Daniel Fuentes Morales

Jorge Flores Velázquez

Ariosto Aguilar Chávez

Rodrigo Roblero Hidalgo

Rev. Fac. Agron. (LUZ). 2022, 39(1):

e223920

ISSN 2477-9407

DOI: https://doi.org/10.47280/RevFacAgron(LUZ).v39.n1.20

Crop Production

Associate editor: Dr. Jorge Vilchez-Perozo

Keywords:

Hydroponics

Photoperiod

Urban agriculture

Fresh weight

Response of LED lights intensity on lettuce production in a home vertical farm

Respuesta de la intensidad de luces LED sobre la producción de lechuga en una granja vertical casera

Resposta da intensidade da luz es LED na produção de alface em uma fazenda doméstica vertical

Posgrado en Ciencias del Agua, Instituto Mexicano de

Tecnología del Agua, Jiutepec, Morelos, México.

Colegio de Postgraduados, Campus Montecillo, Montecillo,

Texcoco, Estado de México, México.

Instituto Mexicano de Tecnología del Agua, Jiutepec,

Morelos, México.

Received: 19-01-2021

Accepted: 24-05-2021

Published: 26-02-2022

Abstract

There are several LED lamps on the market that were not designed to

produce lettuce; however, they can be purchased at a low cost. In Mexico

there is a lack of research on its use in small-scale vertical farms with

hydroponics established in urban agriculture. The objective of this work

was to measure the response of three commercial lamps with LED lights

on the growth and morphogenesis of sangria lettuce grown in a low-

cost home vertical farm. The sangria lettuce was subjected to three light

intensities with different spectra and photoperiod. After 30 days of growth

the plant height, leaf length, leaf width, number of leaves and fresh weight

were measured, with these measurements a one-way analysis of variance

(ANOVA) was performed and Duncan’s multiple comparison method was

used through an algorithm designed in Python. According to the results, the

highest fresh weight was observed for a light intensity of 5700 lux. In the

2100 lux treatment, the highest height of the plant was observed and it was

the treatment with the highest energy consumption. The production of lettuce

at home is technically feasible, however, in addition to the light intensity, it

is necessary to evaluate the quality (wavelengths) from the agronomic point

of view.

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

Rev. Fac. Agron. (LUZ). 2022, 39(1): e223920. January - March. ISSN 2477-9407.

2-5 |

Resumen

En el mercado existen diversas lámparas LED que no fueron

diseñadas para producir lechuga, sin embargo, se pueden adquirir a

un bajo costo. En México se carece de investigaciones sobre su uso en

granjas verticales de pequeña escala con hidroponía establecidas en

agricultura urbana. El objetivo de este trabajo fue medir la respuesta

de tres lámparas comerciales con luces LED sobre el crecimiento

y morfogénesis de lechuga sangría cultivada en una granja vertical

casera instrumentada a bajo costo. La lechuga sangría se sometió

a tres intensidades de luz con diferentes espectros y fotoperiodo.

después de 30 días de crecimiento se midió la altura de la planta,

largo de la hoja, ancho de la hoja, numero de hojas y peso fresco, con

dichas mediciones se realizó un análisis de varianza unidireccional

(ANOVA) y se empleó el método de comparación múltiple de

Duncan mediante un algoritmo diseñado en Python. De acuerdo a

los resultados el mayor peso fresco se observó para una intensidad

de luz de 5700 luxes. En el tratamiento de 2100 luxes se observó la

mayor altura de la planta y fue el tratamiento con mayor consumo

energético. La producción de lechuga en casa es técnicamente viable,

sin embargo, además de la intensidad de luz se requiere evaluar la

calidad (longitudes de onda) desde el punto de vista agronómico.

Palabras clave: Hidroponía, fotoperiodo, agricultura urbana, peso

fresco.

