Hydroponics vs. Soil Cultivation: Functional and Taste Compound Comparison

Original Paper
Tamura Y, Mori T, Nakabayashi R, Kobayashi M, Saito K, Okazaki S, Wang N and Kusano M (2018) Metabolomic Evaluation of the Quality of Leaf Lettuce Grown in Practical Plant Factory to Capture Metabolite Signature. Front. Plant Sci.9:665.
doi: 10.3389/fpls.2018.00665


Cultivation of certain crops is moving out of the field. Indoor production has taken the form of greenhouses, tunnels, and plant factories. These growing methods been collectively deemed controlled environment agriculture (CEA). The attraction is in the name – control. Moving crops out of the field helps remove risk of unpredictable weather and can allow for optimized conditions for crop production. It even enables year-round growing that provides a steady source of fresh food to the public and income to the growers instead of the seasonal flux of traditional agriculture.

With food moving indoors under controlled conditions, crops are receiving different types of input in terms of nutrients, lighting, day/night cycling, temperatures, disease and pest stresses, and other variables. Some crops are growing differently and looking different as well. It all depends on the control conditions.

As plant growth and development changes due to these controlled environments, the metabolic processes dictating that growth and development are probably varying as well. As a result, there may be changes in the plant’s profile of chemical compounds, or metabolites, which take part in and are produced by plant metabolism. These compounds are integral to the structure and general function of the plant as well as its defense against pests and disease. Again, with cultivation conditions changing, the metabolite compositions of the plants are likely changing simultaneously.

The Experiment

If we want to know if or how CEA is changing the metabolite profiles of our food compared to field cultivation, we need to isolate each element of the “control” to determine what changes are being caused by which conditions. To this end, a group from RIKEN in Japan that studies metabolite profiles (an analytical chemistry practice called metabolomics) chose to compare compounds of lettuce grown in a hydroponic system (plant roots growing directly into water with an added nutrient solution) within a Keystone Technology Inc. (Japan) plant factory to lettuce grown in a similarly-controlled growth chamber except planted traditionally in soil (Table 1).

Table 1. Plant factory conditions for hydroponic cultivation treatment compared to growth chamber conditions for the soil treatment.

This group chose two lettuce cultivars, ‘Black Rose’ and ‘Red Fire’, with one head of each cultivar grown per treatment – hydroponics and soil – for a total of 4 heads of lettuce in the experiment. Tamura et al. observed smaller and more pigmented leaves from the soil-grown lettuce compared to the hydroponic production. To detect the metabolites present in the lettuce they used precise instruments (gas and liquid chromatography mass spectrometry). They included samples from leaves on the outside of the head and the middle to account for variation in metabolite production in different parts of the plant.


Analysis resulted in 133 identified compounds and 185 unidentified. Based on the relative abundances of all 318 metabolites, they were able to clearly separate samples of hydroponically grown lettuce from those grown in soil.

Upon further study, they determined that hydroponic lettuce had higher amounts of amino acids (protein building blocks) than the soil-cultivated lettuce. On the other hand, lettuce grown in soil contained more sugars and compounds that contribute to taste and possible health-benefits, such as sesquiterpenes and organic acids. Particularly, glutamate, a metabolite contributing to the umami (or savory) taste profile of a food, was significantly higher in ‘Red Fire’ lettuce grown hydroponically. However, a sugar, sucrose, and a compound associated with bitterness, lactucopicrin-15-oxalate, were both significantly lower in the hydroponic lettuce.

Conclusions and Considerations

This study is valuable due to it being the first of its kind—applying metabolomics to understand how our crops are changing in CEA systems. These results need to be validated by another experiment in which the conditions other than soil/hydroponics are identical. Previous work by Li and Kubota (2009) demonstrated that differences in light intensity and quality can affect metabolite production in a CEA setting.

Additionally, different fertilization regimes largely influence the amount of nitrogen plants can access to produce amino acids. With the hydroponic lettuce receiving almost 3x the fertilizer compared to the lettuce in soil, a higher amino acid content in hydroponic lettuce cannot be completely attributed to hydroponic production itself. Therefore, the differences in control conditions presented in Table 1 above are confounded with the soil/hydroponic treatments, making interpretation of results complicated. This also points to the importance of collaboration across scientific disciplines to ensure the most effective and efficient experiments are conducted.


