conducted in the semi-controlled Venlo-type greenhouse of Jiangsu University P.R. China (33.57° N, 118.16° E), during November–December 2019. For the determination of climate conditions, an automatic weather station (Hobo Agronomy 2021, 11,U12-012, 97 Onset Computer Corp.) was placed in the center of the aeroponics systems. The 3 of 17 measured maximum and minimum average temperatures were 20.13 ± 1.32 °C and 7.65 ± 0.89 °C respectively, in the greenhouse, and 16.46 ± 0.72 °C and 4.17 ± 1.02 °C respec- maximum and minimum average temperatures were 20.13 ± 1.32 ◦C and 7.65 ± 0.89 ◦C tively, in the growthrespectively, boxes in of the the greenhouse, aeroponics and systems. 16.46 ± 0.72Likewise,◦C and the 4.17 maximum± 1.02 ◦C respectively,and in the minimum relativegrowth humidity boxes were of the 68.43% aeroponics ± 5.13% systems. and 45.73% Likewise, ± 2.73% the maximumrespectively, and in minimumthe relative greenhouse, andhumidity 89.4% ± were 3.56% 68.43% and 52.35%± 5.13% ± 1.98% and 45.73% respectively,± 2.73% in respectively, the growth chamber in the greenhouse, and of the aeroponics89.4% systems.± 3.56% and 52.35% ± 1.98% respectively, in the growth chamber of the aeroponics systems. 2.2. Aeroponic System and Nozzles For present2.2. study, Aeroponic the aeroponic System and system Nozzles previously reported by our research team [37] was utilized. TheFor presentaeroponic study, systems the aeroponic were manufactured system previously with reportedblue polyethylene by our research team [37] plastic containerswas with utilized. a Styrofoam The aeroponic lid fixed systems in a steel were frame. manufactured Additionally, with bluethe system polyethylene plastic was composedcontainers of four different with a Styrofoam atomizers lid(droplet fixed insizes), a steel a pressure frame. Additionally, pump (model the system was PLD-1206, Prandycomposed Electromecha of fournical different Equipment atomizers Co., (droplet Ltd., Shijiazhuang sizes), a pressure City, China), pump (modelair PLD-1206, compressors (modelPrandy 750-30<2530>, Electromechanical Shengyuan Equipment Air Compressor Co., Ltd., Shijiazhuang Manufacturing City, Co., China), Ltd., air compressors Wenling, Zhenjiang(model, China), 750-30<2530>, a mist pipeline, Shengyuan a reflux Air pipeline, Compressor a fluid Manufacturing infusion measuring Co., Ltd., Wenling, pump, an axialZhenjiang, flow fan, China),plastic cups a mist to pipeline, hold the a plants, reflux pipeline,a nutrient a fluidsolution infusion tank, measuring a de- pump, an mographic pumpaxial pressure flow fan, plasticregulator, cups a to st holdainless-steel the plants, net a nutrientand connectors solution tank, (two- a demographicand pump three-way connectors).pressure The regulator, schematic a stainless-steel view of the netaeroponics and connectors system is (two- displayed and three-way in Fig- connectors). ure 1. The schematic view of the aeroponics system is displayed in Figure1.

Figure 1. LayoutFigure of 1. the Layout experimental of the experimental setup: 1, plants; setup: 2, 1, plant plants; holder; 2, plant 3, lid; holder; 4, plant 3, lid; roots; 4, plant 5, fog roots; circulation 5, fog fan; 6, nutrient solution mist;circulation 7, nutrient fan; solution 6, nutrient fog; solution 8, nutrient mist; misting 7, nutrient line; 9,solution drainage fog; line; 8, nutrient 10, nutrient misting solution; line; 9, 11, drain- nutrient solution tank; 12, airage compressor; line; 10, nutrient 13, timers; solution; 14, pressure 11, nutrient pump; solution 15, growth tank; boxes; 12, air 16 compressor; low-pressure 13, nozzle; timers; 17, 14, low-pressure pressure nozzle; 18, ultrasonicpump; nozzles; 15, growth 19, high-pressure boxes; 16 low-pressure nozzle with air; nozzle; 20, electric 17, low-pressure box; 21, air nozzle; inlet; 22, 18, connectors. ultrasonic nozzles; 19, high-pressure nozzle with air; 20, electric box; 21, air inlet; 22, connectors.

2.3. Measurement2.3. of MeasurementDroplet Sizes of Droplet Sizes The droplet sizeThe distribution droplet size is distribution known as isa knownvery important as a very importantparameter parameter for the hy- for the hydraulic draulic performanceperformance of any nozzle/atomizer. of any nozzle/atomizer. A laser particle A laser particlesize analyzer size analyzer (Winner318B, (Winner318B, Jinan Jinan Winner WinnerParticle ParticleInstruments Instruments Stock Co., Stock Ltd., Co., Jinan, Ltd., Jinan,China), China), a computer, a computer, an air an air compres- compressor (OTS-550,sor (OTS-550, Taizhou Taizhou Outstanding Outstanding Industry Industry and andTrade Trade Co., Co., Ltd., Ltd., Taizhou, Taizhou, China) and China) and a pressurea pressure pump pump (PLD-1206, (PLD-1206, Shijiazhuang Shijiazhuang Pulandi Pulandi Machine Machine Equipment Equipment Co., Co., Ltd., Wen- Ltd., Wenling, ling,Zhenjiang Zhenjiang,, China) China) were wereused to used determine to determine the particle the particle size distribution, size distribution, as as shown shown in Figurein 2.Figure The 2instrument. The instrument was comp wasosed composed of a photodiode of a photodiode detector, detector,laser trans- laser transmitter mitter and storageand storagecircuit imaging circuit imaging system. system.More importantly, More importantly, the working the working pressures pressures of of the air the air compressor,compressor, water pump water and pump water and flow water rate flow for the rate air-assisted for the air-assisted atomizeratomizer were 0.4 were 0.4 MPa, −1 MPa, 0.2 MPa and0.2 MPa 4 L min and−14, Lrespectively. min , respectively. A pump Apressure pump pressureof 0.2 MPa of 0.2and MPa a water and aflow water flow rate of −1 rate of 1 L min−11 Lwere min stablewere for stable each forairless each atomizer, airless atomizer, and the andflow the rate flow of each rate ultrasonic of each ultrasonic fogger was 1 L min−1. More importantly, the flow rates and pressures were kept constant for the air compressor and water pump throughout the cultivation period.

Agronomy 2021, 11, x FOR PEER REVIEW 4 of 18

Agronomy 2021, 11, 97 fogger was 1 L min−1. More importantly, the flow rates and pressures were kept constant4 of 17 for the air compressor and water pump throughout the cultivation period.

FigureFigure 2.2. Layout of experimentalexperimental setup for the measurementmeasurement of dropletdroplet size:size: (A) high-pressurehigh-pressure nozzles (with air), (B) low-pressurelow-pressure nozzlesnozzles (without(without air).air).

