Foliar application of lithium and zinc combined with humic substances in curly lettuce

Aplicação foliar de lítio e zinco combinados com substâncias húmicas em alface crespa

ABSTRACT

Lettuce is a staple vegetable in the Brazilian diet. This crop is an excellent source of nutrients, vitamins, and minerals. Due to its short growth cycle, lettuce requires high levels of nutrients. Therefore, this study aimed to evaluate lettuce production as affected by foliar application of lithium and zinc in combination with humic substances. Two simultaneous experiments were conducted during two seasons (spring/summer and autumn/winter) with the cultivar SVR 2005^Ⓡ, and two experiments were conducted with the cultivar Rubinela^Ⓡ. The experimental design was a randomized block design with 11 treatments plus one control (no application), each replicated four times. For each cultivar, two sources of micronutrients (Zn and Li) and two sources of humic substances (HS1 and HS2), plus a control, were evaluated. Agronomic assessed traits included stem length, head diameter, fresh leaf mass, dry leaf mass, number of marketable leaves, head volume, and yield. No significant interaction was observed between seasons and treatments; however, significant effects were found for treatments within each cultivar. The control treatment had the lowest values for all evaluated traits, while the highest means were observed in treatments combining a micronutrient source with humic substances.

Keywords:
Lactuca sativa; growth promoter; foliar application; organic matter

RESUMO

A alface é uma das principais hortaliças consumida no Brasil, é bastante rica em nutrientes, vitaminas e sais minerais. Bastante exigente em relação a sua nutrição, devido ao seu ciclo bastante curto. Desse modo, o presente estudo teve como objetivo avaliar a produção de alface em função da aplicação foliar de doses de lítio e zinco associado ao uso de substâncias húmicas. Foram conduzidos dois experimentos simultâneos durante duas épocas (primavera/verão e outono/inverno), para a cultivar SVR 2005Ⓡ e dois experimentos para a RubinelaⓇ. O delineamento experimental adotado foi em blocos ao acaso, em que os 11 tratamentos + 1 testemunha (sem aplicação) foram repetidos quatro vezes. Foram avaliadas duas fontes de micronutriente (Zn e Li) e duas fontes de substâncias húmicas (SH1 e SH2) + testemunha para cada cultivar avaliada. As características agronômicas avaliadas foram comprimento do caule, diâmetro da cabeça, massa fresca das folhas, massa seca das folhas, número de folhas comerciais, volume de cabeça e produtividade. Não houve efeito significativo para a interação entre as épocas avaliadas e os tratamentos avaliados, no entanto houve efeito significativo para os tratamentos aplicados sobre cada uma das cultivares. O tratamento testemunha obteve os menores resultados para todas as características avaliadas, e as melhores médias foram observadas com os tratamentos nos quais houve a associação de alguma fonte de micronutriente com substâncias húmicas.

Palavras-chave:
Lactuca sativa; promotor de crescimento; aplicação foliar; matéria orgânica

Lettuce (Lactuca sativa L.) is a vegetable widely consumed in Brazil and is an important source of nutrients in the Brazilian diet. It is rich in minerals and vitamins, as well as other phytochemicals such as carotenoids and anthocyanins (Silva et al., 2018a). According to Clemente (2015), approximately 64.4% of the lettuce produced in Brazil comes from family farms, with production areas located close to distribution centers due to the crop’s highly perishable nature (Souza et al., 2021). The crop has a short growth cycle, typically ranging from 30 to 45 days depending on the cultivar and environmental conditions (Silva et al., 2020).

Optimal growth of lettuce requires favorable edaphoclimatic conditions and an adequate supply of water and nutrients. Lettuce grows best at temperatures ranging from 15 to 25°C in medium-textured soils with good water-holding capacity (Demartelaere et al., 2020). The main nutritional requirements are for macronutrients such as nitrogen, phosphorus, potassium, and calcium, all directly associated with increased yield and improved leaf biomass quality (Silva et al., 2018c; Santos et al., 2021; Martins et al., 2022).

