Application of Microbubbles to Hydroponics

Application of Microbubbles to HydroponicsSolution Promotes Lettuce Growth

Jong-Seok Park1,2 and Kenji Kurata1

ADDITIONAL INDEX WORDS. Lactuca sativa, macrobubbles, deep flow technique,culture

SUMMARY. We investigated the effects of microbubbles, generated by a swivellingmicrobubble generator in hydroponics nutrient solution, on the growth of leaflettuce (Lactuca sativa). Twenty-four lettuce seedlings at the four- to five-leaf stageeach were transplanted into two culture containers at 21 ± 1 �C (day) and 18 ± 1 �C(night) under fluorescent lamps that provided a photosynthetic photon flux of 173 ±18 and 171 ± 16 mmol�m–2�s–1 averaged at eight points at the canopy level for micro-and macrobubbles conditions, respectively, during a photoperiod of 16 h per day.Seedlings were cultivated for 2 weeks in two deep flow technique (DFT) hydro-ponics culture systems in which micro- or macrobubbles were produced, respec-tively, by a microbubble aerator and aquarium aeration stones. The nutrientsolution was maintained at a temperature of 22 ± 1 �C during the experiment. Freshand dry weights of the microbubble-treated lettuce were 2.1 and 1.7 times larger,respectively, than those of the macrobubble-treated lettuce. Although the reasonsfor growth promotion by microbubbles are still under investigation, we speculatethat the larger specific surface area of the microbubbles and negative electroniccharges on the microbubbles surfaces may promote growth because microbubblescan attract positively charged ions that are dissolved in the nutrient solution. Theseresults indicate that microbubbles generated in a DFT hydroponics culture systemcan remarkably promote plant growth.

Microbubble technology hasbeen gathering much at-tention in many fields, in-

cluding foam fractionation, foodprocessing, purification processingof polluted water, and marine cultureof oysters (Kodama et al., 2000;Ohnari and Tsunami, 2006; Takahashi,2005). Successful applications ofmicrobubbles to bioprocessing ofpolluted water and to aquaculturehave been reported (Ohnari, 2007).Microbubbles are tiny gas bubbles

with a mean diameter of 50 mm orless in water (Ohnari and Tsunami,2006; Takahashi, 2005). One of themost significant characteristics of amicrobubble is that it shrinks in waterand ultimately collapses because itresides in water for a long time

(Zhang et al., 2007); in contrast,ordinary macrobubbles quickly riseand burst at the surface of water(Takahashi et al., 2003). Therefore,microbubbles are a highly efficientmeans of delivering dissolved gas intoa solution. A crucial characteristic ofthe microbubble is that they are elec-trically negative charged (z potential)on their surface (Ohnari and Tsunami,2006; Takahashi, 2005); due to thesecharges, the bubbles do not attracteach other and thereby get largerand attract positively charged materi-als. Because of these characteristics,microbubbles have significant poten-tial to be used for a variety of practicalpurposes.

However, almost no informationis available regarding how microbub-bles affect vegetative plant growth inhydroponic culture systems. Theobjective of the present study was toexamine the effect of microbubbleson the growth of leaf lettuce in a deepflow technique (DFT) hydroponicsculture system. To understand theeffects of microbubbles on plantgrowth, we compared lettuce treatedwith macrobubbles generated byaquarium aeration stones at a similarlevel of aeration.

Material and methodsPLANT MATERIALS AND CULTURE

CONDITIONS. The seeds of ‘Coslet-tuce’ leaf lettuce (Takii Seed Co.,Tokyo) were sown in vermiculite andpeat (1:1) mix and were placed in apart of a plant culture room (4 · 5.7m) in which temperature was con-trolled with four air conditioners(SAP-V22B; SANYO Electric Co.,Tokyo) at 21 ± 1 �C (day) and 18 ±1 �C (night). Fluorescent lamps(CREA1-FPL55EX-L; Iwasaki Elec-tric Co., Tokyo) provided a photo-synthetic photon flux (PPF) of 160 ±15 mmol�m–2�s–1 at the surface of theculture media. On day 14 after sow-ing, the seedling roots were detached

UnitsTo convert U.S. to SI,multiply by U.S. unit SI unit

To convert SI to U.S.,multiply by

0.3048 ft m 3.28083.7854 gal L 0.26422.54 inch(es) cm 0.39371 mmho/cm dS�m–1 1

28.3495 oz g 0.03531 ppm mg�L–1 1

(�F – 32) O 1.8 �F �C (1.8 · �C) + 32

Preliminaryand Regional

Reports

1Department of Bio-Environmental Engineering, TheUniversity of Tokyo, Bunkyo-Ku, Tokyo 113-8657,Japan

2Corresponding author. E-mail: [email protected].