Resumo

No mercado existem várias lâmpadas LED que não foram

projetadas para produzir alface, porém, podem ser adquiridas a baixo

custo. No México faltam pesquisas sobre sua utilização em fazendas

verticais para produzir alface em climas quentes. O objetivo deste

trabalho foi medir a resposta de três lâmpadas comerciais com LED

no crescimento e morfogênese da alface sangria cultivada em uma

fazenda vertical residencial de baixo custo. A alface sangria foi

submetida a três intensidades de luz com diferentes espectros e após

30 dias de crescimento foram medidos a altura da planta, comprimento

da folha, largura da folha, número de folhas e massa fresca, com

medições, uma análise de variância (ANOVA) unilateral foi realizada

e o método de comparação múltipla de Duncan usando Python foi

usado. O maior peso fresco foi observado para uma intensidade de

luz de 5700 lux. No tratamento de 2100 lux, foi observada a maior

altura de planta e foi o tratamento com maior consumo de energia.

Cabeças mais soltas desenvolvidas devido à temperatura do edifício.

A produção de alface em casa é tecnicamente viável, porém, além da

intensidade da luz, é necessário avaliar a qualidade (comprimentos de

onda) do ponto de vista agronômico.

Palavras chave: Hidroponia, fotoperíodo, agricultura urbana, peso

fresco.

Introduction

It is estimated that by 2050, 70% of the world’s population will

live in urban areas and will increase from 7 billion people to more

than 9 billion (OECD, 2012), requiring approximately 50% more food

(Pinstrup-Andersen, 2018). In Mexico, the population is expected to

reach 150.8 million people (Anda and Shear, 2017). The use of urban

vertical farms has been proposed to meet the growing demand for

food by making efcient use of resources (Pinstrup-Andersen, 2018;

Benke and Tomkins, 2017; Beacham et al., 2019).

Vertical farms consume up to 97% less water during a year

compared to traditional agriculture, due to the use of hydroponic

systems and microclimate control inside the buildings, allowing to

reduce evapotranspiration (Kalantari et al., 2018). Producing crops

vertically allows to obtain a higher yield by making an efcient use

of soil (Eigenbrod and Gruda, 2015). The use of LED lights allows

food to be grown in warehouses, buildings and houses, contributing

to the non-deforestation of new areas for food production (Kalantari

et al., 2018).

Lettuce is one of the vegetables that, due to its agronomic

requirements is most cultivated in systems under disruptive

technologies in urban agriculture or indoor (Specht et al., 2014). For

its production in vertical farms, it is necessary to nd the intensity,

luminous spectrum and photoperiod of LED lights that allow to

obtain the maximum yield with the minimum energy consumption

(Loconsole et al., 2019). Zhang et al. (2018) recommend using a

light intensity of 250 μmol.m

-2

-1

PPFD (Photosynthetic photon ux

density) with a photoperiod of 16/8 h.d

-1

(light/dark), according to the

growth and energy consumption of LED lights used in their study of

purple leaf lettuce (Lactuca sativa L. cv. Ziwei). In another study the

best growth was obtained with a light intensity of 290 μmol.m

-2

-1

PPFD and a photoperiod of 6/2 h.d

-1

, for lower light intensities, the

best growth was observed with the combination of 230 μmol. m

-2

-1

PPFD and the photoperiods of 18/6 h.d

-1

and 9/3 h.d

-1

(Kang et al.,

2013).

Another important factor that inuences the development of

lettuce is the air temperature, being a cold season crop, if it is

exposed to high temperatures, biomass accumulation is reduced, stem

elongation occurs and looser heads develop, if the temperature is

not reduced, bolting and emission of oral stem will be caused (Al-

said et al., 2018). During the germination, the optimal temperature

is from 15 °C to 20 °C (Resh, 2013). After transplanting, optimal

temperatures vary from 17 to 28°C during the day and from 3 to 12

°C at night (Wurr et al., 1992).