2.4. Plant Material and Experimental Arrangement 2.4. Plant Material and Experimental Arrangement Butter-head lettuce seeds were cultivated in expanded polystyrene (EPS) trays with Butter-head lettuce seeds were cultivated in expanded polystyrene (EPS) trays with 72 cells containing an equal quantity of material. Emerged lettuce seedlings were 72 cells containing an equal quantity of perlite material. Emerged lettuce seedlings were watered four times a week for two weeks. On the sixteenth day of cultivation, the lettuce watered four times a week for two weeks. On the sixteenth day of cultivation, the lettuce seedlings were transplanted into the aeroponics systems. The aeroponics systems were seedlings were transplanted into the aeroponics systems. The aeroponics systems were ar- arranged in a randomized complete block design (RCBD) with 12 treatments. The ex- ranged in a randomized complete block design (RCBD) with 12 treatments. The experiment periment was comprised of four atomizing nozzles (one with air, two without air and one was comprised of four atomizing nozzles (one with air, two without air and one ultrasonic ultrasonic nozzle) and three nutrient solution spraying intervals (30, 45 and 60 min). nozzle) and three nutrient solution spraying intervals (30, 45 and 60 min). Moreover, the Moreover, the systems spraying time of 5 min was constant throughout the experiment. systems spraying time of 5 min was constant throughout the experiment. The details of The details of the experimental treatments are: nozzles with air (N1) implemented with the experimental treatments are: nozzles with air (N1) implemented with 30-, 45- and 30-, 45- and 60-min spraying intervals (I) denoted as N1I1, N1I2 and N1I3, respectively. 60-min spraying intervals (I) denoted as N1I1, N1I2 and N1I3, respectively. The nozzles withoutThe nozzles air (N2 without and N3) air operated(N2 and N3) at the operated same three at the spraying same three intervals spraying and wereintervals denoted and aswere N2I1, denoted N2I2 andas N2I1, N2I3, N2I2 and N3I1, and N2I3, N3I2 andand N3I3, N3I1, respectively. N3I2 and N3I3, The ultrasonic respectively. nozzle The (N4) ul- mistingtrasonic atnozzle 30-,45- (N4) and misting 60-min at spraying 30-, 45- intervalsand 60-min was spraying denoted intervals as N4I1, was N4I2 denoted and N4I3, as respectively.N4I1, N4I2 and Each N4I3, aeroponic respecti systemvely. Each consisted aeroponic of 12 lettucesystem seedlingsconsisted andof 12 was lettuce replicated seed- threelings times,and andwas therereplicated were 432 three plants times, in total. an Thed there plant-to-plant were 432 distance plants was in maintainedtotal. The 2 atplant-to-plant 14 × 16 cm 2distancein each aeroponicwas maintained system. at The 14 × Hoagland’s 16 cm in chemicaleach aeroponic composition system. of The nu- Hoagland’s chemical composition−1 of nutrient solution with−1 945 mg L-1 of Ca−1 trient solution with 945 mg L of Ca (NO3)2•4H2O, 607 mg L of KNO3, 115 mg L (NO3)2•4H2O, 607 mg L−1 −of1 KNO3, 115 mg L−1 of NH4H2PO−41, 493 mg L−1 of MgSO4•7H−1 2O, of NH4H2PO4, 493 mg L of MgSO4•7H2O, 2.86 mg L of H3BO3, 2.13 mg L of 2.86 mg L−1 of H3BO3, 2.13− 1mg L−1 of MnSO4•4H2O, 0.22 −mg1 L−1 of ZnSO4•7H2O, 0.08 mg−1 MnSO4•4H2O, 0.22 mg L of ZnSO4•7H2O, 0.08 mg L of CuSO4•5H2O, 0.02 mg L −1 4 2 −1 −1 4 6 7 24 2 −1 ofL (NHof 4CuSO)6Mo7•5HO24•4HO, 2O0.02 and mg 28 mgL Lof of(NH ferrum-Ethylenediaminetetraacetic) Mo O •4H O and 28 mg L Acidof (Fe-fer- EDTA)rum-Ethylenediaminetetraacetic was misted though atomizers Acid at(Fe-ED the proposedTA) was spraying misted though intervals atomizers throughout at thethe experiment.proposed spraying A fresh intervals nutrient solutionthroughout with the pH ex (5.8–6)periment. and ECA fresh (1.6–2.2 nutrient dS m− solution1) values with was replacedpH (5.8–6) with and recycled EC (1.6–2.2 nutrient dS m− solution1) values ofwas each replaced system with on therecycled fifth day nutrient throughout solution the of experiment.each system on the fifth day throughout the experiment.

2.5. Measurement of the Power of Hydrogen (pH) andand ElectricalElectrical ConductivityConductivity (EC)(EC) The EC and pHpH valuesvalues ofof thethe atomizedatomized nutrientnutrient solutionsolution werewere measuredmeasured inin eacheach treatment group every day. On every fifth fifth day,day, aa freshfresh nutrientnutrient solutionsolution waswas replacedreplaced with newnew nutrient nutrient solution. solution. A conductivity A conducti metervity meter (ProfiLine (ProfiLine Cond 3110, Cond WTW, 3110, Weilheim, WTW, −1 Germany)Weilheim, withGermany) accuracies with ofaccuracies 0.001 and of 0.1 0.001 mS cmand 0.1was mS used cm− to1 was measure used theto measure EC value the of theEC Hoagland’svalue of the nutrientHoagland’s solution. nutrient For solution the determination. For the determination of the pH value of the of thepH nutrientvalue of solutionsthe nutrient of solutions all treatments, of all atreatments, pH meter a (ProfiLine pH meter pH(ProfiLine 3110, WTW, pH 3110, Weilheim, WTW, Germany)Weilheim, withGermany) accuracy with of accuracy 0.01 was of used. 0.01 was used. 2.6. Vegetative Growth Parameters of Lettuce Plants To compare the vegetative growth parameters of 40 days after transplant (DAT) under all treatments, the number of leaves (NL), stem diameter (SD, mm), leaf length (LL, cm), leaf width (LW, cm), leaf area (LA, cm2), total biomass yield (TBY, g plant−1) and edible yield (EY, g plant−1) were measured using previously reported procedures in References [38–41].

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2.6. Vegetative Growth Parameters of Lettuce Plants To compare the vegetative growth parameters of 40 days after transplant (DAT) under all treatments, the number of leaves (NL), stem diameter (SD, mm), leaf length (LL, Agronomy 2021, 11, 97 cm), leaf width (LW, cm), leaf area (LA, cm2), total biomass yield (TBY, g plant−51) of and 17 edible yield (EY, g plant−1) were measured using previously reported procedures in Ref- erences [38–41]. The NL was measured manually by counting the leaves per plant. A Thedigital NL wasVernier measured caliper manually was used by to counting calculate the the leaves SD (mm). per plant. The ALL digital (cm) and Vernier LW caliper(cm) of waslettuce used plants to calculate were measured the SD (mm).using a Thesteel LL measuring (cm) and scale. LW Additionally, (cm) of lettuce a laser plants leaf were area measuredmeter (CI-203, using CID a steel Bio-Science, measuring Inc. scale. USA) was Additionally, used to calculate a laser the leaf LA area (cm meter2). TBY (CI-203, (stem + 2 CIDleaves Bio-Science, + root) in Inc.grams USA) and wasEY (stem used + to leaves) calculate in grams the LA were (cm measured). TBY (stem by an + electronic leaves + root)analytical in grams balance and EYwith (stem an accuracy + leaves) of in 0.1 grams mg. were The fresh measured weighed by an samples electronic were analytical inserted balancecarefully with into an transparent accuracy of paper 0.1 mg. envelopes The fresh and weighed oven-dried samples at were105 °C inserted for 24 h carefully to determine into transparent paper envelopes and oven-dried at 105 ◦C for 24 h to determine dry weight. dry weight. Figure 3 presents the shoot growth of butter-head lettuce under all treat- Figure3 presents the shoot growth of butter-head lettuce under all treatments. ments.

FigureFigure 3. 3.Shoot Shoot development development of of butter-headbutter-head lettucelettuce inin aeroponicsaeroponics systems,systems, (A) N1I1, (B) N2I1, ( C) N3I1,N3I1, ( D(D)) N4I1, N4I1, ((EE)) N1I2,N1I2, ((FF)) N2I2, ( G) N3I2, N3I2, ( (HH)) N4I2, N4I2, (I (I) )N1I3, N1I3, (J ()J )N2I3, N2I3, (K (K) N3I3) N3I3 and and (L) ( LN4I3) N4I3 treatments. treatments.

2.7.2.7. Root Root Characteristic Characteristic Analysis Analysis ForFor the the analysis analysis of of average average root root diameter diameter (cm), (cm), root root length length (cm), (cm), root root area area (cm (cm2),2), root root volumevolume (cm (cm3),3), maximum maximum number number of roots,of roots, median median number number of roots, of roots, network network perimeter perimeter (cm) and(cm) root and fresh root weight fresh weight (g plant (g− 1plant) of four−1) of plants four plants from eachfrom treatment each treatment group group of 40 day of 40 after day transplantafter transplant (DAT) (DAT) were randomly were randomly selected select for theed measurementfor the measurement of root characteristics. of root characteris- The rootstics. wereThe roots washed were and washed cleaned and with cleaned paper towels.with paper The towels. roots were The placed roots intowere a placed blue plastic into a containerblue plastic and container photographed and photographed carefully from carefully above with from a digital above camera.with a digital The photographs camera. The werephotographs scaled and were treated scaled with and GiA treated Roots softwarewith GiA (Georgia Roots software Tech Research (Georgia Corporation Tech Research and DukeCorporation University, and USA)Duke toUniversity, measure theUSA) root to characteristicsmeasure the root of allcharacteristics treatments. of The all same treat- Agronomy 2021, 11, x FOR PEER REVIEWprocedure as shoot fresh weight was used to measure the root fresh weight. The6 root of 18 ments. The same procedure as shoot fresh weight was used to measure the root fresh growthweight. pattern The root of lettucegrowth plants pattern during of lettuce the experiment plants during are depictedthe experiment in Figure are4 .depicted in Figure 4.