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from the medium, washed, and wrap-ped in a urethane cube. Sixty seedlingswere placed in hydroponics solution,half-strength Yamazaki nutrient solu-tion, containing 45.4 mg�L–1 nitrogen(41.9 mg�L–1 nitrate and 3.5 mg�L–1

ammonium), 7.7 mg�L–1 phosphorus,78.1 mg�L–1 potassium, 20.1 mg�L–1

calcium, 6.1 mg�L–1 magnesium, and8.0 mg�L–1 sulfur, and other micro-nutrients (Yamazaki, 1978). After 1week, 24 seedlings at the four- to five-leaf stage were selected for micro- andmacrobubble treatments (n = 12 foreach condition) and were transplantedwith a planting distance of 13 cm.They were cultured in the room for 2weeks at 21 ± 1 �C day and 18 ± 1 �Cnight temperatures under fluorescentlamps (CREA1-FPL55EX-L, IwasakiElectric Co.) providing a PPF of 173 ±18 and 171 ± 16 mmol�m–2�s–1 (mean ±SD) averaged at eight points at thecanopy level for micro- and macro-bubble conditions, respectively, with a16-h day and 8-h night photoperiod.For uniformity of light intensity, ashading net was set up between thelight source and the lettuce. Plantsgrew in two culture containers (46cm long · 66 cm wide · 33 cm high)filled with its full strength solution of70 L in a DFT hydroponics culturesystem. A thermoregulating systemwith a heater (CTR-420; AGCTechno Glass Co., Chiba, Japan) andcooler (Coolpipe 150LF; Taitec Co.,Saitama, Japan) continuously main-tained the nutrient solution temper-ature at 22 ± 1 �C. Micro- andmacrobubbles were continuously gen-erated at the bottom of the contain-ers throughout the experiment. Weexchanged the position of the two cul-ture containers for each experiment.

MICRO- AND MACROBUBBLE

GENERATION. Microbubbles were gen-erated using a swiveling microbubbleaerator (3.5 cm diameter · 7.0 cmlong; M2-MS/SUS; NanoplanetResearch Institute Co., Yamaguchi,Japan) with a pump (PMD-1521B6E;Sanso Electric Co., Hyogo, Japan).Figure 1 shows a schematic diagramof the experimental setup used for thisstudy. Water introduced into the aer-ator by the pump is spiraled up alongthe wall and down to an outlet alongthe center of the aerator. The centri-fugal force caused by the circulationflow automatically introduces airfrom the gas inlet, and a vortex ofair is formed along the center axis.

The gas is separated into fine bubblesat the outlet of the aerator by thestrong shearing force of the dispersedwater and the circulation power(Takahashi, 2005). Macrobubbleswere generated through aquariumaeration stones by an air pump (MV-70G; Enomoto Micro Pump, Tokyo).In both conditions, air was suppliedat a rate of 1 ± 0.1 L�min–1 through-out the cultivation period.

MEASUREMENT AND STATISTICAL

ANALYSIS. Dissolved oxygen (DO)concentration, electrical conductivity(EC), oxidation reduction potential(ORP), and pH of the nutrient

solution were measured with a DOelectrode (9520–10D with a D25reading unit; Horiba, Kyoto, Japan),a EC meter (B-173, Horiba), a ORPelectrode (9319–10D with a D25reading unit, Horiba), and a pHelectrode (6377–10D with a D21reading unit, Horiba), respectively,every 2 d throughout the cultivationperiod. Measured data showed onlyone selected replication because ofnot much difference in the valuesamong the three replications. Afterthe end of the culture period, freshleaf weight, leaf number, leaf length,and leaf width were recorded. Leaves

Fig. 1. Schematic diagram of deep flow technique hydroponics culture systemwith a microbubble aerator and a thermoregulator.

Fig. 2. Time course of pH, electrical conductivity (EC), dissolved oxygenconcentration (DO), and oxidation-reduction potential (ORP) in nutrient solutionwith generation of macro- and microbubbles for 2 weeks. Data show only oneselected replication because of not much difference in the values among threereplications; 1 mg�L21 = 1 ppm, 1 dS�m21 = 1 mmho/cm.

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and roots were placed in a dry oven(DX400; Yamato Scientific Co.,Tokyo) at 66 �C for 4 d, and the dryweight was also measured. Theexperiment was repeated three timeswith completely randomized designs.Statistical analysis was subjected toanalysis of variance followed by t testat P £ 0.01 using JMP software (SASInstitute, Cary, NC).