There are several LED lamps on the market that were not

designed to cultivate lettuce, however, they can be purchased at a low

cost, in Mexico there is a lack of research about its use in small-scale

vertical farms in warm weather. The objective of this study was to

optimize the LED light according to growth and morphogenesis of

sangria lettuce caused by different commercial lamps in a low-cost

instrumented vertical farm.

Materials and methods

Experiment description

A vertical farm prototype consisting of a 5-level shelf (Length x

width x height, 40.4 x 90.4 x 183.6 cm) was acquired. It was equipped

with a oating root hydroponic system made up of 39 L plastic

tanks with 1” polystyrene plates and 2” hydroponic baskets. For

oxygenation, it had an aeration system composed of 80 L air pumps

(1588, Aquakril, Mexico) and exible translucent silicone hose with

air diffusers. Each level had 6” 117 VAC fans with 14 W of power

(Steren, Mexico) to extract the hot air from the LED lamps (gure 1).

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

3-5 |

Sensors were also installed to monitor climatic variables (table 1)

and an Arduino-based automation system to operate the illumination,

ventilation and oxygenation system.

Figure 1. Characteristics of the vertical home farm with

management and monitoring subsystems and sangria

lettuce in production.

Table 1. Sensors for monitoring and control of the vertical farm.

Sensor Variable Accuracy

Dht22 (UNIT Electronics,

CDMX, Mexico)

Ambient temperature

and humidity

±0.5 °C

±3.5 %

Dsb1820 (UNIT Electronics,

CDMX, Mexico)

Solution temperature ±0.5 °C

BH1750 (UNIT Electronics,

CDMX, Mexico)

Light intensity ±0.5 lx

DS3231 (UNIT Electronics,

CDMX, Mexico)

Date and hour 172 ms.day

-1

The study was carried out inside a building located in Morelos,

Mexico, built with blocks and cement, which are typical materials of

the region. Sangria lettuce seeds (Lactuca sativa L.) were germinated

in 144-cavity agricultural foam tray (peat FOAM, CDMX, Mexico)

for 10 days in the vertical farm with a light intensity of 8500 lux

provided by LED lamps (Venoya International Co., Lt, Hong Kong,

China, Table 3) at a photoperiod of 18/6 h.d

-1

. Eight seedlings per

tray were transplanted at a density of 20 x 20 cm, where the crop was

monitored for 30 days to complete a 40-day cycle.

Plant nutrition was carried out with a commercial nutrient solution

for the production of leafy vegetables composed of a solution A with

micronutrients and a solution B with micronutrients (table 2). For the

rst 10 days of sowing 2.5 mL of A and B were used per liter of water,

after 10 days 5.0 mL of A and B were used. After transplanting, the

nutrient solution was completely replaced every 10 days. The pH of

the nutrient solution was maintained in the range of 5.5-6.5.

Fuentes et al. Rev. Fac. Agron. (LUZ). 2022, 39(1): e223920

Table 2. Mineral composition of the nutrient solution.

Macronutrients mg.L

-1

Micronutrients mg.L

-1

N 180 Fe 1.5

P 41 Mn 0.7

K 172 B 0.13

Ca 158 Cu 0.06

Mg 25 Zn 0.14

S 55 Mo 0.05

Treatments and experimental design

After transplanting, the plants were subjected to a photoperiod

of 18/6 h.d

-1

with three treatments of intensities and light spectrum

according to commercial LED lamps in Mexico (table 3). The

commercial lamps have a driver inside in order to connect them to

the 127 V network, in the case of LED strips, they were connected to

a 12V and 20 A driver (TF12V10A, ÁguiLed, Mexico). A wattmeter

(HER-432, Steren, Mexico) was used to measure the energy

consumption of each treatment and the energy used to obtain 100 g.m

of fresh weight was estimated. All treatments were subjected to the

temperature inside the building.

Table 3. Treatments to which the lettuce was subjected.