Li, Q., and Kubota, C. (2009). Effects of supplemental light quality on growth and phytochemicals of baby leaf lettuce. Environ. Exp. Bot. 67, 59–64. doi: 10.1016/j.envexpbot.2009.06.011

Design for an Improved Temperature Integration Concept in Greenhouse Cultivation

Original paper: O. Ko¨rner *, H. Challa Design for an improved temperature integration concept in greenhouse cultivation Farm Technology Group, Department of Agrotechnology and Food Sciences, Wageningen University, Mansholtlaan 10, 6708 PA Wageningen, The Netherlands Received 22 July 2002; received in revised form 15 November 2002; accepted 28 December 2002.

     Heating energy represents more than two-thirds of a typical greenhouse total energy consumption. Is currently well known that the average day and night temperatures are what controls how fast plants develop. As the temperature increases, crops develop much faster, but there is a significant cost associated, which typically leads to an increase in energy consumption. To mitigate the cost and become more efficient in the control environment production, an approach to improve temperature integration concept could play an essential role in energy savings. Temperature Integration Concept is based on the ability of crops to tolerate temperature deviation from their biological set points. The integration concept manipulates temperature, aiming to be compensated within a pre-set period without having adverse effects on plant growth.

     Theoretically, a crop with more dynamic and flexible temperature boundaries could potentially play an important role, so this study aimed to improve the temperature integration concept by introducing dynamic temperature constraints. A modified temperature integration procedure was designed combining the usual long-term temperature average over several days and fixed boundaries for daily average temperature with short-term temperature averages over 24 hours with a very flexible temperature limit. The overall idea is based on a concept called the Freedom for temperature fluctuations. This concept allows the temperature to freely fluctuate due to the environment without being controlled by heating or ventilation. Temperature fluctuation increases with longer averaging period and increasing temperature bandwidth, which allows longer periods of several days, which enables compensation of warm or cold periods resulting in higher energy savings.

     The proposed regimen for temperature integration was performed by modeling and simulation techniques (MATLAB version 6.0) using tomato as a model crop. Variables such as air temperature, outside radiation, relative humidity, and CO2 measurements were input in the model with a fixed time of 5 min over one year. Measurements such as setpoints for heating, ventilation and CO2 concentrations were calculated with a climate control model (CCM) which provide enough information for calculating relative humidity, air temperature, energy consumption, and natural gas consumption. For accuracy, energy loses where also consider into the model. Two reference temperature regimens were used for comparison:  BP= commercial standards, setpoint increase linearly, and a Bpfix= night and daytime heating and ventilation temperature setpoints were fixed (uncommon practice). The heating setpoints were 18, and 19 °C and ventilation set points were 19 and 20 °C for night and day, respectively. The weather prediction was also used for providing data into the model simulation. Validations of the CCM model was performed in four semi-commercial Venlo-type greenhouse compartments.

     Two temperature integration regimens were model by the Autor: RTI (regular temperature integration) and MTI (modified temperature integration) both with a bandwidth of +/- 2, +/- 4 and +/- 6. The modified regime model (MTI) resulted in more energy saved when compared with regular temperature integration model (RTI) and the BP controls. Energy-saving increased with temperature bandwidth in all cases evaluated. Fluctuation during a cold time (winter) was observed. Overall, yearly greenhouse energy saving increased by up to 23% compared with the BP regime (temperature with a bandwidth of +/- 6 C). Compared with regular temperature integration energy-saving increased relatively with 14%. Interestingly, the setpoint for relative humidity profoundly influenced energy-saving suggesting further focus in future evaluations. When evaluating the different temperature dose-response data, they observe than an increase in the duration of maximum and minimum temperature increase energy saving and gross photosynthesis of tomato plants, which can be traduced to more photosynthetic efficiency. In conclusion, the conceptual design for advance temperature integration control seems to be promising for energy reduction. The distinction between short- and long-term processes in temperature integration lead to an increase in energy savings. A more advanced flexible humidity control concept could probably help to decrease energy consumption further since the highest energy saving was achieved when no humidity control was used.