A greater understanding of micronutrient management and absorption mechanisms is needed to optimize lettuce production. Although required in smaller amounts than macronutrients, micronutrients are also essential for proper crop development (Reetz, 2017).

Among micronutrients, zinc (Zn) is notable for its role in enzyme activation (including dehydrogenases, synthetases, carboxylases, and isomerases) and plant nitrogen metabolism (Teixeira Filho, 2011). Zinc is also necessary for the synthesis of tryptophan, a precursor in the biosynthesis of indole-3-acetic acid (IAA) (Reis & Macagnan, 2021), and for maintaining plasma membrane integrity. Zinc deficiency causes symptoms such as small and distorted leaves, shortened shoots, and leaf clustering during growth (Diniz, 2017).

Lithium (Li), though less studied in lettuce nutrition, is currently considered a trace element rather than a micronutrient (Reis & Macagnan, 2021). Its essentiality for plant growth has not yet been established, but low doses can stimulate growth (Schweigart, 1962; Nascimento, 2014; Ribeiro et al., 2019).

Both elements (Zn and Li) are usually absorbed from the soil solution via mass flow to the roots in the presence of water. However, they exhibit moderate to low mobility in the plant (Li et al., 2017; Gonçalves et al., 2018), which makes it interesting to investigate the use of biostimulants to facilitate foliar absorption and assimilation.

Humic substances are the final products of microbial decomposition and chemical degradation of soil organic matter (Schiavon et al., 2010). These substances mainly comprise humic acids, fulvic acids, and humins, classified according to their solubility in alkaline and acidic media (Benites et al., 2003). Previous studies have demonstrated that humic substances can stimulate nutrient uptake and improve nutrient use efficiency in various vegetables, positively influencing agronomic traits and increasing resistance to environmental stresses (Rodda et al., 2006; Rosa et al., 2009; Lima et al., 2011; Calvo et al., 2014; Silva et al., 2018b).

In this context, the objective of this study was to assess lettuce yield as affected by foliar applications of lithium and zinc in combination with humic substances.

MATERIAL AND METHODS

The experiments were conducted during the spring/summer and autumn/winter seasons of 2022, in the Vegetable Production Sector at the Gurupi Campus of the Federal University of Tocantins (UFT) (11°43'45"S, 49°04'07"W, 287 m altitude). The regional climate is classified as Aw (tropical with a wet summer and dry winter) according to the Köppen & Geiger (1928) classification. The climatic conditions during the experimental period are presented in Figure 1.

Two simultaneous experiments were conducted in two seasons with the cultivar SVR 2005^Ⓡ (Seminis; curly leaf type; cycle of 40 to 55 days; intense light green color) and in two seasons with Rubinela^Ⓡ (Feltrin; curly leaf type; cycle of 45 to 50 days; bright red color). The experimental design was a randomized block design, with 11 treatments plus one control (without application), each replicated four times (Table 1). For each cultivar, treatments included two micronutrient sources (Zn and Li), two humic substance sources (HS1 and HS2), and a control.

Micronutrient applications were carried out at the stage of full vegetative development (25 days after transplanting) using a CO₂-pressurized backpack sprayer at a rate of 200 L/ha. The foliar fertilization doses for Zn and Li were determined based on previous studies, using zinc sulfate at the dose of 200 g/ha (Sousa et al., 2022) and lithium sulfate at 50 g/ha (Faria et al., 2019). The humic substances were provided by the research group Center for Studies on Organic Residues (NERO - Núcleo de Estudos de Resíduos Orgânicos), consisting of a liquid source (SH1), commercially known as Stimugreen (a compound of amino acids, plant extracts, humic substances, and hormonal precursors) applied at the recommended dose of 100 mL/L, and a solid source (SH2), the leonardite, a humified plant-derived material rich in organic matter and humic acids, applied at 120 g/L.

Seedlings were produced in Agrofértil^Ⓡ polystyrene trays containing 200 cells, each with internal dimensions of 2.7 x 2.7 cm. The trays were filled with Biomix^Ⓡ commercial substrate, composed of ground and composted pine and eucalyptus bark and coconut fiber or coir dust. Seedlings remained in the nursery for 30 days until transplanting, at which point they had developed four to five true leaves.