Figure 4. Root growth pattern under different treatments. Figure 4. Root growth pattern under different treatments. 2.8. Nutrient Uptake Four plants of lettuce from each treatment were randomly selected and used to de- termine the nutrient uptake. The total nitrogen (N) concentration was determined by using a Kjeldahl digestion method. The total phosphorous (P) concentration (molybdo- vanadate method) was measured by automated colorimetry, and the total potassium (K) concentration was determined by a flame photometric method [42]. Finally, (Ca) and magnesium (Mg) were analyzed by using an optical emission spectrometer (Optima 5300 DV Spectrometer, Shelton, CT, USA) [33].

2.9. Statistical Analysis SPSS Statistics 19.0 and Microsoft Excel 2016 software were used to analyze the data. The analysis of variance (ANOVA) was employed to determine the effect of different droplet sizes (nozzles) and nutrient solution spraying intervals on aeroponically grown butter-head lettuce. The correlation of the proposed parameters was tested by regression analyses and Duncan’s multiple tests at the p < 0.05 level of significance.

3. Results 3.1. Droplet Size Measurement The droplet size measurement of nozzles with and without air and ultrasonic noz- zles operated at the same operational pressure is presented in Figure 5. The droplet sizes were calculated at frequencies of 10%, 25%, 50%, 75% and 90%. The results revealed that N2 had maximum droplet size, while the minimum and median droplet sizes were ex- hibited by N4 and N1, respectively. The average droplet size diameter (Dav.) for N1, N2, N3 and N4 was 11.24, 26.35, 17.38 and 4.89 µm, respectively.

N1 N2 N3 N4 80

60

40

20

Droplet diameter (µm) diameter Droplet 0 d10 d25 d50 d75 d90 Dav Droplet frequency (%) Figure 5. Comparison of droplet sizes (µm) distribution of nozzles with air (N1), without air (N2 and N3) and ultrasonic nozzles (N4).

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Agronomy 2021, 11, 97 Figure 4. Root growth pattern under different treatments. 6 of 17

2.8. Nutrient Uptake 2.8. Nutrient Uptake Four plants of lettuce from each treatment were randomly selected and used to de- termineFour the plants nutrient of lettuce uptake. from The each total treatment nitrogen were (N) randomlyconcentration selected was anddetermined used to de-by termineusing a Kjeldahl the nutrient digestion uptake. method. The total The nitrogentotal phosphorous (N) concentration (P) concentration was determined (molybdo- by usingvanadate a Kjeldahl method) digestion was measured method. by automat The totaled phosphorouscolorimetry, and (P) the concentration total potassium (molyb- (K) dovanadateconcentration method) was determined was measured by a byflame automated photom colorimetry,etric method and [42]. the Finally, total potassium calcium (Ca) (K) concentrationand magnesium was (Mg) determined were analyzed by a flame by using photometric an optical method emission [42]. spectrometer Finally, calcium (Optima (Ca) and5300 magnesium DV Spectrometer, (Mg) were Shelton, analyzed CT, USA) by using [33]. an optical emission spectrometer (Optima 5300 DV Spectrometer, Shelton, CT, USA) [33]. 2.9. Statistical Analysis 2.9. Statistical Analysis SPSS Statistics 19.0 and Microsoft Excel 2016 software were used to analyze the data. SPSS Statistics 19.0 and Microsoft Excel 2016 software were used to analyze the data. The analysis of variance (ANOVA) was employed to determine the effect of different The analysis of variance (ANOVA) was employed to determine the effect of different droplet sizes (nozzles) and nutrient solution spraying intervals on aeroponically grown droplet sizes (nozzles) and nutrient solution spraying intervals on aeroponically grown butter-head lettuce. The correlation of the proposed parameters was tested by regression butter-head lettuce. The correlation of the proposed parameters was tested by regression analysesanalyses andand Duncan’sDuncan’s multiplemultiple teststests atat thethe pp << 0.050.05 levellevel ofof significance.significance.

3.3. Results 3.1.3.1. Droplet SizeSize MeasurementMeasurement TheThe dropletdroplet sizesize measurement measurement of of nozzles nozzles with with and and without without air air and and ultrasonic ultrasonic nozzles noz- operatedzles operated at the at same the same operational operational pressure pressure is presented is presented in Figure in Figure5. The 5. droplet The droplet sizes were sizes calculatedwere calculated at frequencies at frequencies of 10%, of 25%, 10%, 50%, 25%, 75% 50%, and 75% 90%. and The 90%. results The revealedresults revealed that N2 hadthat maximumN2 had maximum droplet size,droplet while size, the while minimum the minimum and median and droplet median sizes droplet were sizes exhibited were ex- by N4hibited and N1,by N4 respectively. and N1, respectively. The average The droplet averag sizee droplet diameter size (Dav.) diameter for N1, (Dav.) N2, N3for andN1, N2, N4 wasN3 and 11.24, N4 26.35, was 11.24, 17.38 and26.35, 4.89 17.38µm, and respectively. 4.89 µm, respectively.

N1 N2 N3 N4 80

60

40

20

Droplet diameter (µm) diameter Droplet 0 d10 d25 d50 d75 d90 Dav Droplet frequency (%) FigureFigure 5.5. ComparisonComparison ofof dropletdroplet sizes sizes ( µ(µm)m) distribution distribution of of nozzles nozzles with with air air (N1), (N1), without without air air (N2 (N2 and N3)and andN3) ultrasonicand ultrasonic nozzles nozzles (N4). (N4).

3.2. Effect of Droplet Sizes and Spraying Intervals on the pH and EC Values of Hoagland’s Nutrient Solution The statistical analyses results for the effect of different droplet sizes (nozzles) and nutrient solution spraying intervals on pH and EC values of atomized nutrient solution are depicted in Figure6. It was observed that the interaction between droplet size and spraying interval had a significant (p < 0.05) effect on the average values of pH and EC throughout the experiment. An increasing trend for pH under N1, N2 and N3, and a decreasing trend for EC, were observed for all four nozzles misted at 30-, 45- and 60-min nutrition solution spraying intervals. However, pH values showed a decreasing trend under the N4 atomizer at 30, 45 and 60 min of spraying interval. The results revealed that the highest and lowest change in pH values were 0.82 and 0.33 for N1, 0.88 and 0.49 for N2, 1.01 and 0.47 for N3 and 1.15 and 0.73 for N4 respectively, at 30-, 45- and 60-min nutrient solution spraying intervals, respectively. It was observed that the EC values were inversely proportional to the spraying interval for all nozzles. The minimum change in EC values was lowest for the 30-min interval and reached a maximum at the 60-min spraying interval for all tested nozzles (N1, N2, N3 and N4). The highest reduction in EC value (0.50 dS m−1) was calculated for the ultrasonic nozzle (N4) misting at 60-min nutrient solution Agronomy 2021, 11, x FOR PEER REVIEW 7 of 18

3.2. Effect of Droplet Sizes and Spraying Intervals on the pH and EC Values of Hoagland’s Nutrient Solution The statistical analyses results for the effect of different droplet sizes (nozzles) and nutrient solution spraying intervals on pH and EC values of atomized nutrient solution are depicted in Figure 6. It was observed that the interaction between droplet size and spraying interval had a significant (p < 0.05) effect on the average values of pH and EC throughout the experiment. An increasing trend for pH under N1, N2 and N3, and a de- creasing trend for EC, were observed for all four nozzles misted at 30-, 45- and 60-min nutrition solution spraying intervals. However, pH values showed a decreasing trend under the N4 atomizer at 30, 45 and 60 min of spraying interval. The results revealed that the highest and lowest change in pH values were 0.82 and 0.33 for N1, 0.88 and 0.49 for N2, 1.01 and 0.47 for N3 and 1.15 and 0.73 for N4 respectively, at 30-, 45- and 60-min nu- trient solution spraying intervals, respectively. It was observed that the EC values were inversely proportional to the spraying interval for all nozzles. The minimum change in EC values was lowest for the 30-min interval and reached a maximum at the 60-min Agronomy 2021, 11, 97 spraying interval for all tested nozzles (N1, N2, N3 and N4). The highest reduction in7 ofEC 17 value (0.50 dS m−1) was calculated for the ultrasonic nozzle (N4) misting at 60-min nu- trient solution spraying intervals, while the lowest change in EC values of 0.23 dS m−1 −1 wasspraying observed intervals, for the while nozzle the lowestwith air change (N1) in misting EC values at 30-min of 0.23 dSspraying m was intervals. observed The for ANOVAthe nozzle results with airindicated (N1) misting that nozzles at 30-min with spraying and without intervals. air Theshowed ANOVA a highly results significant indicated effectthat nozzleson the withpH value and withoutat all evaluated air showed intervals a highly (p significant< 0.001), and effect the on ultrasonic the pH value nozzle at presentedall evaluated a nonsignificant intervals (p < (p 0.001), > 0.05) and effect. the ultrasonicAccordingly, nozzle the interaction presented aof nonsignificant nozzles and spraying(p > 0.05 )intervals effect. Accordingly, indicated a thesignificant interaction effect of nozzles(p < 0.05) and on sprayingEC values intervals of the butter-head indicated a lettucesignificant variety. effect (p < 0.05) on EC values of the butter-head lettuce variety.