Maximizing crop photosynthesis across the entire canopy requires the optimization of many environmental factors

Original paper: Körner, O., Heuvelink, E., and Niu, Q. 2009. Quantification of temperature, CO2, and light effects on crop photosynthesis as a basis for model-based greenhouse climate control. The Journal of Horticultural Science and Biotechnology. 84:233-239.


Photosynthesis is impacted by multiple environmental factors including temperature, light intensity, and carbon dioxide (CO2) concentration. If optimal environmental conditions that maximize photosynthesis are quantified, they can be employed in controlled environments to increase crop productivity. Attempts to measure such optimal conditions have been undertaken in the past. Environmental setpoint measurements from these studies have even been compiled and implemented into various mathematical models known as “crop photosynthesis models” (CPMs) that can predict potential photosynthetic activity based on a plant’s environment. However, many of the environmental setpoints used in CPMs have relied on leaf-level photosynthesis measurements and optimization which are not always compatible with canopy-wide photosynthesis optimization. This potential incompatibility is caused by differences in the microclimate between the various levels in a crop’s canopy. For example, light intensity generally decreases as you move from the top of the canopy down to lower leaves. Also, there can be large variations in individual leaf temperature and humidity throughout the canopy which will affect photosynthesis. Other studies have investigated canopy-wide photosynthesis, but many were performed in poorly-sealed greenhouses where conditions could potentially fluctuate. Oliver Körner and his colleagues sought to more accurately quantify optimal environmental conditions for canopy-wide photosynthesis by using well-sealed greenhouses equipped with air conditioning and CO2 supplementation. These environmental control measures allowed for experiments in which temperature and CO2 concentration could be effectively manipulated and accurately maintained. The ability to control CO2 concentration and measure CO2 consumption in the greenhouse system was critical to this study. Photosynthesis was quantified by monitoring the amount of CO2 consumed by the plants in the greenhouse. Minimizing any gas exchange with the natural environment was crucial to ensure any measured CO2 change was a result of photosynthesis.

The photosynthetic responses of two different crops (cut-chrysanthemum and tomato) were quantified under different temperatures and CO2 concentrations. ‘Reagan Improved’ chrysanthemum plants were exposed to different combinations of three temperature setpoints (23, 28, and 33 °C) and three CO2 concentrations (400, 700, and 1000 µmol CO2 mol-1) under natural light levels. Similarly, CO2 consumption was measured in ‘Moneymaker’ tomatoes under different combinations of three temperature setpoints (20, 26, and 32 °C) and two CO2 concentrations (400 and 1000 µmol CO2 mol-1). Increasing CO2 concentration raised the maximum potential photosynthetic rate in both crops across all tested temperature setpoints, and this effect was greater in chrysanthemum than tomato. Additionally, higher CO2 levels led to a higher photochemical efficiency (µmol CO2 µmol photons-1) in both chrysanthemum and tomato. Temperature effect on photosynthetic rate was more complicated although photochemical efficiency in both crops consistently decreased as temperature increased. In chrysanthemum and tomato, both light intensity and CO2 concentration affected how temperature affected maximum photosynthetic rate. Using discrete light intensities (600, 900, and 1200 µmol m-2 s-1), optimum temperatures for maximum photosynthesis at 400 and 1000 µmol CO2 mol-1 were calculated. In chrysanthemum, the optimum temperature at all three light intensities was below 23 °C at 400 µmol CO2 mol-1 so a trend was not clear. At the same CO2 concentration in tomatoes, optimum temperature for tomato photosynthesis increased with higher light levels, and the largest increase in optimum temperature occurred between 900 and 1200 µmol m-2 s-1 (25.3 to 27.1 °C). Optimum temperature for chrysanthemum and tomato photosynthesis at 1000 µmol CO2 mol-1 both increased when light intensities increased. At this CO2 concentration, optimum temperature changed the most in both crops when light intensity was changed from 600 to 900 µmol m-2 s-1. Specifically, chrysanthemum optimum temperature changed from 23.5 °C to 26.9 °C while tomato optimum temperature increased from 26.6 °C 28.4 °C.

