Hydroponic lettuce cultivated under different electrical conductivities in the Kratky methodABSTRACT
The Kratky method of non-circulating hydroponics is little studied in Brazil. Our objective was to test this method to produce lettuce grown under different electrical conductivities (EC) of the nutrient solution during both winter and summer seasons. There were four EC, which comprised four treatments (0.6, 1.5, 2.5, and 3.5 mS·cm-1) and 13 replications, totaling 52 plants. Throughout the experiments, maximum and minimum air temperature and humidity were monitored, as well as solution pH, EC, and temperature. After 45 days of transplanting in winter and 31 days in summer, lettuce was harvested, and agronomic variables of the aerial part and roots were measured. Data collected was subjected to polynomial regression analysis that the related variables measured to the EC evaluated according to the growing season. For winter, ideal EC ranged between 2.3 and 2.6 mS·cm-1 and, for summer, from 2.8 to 3.3 mS·cm-1. The non-circulating hydroponic cultivation system Kratky method is suitable for lettuce ‘Crespa Itapuã 401 Super’ production during both winter and summer seasons.
Key words
non-circulating hydroponics; nutrient solution; Lactuca sativa L.
INTRODUCTION
Hydroponics emerged to provide the ability to grow vegetables in inappropriate areas for continuous soil-based cultivation (Sebring et al. 2022). Therefore, it also reduces demand for new arable areas and allows sustainable cultivation throughout the year. Vegetable production through hydroponics minimizes 70–80% water consumption and eliminates the use of soil, increasing productivity and taking agriculture to a level of sustainable food production (Kannan et al. 2022, Majid et al. 2021). Lettuce is currently the most widely grown vegetable in hydroponic systems due to its short cultivation period, low production and equipment costs, high consumer preference, and beneficial health characteristics (Manos and Xydis 2019).
Hydroponics techniques have been increasingly adapted to grow several vegetable and fruit crops, as well as wild plants (Lucena et al. 2012, Puccinelli et al. 2023, Sahoo et al. 2024). However, considering the Brazilian scenario, most hydroponic crops are shaped to meet market demands, reducing such crop variety to four main species: lettuce, rocket, watercress, and tomatoes (Lopes et al. 2015).
Usually, the time required for crop development by hydroponic cultivation is shorter due to complete nutrient availability in the solution and no mechanical obstacles for roots (Keswani et al. 2019). However, nutrient availability must be appropriate for each crop’s requirements because, according to Kannan et al. (2022), excess of salts, i.e., high electrical conductivity (EC), makes root nutrient absorption difficult due to osmotic pressure, while the low presence of nutrients, i.e., low EC, drastically affects crop health and yield. For growing hydroponic lettuce, EC may range from 1.5 to 2.5 mS·cm-1 (Costa et al. 2001, Pantanella et al. 2012), which may also vary according to cultivar and environmental conditions.
Apart from controlling EC in hydroponic cultivation, the hydrogen potential (pH) of the nutrient solution also needs controlled, as it is an important factor affecting plant growth and development. According to Taylor et al. (2021), to optimize cultivation, the ideal pH ranges between 6.5 and 7.0.
Among hydroponics, the Kratky method is a non-circulating system developed by Kratky et al. (1988). The main advantages lie in the need for no energy source, as there is no nutrient solution movement, which is added completely at the beginning, with no subsequent interference. Plants then develop two types of roots, called water and nutrient roots, responsible for absorbing nutrients from the solution, and oxygen roots, responsible for absorbing oxygen from the void formed between the solution and the top of the system. This void prevents plants from coming into direct contact with the nutrient solution, reducing the risk of diseases in the collar region. Roots responsible for absorbing oxygen agglomerate at the top of the root system, while those responsible for absorbing nutrients grow downwards as the level of the nutrient solution decreases (Kratky 1993).
As no studies were found on this method in Brazil, we hypothesized that it is possible to cultivate lettuce using the Kratky method by adjusting the EC properly. Our objective was to evaluate the non-circulating hydroponic cultivation system Kratky method to produce lettuce grown under different nutrient solution EC during both winter and summer seasons.