A N1 N2 N3 N4 B N1 N2 N3 N4 7.2 1.5 y1 = 0.016x + 6.032 y2 = 0.011x + 6.12 y1 = −0.006x + 1.63 y2 = −0.008x + 1.62 h R² = 0.996 R² = 998 e R² = 0.97 R² = 0.96 fg 7.0 1.4 d f d e c cd 6.8 de d 1.3 cd cd pH d ab d d 6.6 b 1.2 b c

c EC (dS. m-1) ab 2 a 6.4 y3 = 0.0002x − 0.025x + 1.83 y3 = 0.013x + 6.24 y4 = 0.013x + 6.032 1.1 a R² = 0.99 y4 = −0.007x + 1.61 R² = 0.52 R² = 0.983 R² = 0.91 6.2 1.0 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) FigureFigure 6. 6. AverageAverage ( (AA)) change change in in pH pH and and ( (BB)) change change in in electrical electrical conductivity conductivity (EC) (EC) values values of of Hoagland’s Hoagland’s nutrient nutrient solution solution for N1, N2, N3 and N4 nozzles misting at 0 (initial), 30-, 45- and 60-min intervals. Whereas yn = Nn. for N1, N2, N3 and N4 nozzles misting at 0 (initial), 30-, 45- and 60-min intervals. Whereas yn = Nn.

3.3. Vegetative Growth Parameters 3.3. Vegetative Growth Parameters 3.3.1. Shoot Growth Parameters 3.3.1. Shoot Growth Parameters The two-way ANOVA analyses results of NL, SD, LL, LW and LA are represented in FigureThe 7. The two-way statistical ANOVA analysis analyses results results showed of NL, that SD, the LL, interaction LW and LAbetween are represented droplet size in andFigure spraying7. The interval statistical had analysis a significant results ( showedp < 0.05) thateffect the on interaction vegetative between growth dropletparameters. size and spraying interval had a significant (p < 0.05) effect on vegetative growth parameters. The results indicated that the average highest number of leaves per plant (28.7) was The results indicated that the average highest number of leaves per plant (28.7) was measured for N1I1, and the lowest (13.0) number of leaves per plant was observed for measured for N1I1, and the lowest (13.0) number of leaves per plant was observed for N4I3. N4I3. Moreover, the maximum and minimum SDs of 3.84 and 0.98 mm were observed Moreover, the maximum and minimum SDs of 3.84 and 0.98 mm were observed under under the N1I2 and N4I1 treatments, respectively. Additionally, the droplet sizes and the N1I2 and N4I1 treatments, respectively. Additionally, the droplet sizes and spraying spraying intervals had positive significant (p < 0.05) effects on the leaf length, leaf width intervals had positive significant (p < 0.05) effects on the leaf length, leaf width and leaf and leaf area of aeroponically grown butter-head lettuce. The maximum LL, LW and LA area of aeroponically grown butter-head lettuce. The maximum LL, LW and LA of 17.15 of 17.15 cm, 11.64 cm and 137.38 cm2 respectively, were calculated under N1I3 treatment. cm, 11.64 cm and 137.38 cm2 respectively, were calculated under N1I3 treatment. However, the minimum LL, LW and LA of 6.28 cm, 2.75 cm and 9.65 cm2 respectively, were observed for the ultrasonic nozzle operating with a 60-min spraying interval. More importantly, the regression analysis results revealed that N1 showed a significant (p < 0.05) effect on NL, SD, LW, LL and LA. Moreover, N2 presented a significant (p < 0.05) effect on LW and LA; additionally, N3 exhibited a significant (p < 0.05) effect on LL and LA. The interaction of the ultrasonic nozzle and spraying interval was nonsignificant (p > 0.05) for all proposed parameters.

3.3.2. Correlation between Shoot Growth Parameter208B The analyzed results of the correlation between growth parameters are presented in Table1. The results showed that the growth parameters had strong positive correlations with each other under all treatments. Agronomy 2021, 11, x FOR PEER REVIEW 8 of 18

However, the minimum LL, LW and LA of 6.28 cm, 2.75 cm and 9.65 cm2 respectively, were observed for the ultrasonic nozzle operating with a 60-min spraying interval. More importantly, the regression analysis results revealed that N1 showed a significant (p < 0.05) effect on NL, SD, LW, LL and LA. Moreover, N2 presented a significant (p < 0.05) effect on LW and LA; additionally, N3 exhibited a significant (p < 0.05) effect on LL and Agronomy 2021, 11, 97 LA. The interaction of the ultrasonic nozzle and spraying interval was nonsignificant8 of(p 17> 0.05) for all proposed parameters.

A N1 N2 N3 N4 B N1 N2 N3 N4 y = −0.13x + 26.33 y = −0.041x + 5.08 30 1 4 1 y2 = −0.033x + 26.06 y = −0.027x + 4.105 R² = 0.99 k R² = 0.99 2 g R² = 0.5192 j R² = 0.96 fg e 25 3 j i e e ef e i d f e 20 d 2 bc c b c d c a 2 15 y3 = −0.013x + 1.067x + 1.33 1 Number of leaves Number a y3= −0.034 + 3.93 R² = 0.99 (mm) diameter Stem y4 = −0.0123x + 1.83 y = −0.01x2 + 0.88x + 0.33 4 R² = 0.89 R² = 0.46 10 R² = 0.62 0 30 45 60 30 45 60 Spraying Interval (min) Spraying Interval (min.) C N1 N2 N3 N4 D N1 N2 N3 N4 14 20 y = −0.0662x + 13.52 y1 = −0.13x + 21.13 2 1 y2 = −0.1843x + 16.177 y2 = −0.0078x + 0.63x + 1.74 18 R² = 0.995 12 h R² = 0.9645 g R² = 0.9938 R² = 0.49 g i fg 16 h 10 g f 14 fg 8 h g e f 12 e 6 d d d c c b c 10 4 a

Leaf length (cm) y = −0.1519x + 17.397 b 3 y3 = −0.22x + 17.58 2 Leaf width (cm) 8 y4 = −0.012x + 0.95x − 8.267 2 R² = 0.99 2 R² = 0.98 y4 = −0.008x + 0.68x −10.19 R² = 0.54 a R² = 0. 053 6 0 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) E N1 N2 N3 N4 y = −1.6685x + 186.59 150 1 k R² = 0.9966 y2 = −2.0222x + 163.33 )

2 120 j R² = 0.9941 i h 90 i g 60 y = −2.39x + 167.49 f e Leaf area (cm 3 d 30 R² = 0.99 2 y4 = −0.08x + 6.9x − 116.02 c b a 0 R² = 0.17 30 45 60 Spraying Interval (min.) Figure 7. ((AA)) Number Number of of leaves, leaves, ( (BB)) stem stem diameter, diameter, ( (C)) leaf leaf length, length, ( (DD)) leaf leaf width width and and ( (EE)) leaf leaf area area under under different different droplet droplet sizes (N1, N2, N3 and N4) misted at 30-, 45- and 60-min intervals. Whereas yn = Nn. sizes (N1, N2, N3 and N4) misted at 30-, 45- and 60-min intervals. Whereas yn = Nn.

Table 1. Correlation analyses results between number of leaves (NL), shoot diameter (SD), leaf length (LL), leaf width (LW) and leaf area (LA).

Parameters NL SD LL LW SD 0.95 LL 0.94 0.93 LW 0.85 0.93 0.85 LA 0.89 0.95 0.93 0.97

3.3.3. Total Biomass Yield and Edible Yield The statistical analysis results of total biomass yield and edible yield are depicted in Figure8. The results indicated that the interaction of droplet sizes and spraying intervals Agronomy 2021, 11, x FOR PEER REVIEW 9 of 18

3.3.2. Correlation between Shoot Growth Parameter208B The analyzed results of the correlation between growth parameters are presented in Table 1. The results showed that the growth parameters had strong positive correlations with each other under all treatments.