Treatment

Light

intensity

(lux)

Spectrum

Illumination Type

T1-2100/

2100 80% R + 20% B

SMD5050 LED strips

(Corp. Res Electronics,

CDMX, Mexico)

T2-5700/

RBIRUV

5700

71,6% R + 26.7%

B + 0.85% IR +

0.85% UV

30x30 cm Lamp (Venoya

International Co., Lt,

Hong Kong, China)

T3-6300/

RBWI-

RUV

6300

62.5% R + 25% B

+ 4.2% W + 4.2%

IR + 4.2% UV

25x25 cm Lamp (ZGSG-

LLM, SUNPIN, United

States)

Light intensity obtained with a BH1750 sensor with an accuracy of ±0.5 lx.

R=Red, B=Blue, W=White, IR=Infrared, and UV=Ultraviolet.

At the end of the crop cycle, 5 plants were randomly selected

for each of the three treatments to measure fresh weight without the

root. The fresh weight of each plant was measured using a balance

(BAPRE-3, Rhino, CDMX, Mexico) with an accuracy of 0.2 g.

Experimental results were subjected to a one-way analysis

of variance (ANOVA) and a Duncan’s multiple range test with a

signicance level of p = 0.05 using an algorithm developed in Python

3.4 (PSF, USA).

The temperature and humidity of the outside were taken from the

automatic meteorological station of the National Water Commission

(CONAGUA, Mexico) located in the facilities of the Mexican

Institute of Water Technology (MTA. Jiutepec, Mor, Mex), which

tracks meteorological variables (Temperature, Relative Humidity and

Radiation). The data obtained both inside and outside were processed

in a software developed in Python 3.4 (PSF, USA).

Results and discussion

Figure 2 shows the crop treatments and Figure 3 shows the

variation in daily temperature and humidity inside and outside the

building. Inside the average temperature remained at 25 °C with a

variation of ± 5 °C and the relative humidity also registered constant

values of 60% with a variation of ± 10%. The variables outside the

Sensors

LED lamps

Floating root

system

Fan

Air pump

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Rev. Fac. Agron. (LUZ). 2022, 39(1): e223920. January - March. ISSN 2477-9407.

4-5 |

house presented higher values and in the case of temperature,

oscillations from 10 to 30°C were measured; relative humidity

varied from 40 to 70%. The relevance of the exterior is that a

building with these characteristics in itself provides greater stability

in the climate variables, in this case temperature and humidity.

Figure 2. "Sangria" lettuce subjected to LED light. Three

treatments with different intensity and light spectrum

were used A) T1-2100/RB, B) T2-5700/RBIRUV and C)

T3-6300/RBWIRUV.

The graphs show that the building is able to reduce the variation

of the outside temperature in addition to conserving humidity which

is a condition for crop development. It has been observed that the

lack of total radiation and keeping the temperature constant at an

average of 25 °C outside the building, favored stem elongation and

the development of looser heads (gure 3). Physiologically, the plant

tries to capture more radiant energy and its geotropism indicates

that it is in the upper part, hence its elongation, which is consistent

with Al-Said et al. (2018) who found that when exposing lettuce

to a temperature of 25°C and 28°C, the exposed characteristics are

presented.

Figure 3. Temporal variation (from Oct 25 to Dec 4) of

temperature and humidity inside (A) and outside

(B) the building during the lettuce cycle (40 days).

The results shown in table 4, show that there are statistical

differences between the agronomic characteristics measured for

lettuce depending on the light intensity of each of the lamps and

photoperiod to which each of the 3 treatments was subjected.

Although it is true that the development of a crop is subjected to the

interaction of climatic variables, with this experiment, it is intended

to show that with conventional articial lights it is possible to meet

minimum requirements to obtain plants with characteristics similar

to those obtained with specic lights in commercial plant factories.

Table 4. Effect of the intensity and spectrum of LED lights on

the growth and development of lettuce.