MATERIAL AND METHODS
The experiment was carried out in a non-acclimatized greenhouse, coated externally by a transparent polyethylene film, internally with a 50% black shading net and sides with a 20% white shading net (23°36’11” S, 51°38’36” W, at 807 m of altitude). According to Köppen classification, the climate in the region is of Cfa type, i.e., it is humid temperate with hot summers, with mean annual minimum and maximum temperatures of 17.4 and 27.2°C, respectively. The experiment lasted 45 days in winter and 31 days in summer. Mean minimum and maximum temperature and relative humidity during winter were 18.2 ± 4.1 and 35.0 ± 4.1°C, and 18.8 ± 12.9% and 79.7 ± 14.5%, respectively. Mean minimum and maximum temperature and humidity during summer were 19.0 ± 3.1 and 35.0 ± 4.2°C, and 18.7 ± 14.1% and 87.3 ± 10.7%, respectively.
For each experiment, pelleted seeds of ‘Crespa Itapuã 401 Super’ lettuce were sown in a phenolic foam plate, one seed per cell measuring 1.9 × 1.9 × 1.9 cm, totaling 345 sown cells. After sowing, the plate was moistened with water and placed in a cool place away from light for 24 hours. Seedlings were then moved and remained in the greenhouse for 20 days before the experiment began, being watered daily. They were then transplanted into the non-circulating hydroponics system Kratky method, which was built with six channels 3-m long × 0.1-m in diameter, closed at the end, isolated from each other, spaced 20 cm apart and filled with approximately 20 L of nutrient solution (Fig. 1). Each channel had the capacity for 15 plants, totaling 90 plants. However, the outermost channels (B1 and B2), as well as plants at the end of each channel, were not considered when obtaining experimental data due to the bordering effect, reducing the data source to four channels with 13 plants each, totaling 52 plants.
Layout of the growing channels for lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivation in the non-circulating hydroponic system Kratky method (B1 and B2 are bordering channels; C1, C2, C3, and C4 are cultivation channels).
Lettuce seedlings, with the phenolic foam cells, were placed individually in polystyrene pots of 50-mL capacity, which were perforated at the base to allow contact between the solution and the phenolic foam cells, and the roots to grow. Each of the four channels had a different nutrient solution EC: 0.6 mS·cm-1 for channel 1 (C1), 1.5 mS·cm-1 for channel 2 (C2), 2.5 mS·cm-1 for channel 3 (C3), and 3.5 mS·cm-1 for channel 4 (C4) (Fig. 1). The mineral salts used were Flex Blue (10% N, 15% Ca, and 2% Mg) and Flex Red (8% N, 8% P, 30% K, 1% Mg, 0.04% B, 0.04% Mn, 0.03% Cu, 0.019% Zn, 3% S, 0.009% Mo, 0.006% Ni, 0.14% Fe, and 0.002% Co), indicated for leafy vegetables (Plantpar), that recommend the EC of 1.5 mS·cm-1 for growing lettuce. From this value, surrounding ones were defined: 0.6, 2.5, and 3.5 mS·cm-1, in addition to 1.5 mS·cm-1, aiming to evaluate a wider range of potential EC ideal for growing lettuce in the non-circulating system Kratky method, comprising four treatments.
During 45 days in winter and 31 days in summer of plant cultivation in the Kratky system, EC, pH, and temperature data were collected from the nutrient solutions in channels 1, 2, 3, and 4. Maximum and minimum temperatures of the nutrient solutions were, in winter, 37.8 and 16.9°C for C1; 36.5 and 16.8°C for C2; 37.0 and 16.8°C for C3; and 38.4 and 16.4°C for C4; and in summer, 39.0 and 17.4°C for C1; 38.2 and 17.0°C for C2; 37.5 and 17.0°C for C3; and 36.8 and 17.0°C for C4. All data were collected daily using a thermo-hygrometer located at the central area of the experiment and a multifunction EC, pH, and temperature meter inserted into each channel manually.