Table 1. Correlation analyses results between number of leaves (NL), shoot diameter (SD), leaf length (LL), leaf width (LW) and leaf area (LA).

Parameters NL SD LL LW SD 0.95 LL 0.94 0.93 LW 0.85 0.93 0.85 LA 0.89 0.95 0.93 0.97

Agronomy 2021, 11, 97 3.3.3. Total Biomass Yield and Edible Yield 9 of 17 The statistical analysis results of total biomass yield and edible yield are depicted in Figure 8. The results indicated that the interaction of droplet sizes and spraying intervals hadhad a a significant significant (p (p << 0.05) 0.05) effect effect on on total total biomass biomass yield yield and and edible edible yield. yield. The Thehighest highest and −1 lowestand lowest total totalbiomass biomass yields yields of 63.85 of 63.85 and and9.29 9.29g plant g plant−1 were weremeasured measured for the for N1I1 the N1I1 and N4I3and N4I3treatments, treatments, respectively. respectively. Furthermore, Furthermore, maximum maximum andand minimum minimum edible edible yields yields of −1 49.48of 49.48 and and 4.92 4.92 g plant g plant−1 werewere observed observed for for the the N1I2 N1I2 and and N4I4 N4I4 treatments, treatments, respectively. respectively. These findings findings indicated that the nozzles utilizing air showed a significantsignificant (p < 0.05) effect onon the total biomass yield and edible yieldyield ofof butter-headbutter-head lettuce.lettuce. Only the ultrasonic nozzlenozzle showed a very highly significant significant (p ≤ 0.01)0.01) effect effect on on total total biomass biomass yield. yield. Moreover, Moreover, thethe nozzles withoutwithout airair indicatedindicated a a nonsignificant nonsignificant (p (p> > 0.05) 0.05) effect effect on on both both yield yield parameters parame- terswhen when misting misting at 30-, at 30-, 45- and45- and 60-min 60-min intervals. intervals.

A N1 N2 N3 N4 B N1 N2 N3 N4 80 60 )

-1 y1 = −0.456x + 55.09 y = −0.74x + 85.5 y2 = −0.2807x + 44.6 1 50 f R² = 0.91 g R² = 0.996 y2 = −0.0973x + 51.426 ) R² = 0.56 -1 f d 60 f R² = 0.149 e 40 d e d de c cd e d 30 c 40 d c 2 d y3 = 0.028x − 3.04x + 111.2 20 c R² = 0.80 y4 = −0.4217x + 23.66 20 b c R² = 0.99 y = −0.43x + 35.08 a 10 b 4 Edible yield (g. plant a y3 = −0.68x + 76.52 Total biomass yield (g. plant R² = 0.99 R² = 0.87 0 0 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) Figure 8. ((A)) Total Total biomass yield and ( B) edible yield yield under under different different droplet droplet sizes sizes (N1, (N1, N2, N2, N3 N3 and and N4) N4) misted misted at at 30-, 30-, 45- 45- and 60-min intervals. Whereas yn = Nn. and 60-min intervals. Whereas yn = Nn.

3.4. Root Characteristics 3.4. Root Characteristics The droplet sizes (atomizers) and spraying intervals showed significant (p < 0.05) effectsThe on droplet the average sizes (atomizers)root diameter, and root spraying length, intervals root area, showed root significantvolume, maximum (p < 0.05) numbereffects on of the roots, average median root numb diameter,er of rootroots, length, network root perimete area, rootr and volume, fresh maximum weight of number lettuce rootsof roots, grown median under number aeroponic of roots,systems network (Figure perimeter 9). A decreasing and fresh trend weight was observed of lettuce under roots allgrown treatments under with aeroponic respect systems to increasing (Figure sp9).raying A decreasing interval. The trend highest was observedaverage root under di- ameterall treatments (0.45 cm) with and respectaverage to root increasing length (4707.8 spraying cm) interval.were observed The highest under N1I1, average and root the lowestdiameter values (0.45 cm)of 0.34 and and average 2147 root cm lengthrespecti (4707.8vely, were cm) were calculated observed under under the N1I1, N4I3 and treat- the ment.lowest Additionally, values of 0.34 the and maximum 2147 cm respectively,average root were area, calculated root volume, under maximum the N4I3 number treatment. of Additionally, the maximum average root area, root volume, maximum number of roots, median number of roots and network perimeter of 1492.04 cm2, 65.55 cm3, 93.9, 47.5 and 8383.8 cm respectively, were observed under the N1I1 treatment. More importantly, the maximum and median numbers of roots for N1 misting at a 30-min interval were two to three times greater than those of N2, N3 and N4 misting at 45- and 60-min intervals. In general, the experimental results of the study indicated that roots could grow better using a nozzle implementing air and misting the nutrient solution at 30- and 45-min intervals. Accordingly, the nozzle with air (N1) showed a significant (p < 0.05) effect, N2 showed a nonsignificant (p > 0.05) effect and N3 and N4 presented a mixed phenomenon of significant (p < 0.05) and nonsignificant (p > 0.05) effects on all measured parameters when misting at 30-, 45- and 60-min intervals. From the regression analysis results of growth parameters, prediction models were developed. Agronomy 2021, 11, x FOR PEER REVIEW 10 of 18

roots, median number of roots and network perimeter of 1492.04 cm2, 65.55 cm3, 93.9, 47.5 and 8383.8 cm respectively, were observed under the N1I1 treatment. More importantly, the maximum and median numbers of roots for N1 misting at a 30-min interval were two to three times greater than those of N2, N3 and N4 misting at 45- and 60-min intervals. In general, the experimental results of the study indicated that roots could grow better us- ing a nozzle implementing air and misting the nutrient solution at 30- and 45-min inter- vals. Accordingly, the nozzle with air (N1) showed a significant (p < 0.05) effect, N2 showed a nonsignificant (p > 0.05) effect and N3 and N4 presented a mixed phenomenon of significant (p < 0.05) and nonsignificant (p > 0.05) effects on all measured parameters Agronomy 2021, 11, 97 when misting at 30-, 45- and 60-min intervals. From the regression analysis results10 of of 17 growth parameters, prediction models were developed.

A N1 N2 N3 N4 B N1 N2 N3 N4 0.50 6000 y1 = −0.0024x + 0.519 y1 = −27.81x + 5556.2 R² = 0.995 2 d R² = 0.996y2 = −2.5376x + 215.35x − 343 0.45 y2 = −0.0004x + 0.425 5000 f c R² = 0.26 c R² = 0.274 ef c c e e 0.40 b c 4000 d c d b ab b bc b b y3 = −22.44x + 4698.5 0.35 y = −0.0017x + 0.478 a 3000 y4 = −1.29x + 2253.9 3 y = −0.0016x + 0.442 R² = 0.993 Avg. root diameter (cm) diameter root Avg. R² = 0.75 4 a R² = 0.123

Avg. root length (cm) a R² = 0.766 a 0.30 2000 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) N1 N2 N3 N4 C D N1 N2 N3 N4 1900 70 y1 = −16.314x + 1983.9 ) y = −0.348x + 67.27

3 1 2 ) y2 = −3.0692x + 1281.9 y = −0.03x + 2.82x − 8.07 2 R² = 0.999 2 g 60 g R² = 0.991 R² = 0.249 fg R² = 0.25 1400 fg e 50 f e f g e e d e c d y3 = −10.98x + 1497.4 d 40 y = −0.2632x + 55.991 900 y = −3.5x + 814.56 3 b R² = 0.94 4 R² = 0.999 y4 = −0.0125x + 27.164 R² = 0.65 c R² = 0.185 Avg. root area (cm area root Avg. 30 a a a a b a Avg. root volume (cm 400 20 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) E N1 N2 N3 N4 F N1 N2 N3 N4 120 60 y = −1.52x + 140.3 y2 = −0.8767x + 92.217 y = −0.92x + 74.667 100 h R² = 0.9963 R² = 0.942 50 g R² = 0.9971 y = 0.0258x2 − 2.5333x + 85.9 g f 2 80 ef 40 R² = 0.47 ef e 60 f d e 30 e b cd c d 40 ab 20 b c c bc a c y3 = −1.14x + 113.8 a 20 y4 = −0.3867x + 50.767 a 10 y = −0.72x + 62.4 y4 = −0.2467x + 24.13 R² = 0.999 Median number roots of 3 Maximum number of roots Maximum number of R² = 0.997 R² = 0.999 R² = 0.858 Agronomy 2021, 110, x FOR PEER REVIEW 0 11 of 18 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.)