Körner and his colleagues sought to quantify the optimum environmental conditions (temperature, CO2 concentration, and light intensity) for canopy-level photosynthesis in two crops (cut-chrysanthemum and tomato). Higher CO2 levels increased maximum photosynthesis and photochemical efficiency in both crops with this effect being greater at higher temperatures. Similarly, higher CO2 concentration led to an increased optimum temperature for photosynthesis, and this occurred at the largest level when light intensity was high. Variability in the canopy microclimate (most notably temperature and light intensity) resulted in different environmental factor effects than those observed in leaf-level photosynthesis models. In general, environmental conditions caused smaller changes in canopy-level photosynthesis when compared to leaf-level photosynthesis. While basic trends were similar in both chrysanthemum and tomato, the results indicate that optimum environmental conditions for photosynthesis must be quantified for individual crops. Differences between crops including leaf area and canopy architecture must be accounted for to create accurate CPMs. In conclusion, this study indicates that crop-specific responses to interactions between multiple environmental factors must be accounted for in CPMs to accurately quantify canopy-level photosynthesis.

A small amount of UV radiation is needed for tomato plant health

Original paper: Kubota, C., T. Eguchi, and M. Kroggel. 2017. UV-B radiation dose requirement for suppressing intumescence injury on tomato plants. Scientia Horticulturae. 226:366-371.


UV-B radiation (300-320 nm) has the shortest wavelength of the sunlight spectrum in the natural environment. Too strong UV radiation can cause issues such as leaf burn, but too little UV radiation also becomes problematic for certain species of plants including tomato. The sensitivity varies among varieties of tomato and a particular rootstock tomato used for grafting, for example, is a very sensitive one that must be grown under light including UV-B.  A typical disorder caused by lack of UV-B radiation are leaf tumors called intumescence (or oedema). In severe cases, plants cannot grow and usually die. This becomes problematic when plants are grown under protected environments (such as tunnels) covered with UV-blocking plastic material or under sole source electric lighting that does not emit UV radiation (such as LEDs). A research group led by Chieri Kubota (currently at the Ohio State University) identified the needed amount of UV radiation to maintain plants without causing such disorder.  This group also worked on other innovative approaches to mitigate intumescence injury, including a discovery of an effective LED lighting protocol to mitigate the intumescence injury.


In their experiment, they grew rootstock tomato ‘Beaufort’ plants as their experimental plant material for its high sensitivity to induce intumescence under UV-B deficient light environment.  The lamps that they tested were red and blue LEDs or cool-white T5 fluorescent lamps. While red and blue LEDs do not emit UV radiation at all, T5 fluorescent lamps do emit small amount of UV-B radiation. When ‘Beaufort’ plants were grown under LEDs, plants developed massive amounts of intumescence, but the injury was less severe under T5 fluorescent lamps due to the UV radiation.  UV-B supplementation was examined for the plants under LEDs at varied hours of UV-B exposure (1.5, 3.0 and 6.0 hours) at a very low intensity of UV-B of 0.12 W m-2 (or 0.31 μmol m-2 s-1). The UV-B exposure was conducted during the night time (every night), rather than day time. Kubota’s group showed that an increasing amount (dose) of UV-B exposure drastically improved ‘Beaufort’ tomato plant health.  The highest daily dose of UV-B they examined was 2.6 kJ m-2 (or 6.7 mmol m-2). This UV-level is less than one-tenth of what you can expect outdoors during the summer.  Under this dose of 2.6 kJ m-2, plants still exhibited a minor injury of intumescence. So the group used a mathematical approach (linear regression) to estimate the dose of UV-B that would have eliminated the intumescence injury, which was 5 kJ m-2 (or 14 mmol m-2), about twice of the highest examined dose in this study, but still far below the natural UV level of sunlight.


The impact of this study includes possible contributions to the design of plant growing facilities that use LED lighting.  The linear dose response they described in this study is particularly useful as one can choose a combination of UV-B intensity and exposure time suitable for their production setting.  A lower intensity for long exposure and a higher intensity for shorter exposure could yield the same effective dose.  Kubota group also showed that night-time application of UV-B is effective and this is helpful for growers who do not want to expose workers to UV-B light while they are in the facility.  The known/Knowing the dose response also makes the application methods more flexible, including using a movable light source over the plant canopy. The finding will be helpful for Indoor farming as well as greenhouse crop production.