Treatment

No. Of

leaves

Plant

height

(cm)

Leaf

Fresh

weight

(g)

Length

(cm)

Width

(cm)

T1-2100/RB 16.4±1.3 a* 28.4±1.6 bc 19.1±1.0 11.5±0.9 a 25.5±5.9 a

T2-5700/RBIRUV 20.8±0.2 c 23.5±0.3 a 20.8±0.3 15.8±0.2 bc 65.7±1.0 b

T3-6300/RBWIRUV 18.0±0.5 ab 27.8±0.7 b 21.2±0.3 15.5±0.6 b 47.5±5.5 c

*Data are mean values ± ES of 5 plants per treatment. Different letters in the

same column were signicantly different by Duncan’s multiple comparison

method (p ≤ 0.05).

In addition to the fact that the lamps used in the present research

provide low light intensity, it was observed that the exposure to

red light (R:B 4:1) of T1-2100 treatment was correlated with stem

elongation, these results are consistent to those obtained by Zhang

et al. (2017) who report a higher height of lettuce when grown under

red light in a R:B ratio 9:1 and 4:1. Hence the relevance of the

combination of the other colors of the solar spectrum, depending on

the type of plant, in this case lettuce.

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5-5 |

The measured plant parameters (height, weight, leaf size) were

comparatively (among the three treatments) higher for the T2-5700

treatment (gure 4) whose spectrum is mainly composed of blue

and red light with a 2.7:1 ratio. Zhang et al. (2018), found that

quality characteristics were better for a suitable ratio of red and blue

light at an R:B ratio 2.2:1 compared to white light.

Figure 4. Effect of agronomic characteristics (size, shape) of

the three treatments with different intensity and light

spectrum, A) T1-2100, B) T2-5700 and C) T3-6300.

The width of leaves also presented a reduction in the size of the

leaf, which is statistically signicant due to the low illumination

of the T1-2100 treatment, which improves with the other two light

intensities T2-5700 and T-6300.

A higher light intensity increased plant growth, obtaining the

maximum fresh weight of 65.7 g for an intensity of 5700 lux. This is

consistent with Kang et al. (2013) who obtained higher fresh weight

with higher light intensities where the maximum value was 81.28

g with 290 μmol.m

-2

-1

PPFD using a photoperiod of 18/6 h.day

-1

and a R:B:W ratio 8:1:1. In another study carried out by Zhang et

al. (2018) the highest fresh weight was obtained for an intensity

of 250 μmol.m

-2

-1

PPFD which was 42.7 g with a photoperiod of

16/8 h.day

-1

and a R:B ratio 2.2:1. Fresh weight of the T1-2200

treatment was signicantly lower compared to the T2-5700 and T3-

6300 treatments.

Due to the low yield and high power consumption of the

driver used in the LED strips, the T1-2100 treatment is the least

efcient for lettuce production and the most efcient is the T2-5700

treatment (table 5).

Table 5. Energy consumption for LED light treatments on

lettuce.

Treatment Consumption (kwh per 100 g.m

-2

of fresh weight)

T1-2100 17.6

T2-5700 2.0

T3-6300 2.4

With a light intensity of 2100 lux, the lettuce has an initial

growth reaching a point where it stops and stops growing; these

plants remained alive the rest of the time than the others, but no

changes in size and weight were observed, so it is inferred that the

amount of light provided by the used lamp is insufcient for it to

perform its physiological functions and continue its development

phases. An intensity of 5700 lux with an R:B ratio of 2.7:1 was the

one that presented the highest fresh weight with the lowest electrical

consumption and also fewer morphological problems were obtained.

The highest fresh weight was related to a higher water consumption,

which makes sense, the higher the evapotranspiration, the higher

the dry matter production, even in indoor conditions.

Conclusions

The home plant factory implemented for lettuce production

indoors in urban buildings is technically feasible. Among the basic

requirements, it is concluded that commercial lamps with light

intensities of 5700 lux and an R:B ratio of 2.7:1 achieve lettuce of

65.7 g, which can be improved by extending the wavelength range

(light quality).

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Rev. Fac. Agron. (LUZ). 2022, 39(1): e223920. January - March. ISSN 2477-9407.