G N1 N2 N3 N4 H N1 N2 N3 N4 20

11,000 ) -1 y = −0.0467x + 15.412 y = −57.927x + 10073 y2 = 0.1833x + 2.7933 y2 = −24.5x + 8070.9 R² = 0.5606 15 f R² = 0.994 9000 g R² = 0.990 R² = 0.368 e e f ef e de c e e 7000 d 10 d c c c b y4 = −1.84x + 4150.3 b a a 5000 y3 = −59.097x + 9239.5 5 a Network perimeter Network R² = 0.029 R² = 0.997 a y = −0.186x + 18.29 3 y4 = -0.009x + 5.0906 a a R² = 0.987

Root fresh weight (g. plant R² = 0.157 3000 0 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.)

FigureFigure 9. (A) Average rootroot diameter,diameter, ( B(B)) average average root root length, length, (C ()C average) average root root area, area, (D )(D average) average root root volume, volume, (E) maximum(E) maxi- mumnumber number of roots, of roots, (F) median (F) median number number of roots of roots (G) network (G) network perimeter, perimeter, and ( Hand) root (H) freshroot fresh weight weight for different for different droplet droplet sizes sizes (N1, N2, N3 and N4) misted at 30-, 45- and 60-min intervals. Whereas yn = Nn. (N1, N2, N3 and N4) misted at 30-, 45- and 60-min intervals. Whereas yn = Nn. Correlation between Root Characteristics The correlation between root growths parameters are presented in Table 2. The an- alyzed results showed a positive correlation between each other under all treatments.

Table 2. Correlation between root growth parameters.

RD RL RA RV Max. Roots Med. Roots Net. Perimeter RL 0.77 RA 0.88 0.96 RV 0.77 1.00 0.96 Max. roots 0.77 0.80 0.85 0.80 Med. roots 0.82 0.78 0.83 0.77 0.94 Net. perimeter 0.81 0.99 0.97 0.99 0.83 0.81 RFW 0.66 0.88 0.86 0.87 0.67 0.69 0.88

3.5. Ratio of Roots to Shoots The analyzed results for the interaction effect of droplet size (nozzles) and spraying interval on the ratio of roots to shoots (fresh and dry) are displayed in Figure 10. The results showed that both parameters had a positive effect on the ratio of roots to shoots. The weight of shoot biomass was higher than that of roots under all treatments. The in- creasing root-to-shoot ratio for N1, N2 and N3 was observed when utilizing fresh weights of roots and shoots with respect to spraying interval, except for N4. Furthermore, a mixed trend of increasing and decreasing ratios was observed when utilizing dry weights. The regression analysis results indicated that N1 and N4 had a significant (p < 0.05) effect on the root-to-shoot ratio (fresh) and that N1 had a significant (p < 0.05) effect on the root-to-shoot ratio (dry). Additionally, N2 and N3 showed nonsignificant (p > 0.05) effects on the root-to-shoot ratios calculated using fresh and dry weights. Agronomy 2021, 11, 97 11 of 17

Correlation between Root Characteristics The correlation between root growths parameters are presented in Table2. The analyzed results showed a positive correlation between each other under all treatments.

Table 2. Correlation between root growth parameters.

RD RL RA RV Max. Roots Med. Roots Net. Perimeter RL 0.77 RA 0.88 0.96 RV 0.77 1.00 0.96 Max. roots 0.77 0.80 0.85 0.80 Med. roots 0.82 0.78 0.83 0.77 0.94 Net. 0.81 0.99 0.97 0.99 0.83 0.81 perimeter RFW 0.66 0.88 0.86 0.87 0.67 0.69 0.88

3.5. Ratio of Roots to Shoots The analyzed results for the interaction effect of droplet size (nozzles) and spraying interval on the ratio of roots to shoots (fresh and dry) are displayed in Figure 10. The results showed that both parameters had a positive effect on the ratio of roots to shoots. The weight of shoot biomass was higher than that of roots under all treatments. The increasing root-to-shoot ratio for N1, N2 and N3 was observed when utilizing fresh weights of roots and shoots with respect to spraying interval, except for N4. Furthermore, a mixed trend of increasing and decreasing ratios was observed when utilizing dry weights. The regression analysis results indicated that N1 and N4 had a significant (p < 0.05) effect on the root-to- shoot ratio (fresh) and that N1 had a significant (p < 0.05) effect on the root-to-shoot ratio Agronomy 2021, 11, x FOR PEER REVIEW(dry). Additionally, N2 and N3 showed nonsignificant (p > 0.05) effects on the root-to-shoot12 of 18

ratios calculated using fresh and dry weights.

A N1 N2 N3 N4 B N1 N2 N3 N4 0.6 0.16

y1 = 0.0002x + 0.0976 y2 = 0.0042x + 0.0489 0.14 y2 = −0.001x + 0.164 0.5 y = 0.0028x + 0.1418 f 1 R² = 0.891 R² = 0.997 R² = 0.989 R² = 0.992 g f 0.4 0.12 e de e de de 0.3 b 0.10 de b c d de b d c d 0.2 b 0.08 b Root:Shoot (dry) b a ab a Root:Shoot (Fresh) y = −0.0009x + 0.257 y = 0.0087x − 0.0582 y = 8E−05x2 − 0.007x + 0.3 0.1 3 4 0.06 4 R² = 0.479 R² = 0.994 2 R² = 0.018 y3 = 5E−05x − 0.004x + 0.12 0.0 0.04 R² = 0.83 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) FigureFigure 10. ((A) Root: Shoot (fresh),(fresh), ((BB)) Root:Root: ShootShoot (dry)(dry) under under different different droplet droplet sizes sizes (N1, (N1, N2, N2, N3 N3 and and N4) N4) misted misted at 30-,at 30-, 45- n n. 45-and and 60-min 60-min intervals. intervals. Whereas Whereas yn =y N =n. N

3.6. Nutrient Uptake 3.6. Nutrient Uptake The experimental results presented in Figure 11 describe the effect of different noz- zles (dropletThe experimental sizes) and results nutrient presented solution in sp Figureraying 11 intervalsdescribe theon nutrient effect of differentuptake. The nozzles re- gression(droplet sizes)analysis and results nutrient revealed solution that spraying the different intervals atomizers on nutrient operated uptake. at Thethe regressionthree nu- trientanalysis solution results spraying revealed intervals that the differentshowed a atomizers significant operated (p < 0.05) at theeffect three on nutrientthe nitrogen solution (N) uptakespraying of intervals lettuce leaves. showed The a significant interaction (p < of 0.05) droplet effect size on theand nitrogen spraying (N) interval uptake ofaffected lettuce plantleaves. metabolism. The interaction A significant of droplet increase size and in spraying N in leaf interval tissues affectedincreases plant photosynthesis metabolism. ef- A ficiencysignificant and increase could result in N inin leafhigh tissues yield. increasesThe highest photosynthesis N uptake (0.36 efficiency mg. g−1 and) was could observed result −1 underin high the yield. N1I1 The treatment. highest N The uptake results (0.36 also mg. re gvealed) was that observed the atomizers under the operating N1I1 treatment. at 30- and 45-min spraying intervals had significantly higher values of N uptake compared to that at the 60-min spraying interval (Figure 11A). Furthermore, it was observed that both parameters affected P. The droplet size and spraying interval also affected plant metabo- lism and helped to simulate outdoor environmental conditions. The decrease in P was observed under N1, N2, N3 and N4 at 30-, 45- and 60-min nutrient solution spraying in- tervals. Moreover, different atomizers operating with a 30-min interval had significantly higher values than those of nozzles operating at 45- and 60-min intervals. Furthermore, the regression analysis results for the effects of different aeroponic nutrient solution spraying intervals on K uptake of the lettuce plants are displayed in Figure 11C. The highest uptake (0.60 mg. g−1) was illustrated under the N1I1 treatment, and the lowest uptake (0.20 g. plant−1) was calculated under N4I1. Mixed fractions of calcium (Ca) and magnesium (Mg) were observed under all treatments (Figure 11D,E). More importantly, the ultrasonic foggers (N4) operating at 30-, 45- and 60-min nutrient solution spraying intervals showed lower Ca and Mg concentrations than those observed for N1, N2 and N3.