In the experimental period, the nutrient solution was replenished as there was a great plant density (30 plants·m-2), around 50% more than recommended (Kratky, 2005). Kratky (2005) reports that replenishing the nutrient solution in the production system should be avoided since there is a possibility of “drowning” the roots responsible for absorbing oxygen and, consequently, causing plant death. Therefore, it is recommended to provide approximately 5.6-L nutrient solution per lettuce at the cultivation beginning, apart from a maximum density of 20 plants·m-2. For these experiments, only 1.3 L of nutrient solution was initially made available per plant, approximately 75% less than recommended. This lower nutrient solution volume was due to the hydroponic system structure, made from polyvinyl chloride (PVC) pipes, which resulted in a much lighter system, around 60% lighter than those tested by Kratky (2005). Nonetheless, the nutrient solution replenishiment was different for each treatment (Table 1), possibly due to different stages of plant development caused by each treatment.
Supplementary nutrient solution added to each treatment/channel (C1, C2, C3, and C4) for lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivation during 45 days in winter and 31 days in summer in the non-circulating hydroponic system Kratky method.
Based on data monitoring, any pH variation out of the 6 to 7 range was corrected, so during the experiment, only the pH decrease was necessary by adding phosphoric acid. The nutrient solutions were also adjusted based on both EC and demand for more water due to plant consumption and evapotranspiration. Throughout cultivation, signs of thrips attack were also observed, which were controlled with a weekly application of a natural insecticide based on flour and water (20 g of flour to 1 L of water).
Lettuce was harvested after 45 days of cultivation in winter and 31 days in summer. Before harvesting, the chlorophyll content was measured from the first fully expanded true leaf with no signs of senescence, as described in Ansorena et al. (2012). Immediately after harvesting, the upper diameter and height of the aerial part were also measured. Plants were then divided into two parts, one being the aerial part and the other the roots, for fresh matter measurements, as well as leaf area, root volume by Archimedes’ principle, and dry matter to fresh matter ratio. The aerial part and roots were then separated in paper bags and placed in a forced air circulation oven at 65°C for drying and later obtaining the dry matter, so the plant material was weighed every day until reaching a constant weight.
Data was analyzed to identify outliers by the interquartile range, so values falling outside the standard of data general distribution were excluded. The subsequent sample data were subjected to polynomial regression to verify the behavior of the variables according to the solution EC, using SAS software (SAS Institute Inc. 2023).
RESULTS AND DISCUSSION
Nutrient solution replenishing and temperatures reached do not seem to have had a negative effect on crop development neither on the variables measured, as harvesting was carried out within the commercial time established for cultivation, during both winter and summer seasons.
Among the variables evaluated, chlorophyll content was the only non-significant one for both periods (Table 2).
Chlorophyll content of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’), according to each treatment/channel (C1, C2, C3, and C4), cultivated during 45 days in winter and 31 days in summer in the non-circulating hydroponic system Kratky method.
Winter cultivation
As for plant development, upper diameter and height of the aerial part were greater for plants grown under both 2.5- and 3.5-mS·cm-1 EC, showing a biased relationship between these parameters and greater salt presence in the solution (Fig. 2). However, aerial part was much higher from 1.5- and 2.5-mS·cm-1 EC treatments, and finally, aerial part fresh matter was higher in plants cultivated under 2.5-mS·cm-1 EC, which showed an inverse relationship with aerial part dry matter, with the lowest dry matter/fresh matter ratio (6.3%). This ratio was higher for 3.5-mS·cm-1 EC (8.6%), indicating that the plant weight did not come only from hydration, but from the plant material formed (Fig. 2). For 0.6- and 1.5-mS·cm-1 EC, the dry matter/fresh matter ratio of the aerial part was 7.5 and 7.8%, respectively.
Polynomial regression of the aerial part variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivated during 45 days in winter in the non-circulating hydroponic system Kratky method: (a) upper diameter, (b) height, (c) leaf area, (d) fresh matter, and (e) dry matter of the aerial part. **p < 0.01.
Cometti et al. (2008) analyzed the effects of different ECs on lettuce production, ranging from 0.29 to 1.84 mS·cm-1, and reported that as the EC of the nutrient solution increased, so did the dry matter production of the aerial part. However, lettuce growth and development are negatively affected by both excessive and deficient salt concentrations in the nutrient solution. Fallovo et al. (2009) described that, due to either an excess or a lack of salts in the nutrient solution, both leaf area and chlorophyll content variables are negatively impacted as a result of osmotic stress and nutrient deficiency, respectively. Also, according to Taiz et al. (2017), plants have an adaptive mechanism of reducing leaf area when grown under excess salts, since, under such conditions, transpiration is reduced, corroborating what happened with the leaf area of ‘Crespa Itapuã 401 Super’ lettuce (Fig. 2).