Results and discussionThe time course of pH, EC, DO,

and ORP in nutrient solution withgeneration of macrobubbles andmicrobubbles for 2 weeks was notmuch different among the three rep-lications. EC, ORP, and pH in thenutrient solution did not differbetween the micro- and macrobubbleconditions at any time during theexperiment (Fig. 2). However, DOin the microbubble condition wasslightly higher than that in the macro-bubble condition (Fig. 2), probablybecause of the excellent gas dissolu-tion ability of microbubbles (Takahashiet al., 2003). Oxygen deficiency inthe root zone can lead to poor rootand plant performance and an in-crease in the infection of disease(Cherif et al., 1997). Research hasfound that aerial fresh weight oftomato (Solanum lycopersicum) plantscultivated under DO concentrationsof 8.5 (ambient air saturated level),20, and 30 mg�L–1 were not differentamong them, but showed a decreasein 40 mg�L–1 conditions (Zheng et al.,2007). Tomato plant is more suscep-tible to pythium root rot (Pythiumspp.) in a greenhouse culture if rootzone oxygen drops below 2.8 mg�L–1

(Cherif et al., 1997). Although themean DO value in the microbubbletreatment was 0.10 mg�L–1 higherthan that in the macrobubble treat-ment for 2 weeks, the difference inDO values would be predicted tohave a negligible effect on plantgrowth in this DFT hydroponic cul-ture system. All things considered,the solution’s properties of EC, pH,ORP, and DO did not cause thedifference in lettuce growth betweentwo conditions.

Significant increases in growthwere observed in the microbubblecondition, where samples showed2.1 times greater fresh leaf weightand 1.7 times greater dry leafweight than the macrobubble samples(Fig. 3). Leaf number, leaf length,

Fig. 3. Aerial fresh weight (AFW), leaves number, leaf length (LL), leaf width (LW),aerial dry weight (ADW), and root dry weight (RDW) per plant of leaf lettuce grownfor 2 weeks in deep flow technique hydroponics culture system in which macro- andmicrobubbles were generated. Data represent means ± SE. Means with an asteriskare significantly different by t test at P £ 0.01 (n = 36); 1 cm = 0.3937 inch, 1g = 0.0353 oz.

Fig. 4. Leaf lettuce grown for 2 weeks in a deep flow technique hydroponicsculture system in which macrobubbles (left) or microbubbles (right) weregenerated in the nutrient solution; 1 cm = 0.3937 inch.

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PRELIMINARY AND REGIONAL REPORTS

and leaf width of lettuce grown inthe microbubble condition werealso significantly higher (Fig. 3). Avisible difference in aerial growth wasobserved 4 d after the start of thebubble generation. Although EC,ORP, pH, and DO did not differ,remarkably, promoted roots wereobserved in the microbubble condi-tion (Fig. 4). This finding was con-sistent with a growth increaseobserved when water hyacinth(Eichornia crassipes) roots were trea-ted with microbubbles, in that casefor processing of polluted water in apond (Ohnari, 2007). Thus, micro-bubbles generated in a DFT hydro-ponic culture system promoted rootand shoot growth.

Although the reasons for growthpromotion by the microbubbles arestill under investigation, one of thepossibilities is the larger specific sur-face area of microbubbles when com-pared with macrobubbles. Numerousmicrobubbles adhered to the root.This seems to positively stimulateroot growth by supplying oxygendirectly to the surface of roots. Neg-

ative electronic charges on microbub-bles surface may help roots absorbnutrient salts because microbubblescan attract positively charged ionsthat are dissolved in the nutrientsolution, such as calcium, magne-sium, potassium, and ammonium(Takahashi, 2005).

In conclusion, the advantage ofusing microbubbles was demonstra-ted in a DFT hydroponics culturesystem, remarkably promoting let-tuce growth. Further study will berequired to elucidate the relationshipbetween the specific properties ofmicrobubbles and plant root growth.

Literature citedCherif, M., Y. Tirilly, and R.R. Belanger.1997. Effect of oxygen concentration onplant growth, lipid peroxidation, andreceptivity of tomato roots to pythiumunder hydroponic conditions. Eur. J.Plant Pathol. 103:255–264.

Kodama, Y., A. Kakugawa, T. Takahashi,and H. Kawashima. 2000. Experimen-tal study on microbubbles and theirapplicability to ships for skin friction

reduction. Int. J. Heat Fluid Flow 21:582–588.

Ohnari, H. 2007. Maikuro baburu gijutsuno konnichi teki kadai. Kagaku K�ogaku71:154–159. (in Japanese).

Ohnari, H. and Y. Tsunami. 2006. Mai-kuro baburu no hassei kikou to sh�ushukutokusei. Mizu K�ogaku Ronbun Sh�u50:1345–1350. (in Japanese).

Takahashi, M. 2005. z Potential of micro-bubbles in aqueous solutions: Electricalproperties of the gas-water interface.J. Phys. Chem. B 109:21858–21864.

Takahashi, M., T. Kawamura, Y. Yamamoto,H. Ohnari, S. Himuro, and H. Shakutsui.2003. Effect of shrinking microbubbleson gas hydrate formation. J. Phys. Chem.B 107:2171–2173.

Yamazaki, K. 1978. Y�oeki saibai zenpen.Hakuy�usha, Tokyo. (in Japanese).

Zhang, X.H., A. Khan, and W.A. Ducker.2007. A nanoscale gas state. Phys. Rev.Lett. 98:136101.

Zheng, Y., L. Wang, and M. Dixon. 2007.An upper limit for elevated root zonedissolved oxygen concentration fortomato. Scientia Hort. 113:162–165.

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Application of Microbubbles to Hydroponics