A A1 A2 A3 A4 B A1 A2 A3 A4 0.40 0.25 y1 = −0.0034x + 0.4717 y1 = −0.003x + 0.2817 y = −0.0011x + 0.1656 R² = 0.9276 y = −0.0002x2 + 0.0198x − 0.11 2 f 2 R² = 0.996 0.35 ) 0.20 R² = 0.824 ) e -1

-1 R² =0.27 e e e 0.30 d 0.15 d c c c 2 b y3 = −0.0003x + 0.0198x − 0.0233 ab 0.25 R² = 0.77 0.10 a b a a b a a y = −0.0002x2 + 0.013x − 0.13 Nitrogen (mg. g Nitrogen (mg. 0.20 y = −0.0001x2 + 0.0133x − 0.0667 0.05 a 4 a R² = 10.25 y4 = −0.0003x + 0.103 R² =0.0001 g Phosphorous (mg. R² = 0.96 0.15 0.00 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.)

Agronomy 2021, 11, x FOR PEER REVIEW 12 of 18

A N1 N2 N3 N4 B N1 N2 N3 N4 0.6 0.16

y1 = 0.0002x + 0.0976 y2 = 0.0042x + 0.0489 0.14 y2 = −0.001x + 0.164 0.5 y = 0.0028x + 0.1418 f 1 R² = 0.891 R² = 0.997 R² = 0.989 R² = 0.992 g f 0.4 0.12 e de e de de 0.3 b 0.10 de b c d de b d c d 0.2 b 0.08 b Root:Shoot (dry) b a ab a Root:Shoot (Fresh) y = −0.0009x + 0.257 y = 0.0087x − 0.0582 y = 8E−05x2 − 0.007x + 0.3 0.1 3 4 0.06 4 R² = 0.479 R² = 0.994 2 R² = 0.018 y3 = 5E−05x − 0.004x + 0.12 0.0 0.04 R² = 0.83 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) Figure 10. (A) Root: Shoot (fresh), (B) Root: Shoot (dry) under different droplet sizes (N1, N2, N3 and N4) misted at 30-, 45- and 60-min intervals. Whereas yn = Nn.

3.6. Nutrient Uptake The experimental results presented in Figure 11 describe the effect of different noz- zles (droplet sizes) and nutrient solution spraying intervals on nutrient uptake. The re- gression analysis results revealed that the different atomizers operated at the three nu- Agronomy 2021, 11, 97 trient solution spraying intervals showed a significant (p < 0.05) effect on the nitrogen (N) 12 of 17 uptake of lettuce leaves. The interaction of droplet size and spraying interval affected plant metabolism. A significant increase in N in leaf tissues increases photosynthesis ef- −1 Theficiency results and also could revealed result thatin high the yi atomizerseld. The highest operating N uptake at 30- (0.36 and mg. 45-min g ) was spraying observed intervals under the N1I1 treatment. The results also revealed that the atomizers operating at 30- had significantly higher values of N uptake compared to that at the 60-min spraying interval and 45-min spraying intervals had significantly higher values of N uptake compared to (Figure 11A). Furthermore, it was observed that both parameters affected P. The droplet that at the 60-min spraying interval (Figure 11A). Furthermore, it was observed that both sizeparameters and spraying affected interval P. The alsodroplet affected size and plant spraying metabolism interval and also helpedaffected to plant simulate metabo- outdoor environmentallism and helped conditions. to simulate The outdoor decrease environmental in P was observed conditions. under The decrease N1, N2, in N3 P andwas N4 at 30-,observed 45- and under 60-min N1, nutrient N2, N3 and solution N4 at 30-, spraying 45- and intervals. 60-min nutrient Moreover, solution different spraying atomizers in- operatingtervals. Moreover, with a 30-min different interval atomizers had operating significantly with a higher 30-min valuesinterval thanhad significantly those of nozzles operatinghigher values at 45- than and those 60-min of intervals.nozzles operating Furthermore, at 45- and the 60-min regression intervals. analysis Furthermore, results for the effectsthe regression of different analysis aeroponic results nutrient for the solution effects sprayingof different intervals aeroponic on Knutrient uptake solution of the lettuce plantsspraying are displayed intervals on in FigureK uptake 11 C.of Thethe highestlettuce plants uptake are (0.60 displayed mg. g− in1) Figure was illustrated 11C. The under thehighest N1I1 treatment,uptake (0.60 and mg. the g−1) lowest was illustrated uptake (0.20under g. the plant N1I1− 1treatment,) was calculated and the lowest under N4I1. −1 Mixeduptake fractions (0.20 g. ofplant calcium) was (Ca) calculated and magnesium under N4I1. (Mg) Mixed were fractions observed of calcium under (Ca) all treatments and magnesium (Mg) were observed under all treatments (Figure 11D,E). More importantly, (Figure 11D,E). More importantly, the ultrasonic foggers (N4) operating at 30-, 45- and the ultrasonic foggers (N4) operating at 30-, 45- and 60-min nutrient solution spraying 60-min nutrient solution spraying intervals showed lower Ca and Mg concentrations than intervals showed lower Ca and Mg concentrations than those observed for N1, N2 and thoseN3. observed for N1, N2 and N3.

A A1 A2 A3 A4 B A1 A2 A3 A4 0.40 0.25 y1 = −0.0034x + 0.4717 y1 = −0.003x + 0.2817 y = −0.0011x + 0.1656 R² = 0.9276 y = −0.0002x2 + 0.0198x − 0.11 2 f 2 R² = 0.996 0.35 ) 0.20 R² = 0.824 ) e -1

-1 R² =0.27 e e e 0.30 d 0.15 d c c c 2 b y3 = −0.0003x + 0.0198x − 0.0233 ab 0.25 R² = 0.77 0.10 a b a a b a a y = −0.0002x2 + 0.013x − 0.13 Nitrogen (mg. g Nitrogen (mg. 0.20 y = −0.0001x2 + 0.0133x − 0.0667 0.05 a 4 a R² = 10.25 y4 = −0.0003x + 0.103 R² =0.0001 g Phosphorous (mg. R² = 0.96 Agronomy 2021, 110.15, x FOR PEER REVIEW 13 of 18 0.00 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.)

C A1 A2 A3 A4 D A1 A2 A3 A4 0.45 0.7 y1 = −0.0047x + 0.736 2 y = −0.0042x + 0.471 g R² = 0.9992 y2 = −0.0007x + 0.0626x − 0.88 0.40 1 0.6 R² = 0.977 f e e R² = 0.016 g y = −0.0002x2 + 0.017x − 0.077 0.5 d 0.35 2 f R² = 0.0048 y = 0.0005x2 − 0.0548x + 1.7167 0.4 3 0.30 e e c R² = 0.027 c c d b 0.25 c 0.3 c c c a b 0.2 2 0.20 b c y4= −0.0003x + 0.0294x − 0.3333 a Calcium (mg. g-1) y3 = −0.002x + 0.36 Potasium (mg. g-1) Potasium (mg. R² = 0.036 0.1 0.15 R² = 0.88 y = −0.0034x + 0.35 4 a R² = 0.99 0.0 0.10 30 45 60 30 45 60 Spraying Interval (min.) Spraying Interval (min.) E A1 A2 A3 A4 0.40

y1 = −0.0036x + 0.438 )

-1 0.35 d R² = 0.9948 y2 = 0.0007x + 0.23 R² = 0.008 0.30 c c c c 0.25 b ab bc y3 = −0.0009x + 0.267 y4 = −0.001x + 0.2308

Magnesium (mg. g 0.20 R² = 0.976 R² = 0.919 ab a a a 0.15 30 45 60 Spraying Interval (min.) FigureFigure 11. (A 11.) Nitrogen,(A) Nitrogen, (B) ( ,B) phosphorus, (C (C)) potassium, potassium, ( D)) calcium calcium and and (E ()E magnesium) magnesium uptake uptake under under different different droplet droplet sizes (N1, N2, N3 and N4) of nutrient solution misted at 30-, 45- and 60-min intervals. Whereas yn = Nn. sizes (N1, N2, N3 and N4) of nutrient solution misted at 30-, 45- and 60-min intervals. Whereas yn = Nn. Correlation between Nutrient Uptakes The analyzed results of the correlation between N, P, K, Ca and Mg uptake are de- picted in Table 3. The results revealed that the measured parameters had moderate cor- relations with each other for the four different nozzles and the three different nutrient solution spraying intervals.

Table 3. Correlation between N, P, K, Mg and Ca.