All root variables, namely root volume, root fresh matter, and root dry matter, performed better when plants were grown under 2.5-mS·cm-1 EC (Fig. 3).
Polynomial regression of root variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivated during 45 days in winter in the non-circulating hydroponic system Kratky method: (a) volume, (b) fresh matter, and (c) dry matter of roots. **p < 0.01.
According to Nezamdoost et al. (2023), lettuce is sensitive to salinity. In addition, Carillo et al. (2021) reported that an increase in solute concentration in the roots, especially ionic solutes, may reduce permeability of the root system, thus increasing the potential for water stress. Furthermore, the excess of ions accumulated in the solution causes a reduction in both osmotic and water potential. For continuing water absorption, plants need to reduce the potential of roots and leaves, thus maintaining a potential gradient in the plant solution system. The effects of higher salinity, implied in plants cultivated under 3.5-mS·cm-1 EC, are evident by leaf area decrease when compared to 1.5- and 2.5-mS·cm-1 EC, and by minor fresh matter in comparison with 2.5-mS·cm-1 EC, which indicates that those plants submitted to 3.5-mS·cm-1 EC were more dehydrated due to osmotic stress.
From the equations generated by the polynomial regression, the best results for the aerial part were obtained when the EC ranged from 2.4 to 3.5 mS·cm-1 (Table 3).
Ideal electrical conductivity (EC) for aerial part variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivated during 45 days in winter and 31 days in summer in the non-circulating hydroponic system Kratky method.
For lettuce, the part of commercial interest is the leaves, i.e., the aerial part, which are mainly reflected by leaf area and fresh matter variables, as they correspond to the largest product area and the largest saleable mass, respectively. These two variables were maximum under the EC range of 2.4 to 2.6 mS·cm-1. Similar results for lettuce were found by Fallovo et al. (2009) from the EC of 2.2 to 2.6 mS·cm-1. Nezamdoost et al. (2023) also obtained better results from an EC of 2.5 mS·cm-1. In addition, the authors indicated that, as the concentration of the nutrient solution increased from 2.5 to 4.0 mS·cm-1, the presence of soluble solids also increased, promoting a similar result for the dry matter variable (ideal EC of 2.9 mS·cm-1); the latter reached the highest average production value under the EC of 3.5 mS·cm-1. This correlation between dry matter and soluble solids was tested by Scalisi and O’Connell (2021), who stated that, for several crops, there is a directly proportional relationship between the presence of soluble solids and dry matter.
The root system is responsible for the connection between the plant and the nutrient solution, so a well-developed root system should reflect greater plant development, as mentioned by Behling et al. (2018) and Segovia et al. (1997). The ideal EC for lettuce root development ranges from 2.5 to 2.6 mS·cm-1 (Table 4), a close value to that one found for aerial part variables. Magalhães et al. (2010), when working with lettuce, also indicated the ideal EC ranges for greater root development as the same as for greater aerial part development.
Ideal electrical conductivity (EC) for root variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivated during 45 days in winter and 31 days in summer in the non-circulating hydroponic system Kratky method.
The effect of the nutrient solution concentration (EC) on the production of aerial part fresh matter may be modeled by a quadratic polynomial (R2 = 0.85, significant at 1%). The maximum production curve resulted in 83.04 g from 2.58-mS·cm-1 EC. For 0.6-mS·cm-1 EC, fresh matter production was 79% lower, while for the highest EC, 3.5 mS·cm-1, production was 17% lower, and for the EC of 1.5 mS·cm-1, recommended by the fertilizer manufacturer, production was 16% lower than the ideal. This result is also evident in the production of root fresh matter (R2 = 0.81, significant at 1%). The maximum production curve resulted in 26.18 g of root fresh matter from 2.51-mS·cm-1 EC. For 0.6-mS·cm-1 EC, production was 77% lower, EC of 3.5 mS·cm-1 caused a 21% drop, and EC of 1.5 mS·cm-1 caused a 32% decrease in the yield of root fresh matter.
Summer cultivation
For the summer crop, upper diameter and height of the aerial part were greater for those plants grown under 2.5- and 3.5-mS·cm-1 EC (Fig. 4), as it happened for the winter crop. However, leaf area, fresh matter, and dry matter of the aerial part were higher from the 3.5-mS·cm-1 EC (Fig. 4). The ratio between dry matter and fresh matter indicated that, when compared with the winter period, the water content in plant composition was greater in summer, resulting in 6.4, 5.3, 4.7, and 4.8% for EC of 0.6, 1.5, 2.5, and 3.5 mS·cm-1, respectively.
Polynomial regression of the aerial part variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivated during 31 days in summer in the non-circulating hydroponic system Kratky method: (a) upper diameter, (b) height, (c) leaf area, (d) fresh matter, and (e) dry matter of the aerial part. **p < 0.01.
Contrary to what happened in the winter period, root variables in summer showed better results when plants were grown under 1.5- and 3.5-mS·cm-1 EC (Fig. 5).
Polynomial regression of root variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) cultivated during 31 days in summer in the non-circulating hydroponic system Kratky method: (a) volume, (b) fresh matter, and (c) dry matter of roots. **p < 0.01.
The polynomial regression analysis indicated that EC values from 3.0 to 3.3 mS·cm-1 were ideal for achieving maximum values of leaf area and fresh matter of the aerial part (Table 3). As for root development, the ideal EC range in summer fell between 2.7 and 2.9 mS·cm-1, slightly higher than the value found for the winter period (Table 4).
This increase in the ideal EC may have been caused by climatic variation during both winter and summer periods, such as higher relative humidity. According to Ahmed et al. (2020) and Tibbitts and Bottenberg (1976), such conditions reduce plant transpiration and, at the same time, decrease stomatal resistance, which results in greater stomata opening. Consequently, there is increase in carbon dioxide absorption and greater water loss. In addition, lettuce may prioritize vegetative growth, consuming more nutrients from nutrient solutions and demanding solutions richer in salts. This shows that increasing salt concentration in the nutrient solution during summer is beneficial for growing lettuce when using the Kratky method.
The effect of the nutrient solution concentration (EC) on the production of fresh matter of the aerial part may also be modeled by a quadratic polynomial regression (R2 = 0.82, significant at 1%), as in the winter period. The maximum point of the production curve resulted in 118.32 g under EC of 3.01 mS·cm-1. For 0.6-mS·cm-1 EC, fresh matter production was 77% lower; for the highest EC of 3.5 mS·cm-1, production was 3% lower; and for the EC of 1.5 mS·cm-1, recommended by the salt manufacturer, production was 20% lower than ideal. This result is also evident in the production of root fresh matter (R2 = 0.51, significant at 1%). The maximum production curve resulted in 19.33 g of root fresh matter with an EC of 2.81 mS·cm-1. For 0.6-mS·cm-1 EC, production was 52% lower, EC of 3.5 mS·cm-1 caused 5% drop, and EC of 1.5 mS·cm-1 18% drop in root fresh matter.
CONCLUSION
The agronomic variables of lettuce (Lactuca sativa ‘Crespa Itapuã 401 Super’) indicate that it may be cultivated during both winter and summer periods in a non-circulating hydroponic system Kratky method.
The ideal EC should vary according to the growing season. For winter, the ideal EC range is between 2.3 and 2.6 mS·cm-1, and, for summer, 2.8 and 3.3 mS·cm-1 was the ideal range to produce lettuce ‘Crespa Itapuã 401 Super’.
ACKNOWLEDGMENTS
To National Treasury of Brazil/Universidade Federal do Paraná (TN/UFPR) for a Technological Initiation Scholarship to the first author.
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How to cite:
Silva, W. A. V., Faria, G., Guedes Filho, O. and Mazzini-Guedes, R. B. (2025). Hydroponic lettuce cultivated under different electrical conductivities in the Kratky method. Bragantia, 84, e20250076. https://doi.org/10.1590/1678-4499.20250076
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FUNDING
Not applicable.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author.
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Edited by
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Section Editor:
Gabriel Constantino Blain https://orcid.org/0000-0001-8832-7734