Parameters N K P Mg K 0.83 P 0.86 0.64 Mg 0.78 0.84 0.59 Ca 0.60 0.81 0.56 0.75

4. Discussion Based on the literature, very few researchers have conducted studies on droplet size and spraying interval effects on the chemical properties of nutrient solutions, the growth of shoots to roots, the root-to-shoot ratio, the biomass yield and the nutrient uptake of lettuce crops grown in aeroponic systems. From the results, it was observed that droplet size (nozzles) had a significant (p < 0.05) effect on the chemical properties (EC and pH) of Hoagland’s nutrient solution, except for the ultrasonic fogger, which showed a nonsig- nificant (p > 0.05) effect on pH value. Simultaneously, the pH values decreased, and EC values increased for all nozzles with respect to increasing spraying interval. The highest change in pH and change in EC were observed for the ultrasonic nozzle, and the lowest

Agronomy 2021, 11, 97 13 of 17

Correlation between Nutrient Uptakes The analyzed results of the correlation between N, P, K, Ca and Mg uptake are depicted in Table3. The results revealed that the measured parameters had moderate correlations with each other for the four different nozzles and the three different nutrient solution spraying intervals.

Table 3. Correlation between N, P, K, Mg and Ca.

Parameters N K P Mg K 0.83 P 0.86 0.64 Mg 0.78 0.84 0.59 Ca 0.60 0.81 0.56 0.75

4. Discussion Based on the literature, very few researchers have conducted studies on droplet size and spraying interval effects on the chemical properties of nutrient solutions, the growth of shoots to roots, the root-to-shoot ratio, the biomass yield and the nutrient uptake of lettuce crops grown in aeroponic systems. From the results, it was observed that droplet size (nozzles) had a significant (p < 0.05) effect on the chemical properties (EC and pH) of Hoagland’s nutrient solution, except for the ultrasonic fogger, which showed a nonsignificant (p > 0.05) effect on pH value. Simultaneously, the pH values decreased, and EC values increased for all nozzles with respect to increasing spraying interval. The highest change in pH and change in EC were observed for the ultrasonic nozzle, and the lowest change in pH and EC values were calculated for the nozzle utilizing air. The findings of our research are in line with those of the authors of References [30,43], who concluded that nozzles (droplet size) could affect the physicochemical properties of nutrient solution and could change with time. In aeroponic systems, the reuse intervals of the nutrient solution vary from days to weeks. Similar results were also reported by the authors of Reference [21], who found that it is very important to evaluate the nutrient solution pH and EC of the nutrient solution regularly to avoid placing unnecessary pressure on the growth of plants. Researchers also reported that EC values lower than a certain range could reduce the availability of nutrients to plants, and EC values higher than this range could cause toxicity, unnecessary stress and an imbalance of nutrient elements in plants [37,44–47]. Additionally, the growth parameters presented a positive response for nozzles utilizing air compared to nozzles not utilizing air, while little growth was observed for the ultrasonic nozzle misting at 30-, 45- and 60-min intervals. Continuous contact with oxygen for air-assisted atomizers stimulates metabolic processes, which has positive effects on the development of shoots, roots and nutrient uptake [48]. The growth trend of all parameters was different between all treatments. The number of leaves was not significantly different between N2, N3 and N4 misting at 30-, 45- and 60-min intervals. The highest stem diameter, leaf length, leaf with and leaf area were observed for N1 compared to those observed for N2, N3 and N4 at the same spraying intervals, and the lowest above-mentioned parameters were found for the ultrasonic nozzle. The number of leaves, stem diameter, leaf length, leaf width and leaf area showed decreasing trends under N1 and N2 with increasing spraying interval; however, these parameters first increased and then decreased with 30-, 45- and 60-min spraying intervals. The findings of our research showed a very close connection with the review literature that the droplet size and spraying interval had powerful relations to plant growth [20,22,26,30,49–51]. In another study, the authors of Reference [21] reported that plant growth could be improved by using proper droplet size and that air-assisted atomizers presented higher yields than those of airless nozzles. Furthermore, the biomass yield and edible yield results showed a decreasing trend for all treatments with increasing spraying interval. The results showed that spraying intervals had an inversely proportional relationship to biomass yield and edible yield. Significantly Agronomy 2021, 11, 97 14 of 17

higher biomass yield and edible yield were observed for the nozzle with air (N1) than those observed for N2 and N3 misting at a 30-min interval. Very poor plant yield was observed for the ultrasonic nozzle. This was because of the high change in pH and EC values of the nutrient solution. These results agree with the findings of Lakhiar et al. [38] that a relatively high biomass yield could be obtained from lettuce with an optimal nutrient solution recycling process. Li et al. [52] also reported that aeroponic systems with proper spraying intervals lead to remarkably improved biomass yield. Regarding the two-way ANOVA results, the root characteristics were more dependent on droplet size and spraying interval than the leaf characteristics. However, N1 showed a significant (p < 0.05) effect on all measured parameters. N3 showed a mixed phenomenon of a significant (p < 0.05) and nonsignificant (p > 0.05) effect, and N2 and N4 presented a nonsignificant (p > 0.05) effect on root characteristics. A greater root length, root diameter, root area and root volume were observed with the air-assisted nozzle and a 30-min spraying interval than with N2, N3 and N4 misting at 30-, 45- and 60-min intervals. It is well known that a relatively large root system could lead to a higher biomass yield than a small root system. Previous studies also present the same type of results: good aeration of the root environment is most advantageous for root growth [53]. Lakhiar et al. [21] also reported similar results, that air-assisted atomizers enhance root growth more significantly (p < 0.05) than airless atomizers. The analyzed results indicated that the droplet size (nozzle) and spraying interval affected the root-to-shoot ratio (fresh and dry). An increase in the fresh root-to-shoot ratio with increasing misting interval was observed for N1, N2 and N4, while for N3, the root- to-shoot ratio first increased and then decreased at 30-, 45- and 60-min spraying intervals. Furthermore, an increasing in the dry root-to-shoot ratio was calculated for N1 and N4 with increasing spraying intervals, and a decrease in this parameter was observed for N2 and N3. The researchers [53–55] also reported corresponding findings that the root-to-shoot ratio was higher in aerated growth boxes with good oxygen circulation than under poor oxygen conditions. Another study concluded that different spraying intervals induce different integrated distribution patterns, shifting assimilates from roots to shoots [56–59]. The interaction effect of droplet size and nutrient solution spraying intervals showed a positive effect on the nutrient uptake of aeroponically grown lettuce. It was observed that the nozzle with air (N1) had a significant (p < 0.05) effect on phosphorus, potassium and magnesium uptake and a nonsignificant (p > 0.5) effect on nitrogen and calcium. N3 showed a significant (p < 0.05) effect only on magnesium uptake, and the ultrasonic nozzle presented a significant (p < 0.05) effect on calcium uptake. More importantly, N2 illustrated a nonsignificant (p > 0.05) effect on all measured parameters of nutrient uptake. These elements benefit plants directly or indirectly by affecting plant metabolism. The findings of our research coincide with the recommendations of Khan et al. [33], Li et al. [52] and Xie et al. [60], who reported that N, P, K, Ca and Mg uptake increases the efficiency of photosynthesis, which is key to increasing crop yield in an aeroponic system. Another study reported that good aeration with proper spraying intervals of the root environment is advantageous in aeroponics and could result in improved uptake of N, P, K, Ca and Mg by plants [61,62].

5. Conclusions This study demonstrated that both droplet size (nozzle) and nutrient solution spray- ing interval are major factors affecting shoot and root growth of lettuce crops grown in aeroponic systems. The biomass yield was significantly higher under the N1I1 treatment than under the N2, N3 and N4 treatments misted at 30-, 45- and 60-min intervals. For the air-assisted nozzle misting at a 30-min spraying interval, shoot development was more con- strained than root development, which was prominent in the alteration of the root-to-shoot ratio (fresh and dry) and nutrient uptake. The authors concluded that the nozzle utilizing air (N1) provides a suitable environment, oxygen availability and is more advantageous for root growth. The greater root growth provided greater shoot biomass (yield) as compared Agronomy 2021, 11, 97 15 of 17

to airless and ultrasonic nozzles misted at different spraying intervals. In addition, to grow crops and vegetables in aeroponic systems, we recommended that the proper droplet size, spraying interval and nutrient solution for each be investigated in the future.

Author Contributions: M.H.T. collected the data, designed the experiment, involved in laboratory analyses and drafted the manuscript. J.G. arranged all tests, designed all nozzles and gave main suggestions how to analyze the data, edited and finalizes the manuscript. I.A.L. gave some important suggestions for the initial manuscript. W.A.Q. was also involved in laboratory analyzes. S.A.S. and K.A.S. helped in data collection and removed the mistakes and J.C. finalized the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge that this work was financially supported by the National Natural Science Foundation of China Program (No. 51975255), Jiangsu Agriculture Science and Technology Innova- tion Fund (CX (18) 3048), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (No. PAPD-2018-87). Data Availability Statement: The data is available in Figures5–11 and Tables1–3 within the article. Conflicts of Interest: The authors declare no conflict of interest.

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