Figure 1
Precipitation (mm), temperature (°C), and air relativehumidity (%) during the experimental period. Gurupi, INMET, 2023.

Table 1
Identification and description of the treatments to which the cultivars SVR 2005 and Rubinela were subjected. Gurupi UFT, 2022.

Nº	Identification	Description
1	Li	Lithium sulfate (50 g/ha)
2	Li+Zn	Lithium sulfate (50 g/ha) + Zinc sulfate (200 g/ha)
3	Control	No application.
4	Sh1	Stimugreen (100 mL/L)
5	Sh1+Li	Stimugreen (100 mL/L) + Lithium sulfate (50 g/ha)
6	Sh1+Li+Zn	Stimugreen (100 mL/L) + Lithium sulfate (50 g/ha) + Zinc sulfate (200 g/ha)
7	Sh1+Zn	Stimugreen (100 mL/L) + Zinc sulfate (200 g/ha)
8	Sh2	Leonardite (120 g/L)
9	Sh2+Li	Leonardite (120 g/L) + Lithium sulfate (50 g/ha)
10	Sh2+Li+Zn	Leonardite (120 g/L) + Lithium sulfate (50 g/ha) + Zinc sulfate (200 g/ha)
11	Sh2+Zn	Leonardite (120 g/L) + Zinc sulfate (200 g/ha)
12	Zn	Zinc sulfate (200 g/ha)

The experiments were conducted in raised beds, with plots measuring 1.0 m in length and 1.0 m in width. Each plot consisted of 20 plants, with the six central plants considered as the usable area for evaluation. Spacing was 20 cm between plants and 20 cm between rows.

The chemical characteristics of the soil for each experimental season are shown in Tables 2 and 3. Based on the results of the soil chemical analysis, pH correction was necessary; therefore, approximately 2 t/ha of dolomitic limestone was applied 30 days before transplanting to adjust soil pH and increase calcium and magnesium levels. Basal fertilization was established according to the crop’s nutritional requirements (Ribeiro et al., 1999). Urea (45% N) was used as the nitrogen source, with a total of 130 kg/ha applied, 20% (26 kg/ha) as basal fertilizer and the remainder split into two topdressing applications via fertigation at 15 and 30 days after transplanting. Single superphosphate (20% P₂O₅) was used as the phosphorus source at a rate of 400 kg/ha, applied entirely as basal fertilizer three days before transplanting. Potassium chloride (60% K₂O) was used as the potassium source, with a total of 120 kg/ha, 20% (24 kg/ha) was applied as basal fertilizer three days before transplanting, and the remainder was split into two topdressing applications via fertigation at 15 and 30 days after transplanting.

Table 2
Chemical attributes and particle size distribution of the soil used in the experiment during the spring/summer, Gurupi, UFT, 2022.

pH	Pmeh	K	K	Ca	Mg	Al	H+Al	OM	OC	V	Clay	Silt	Total sand
CaCl2	mg/dm3		cmolc/dm3					dag/kg		%
5.4	9.7	42	0.11	1.6	0.4	0.0	1.5	1.6	0.9	58	25	5	70

• OM = organic matter.

Table 3
Chemical attributes and particle size distribution of the soil used in the experiment during the autumn/winter, Gurupi, UFT, 2022.

pH	Pmeh	K	K	Ca	Mg	Al	H+Al	OM	OC	Base saturation	Clay	Silt	Total sand
CaCl2	mg/dm3		cmolc/dm3					dag/kg		%
5.2	40.1	39	0.10	2.1	0.7	0.0	2.50	1.5	0.9	54	22.5	5	72.5

• Source: Sellar (2022).

Irrigation was performed daily using a drip system, with a watering schedule of 3 hours per day (one and a half hours in the morning and one and a half hours in the afternoon). Phytosanitary treatments were not necessary. Weed control was carried out manually as needed.

Harvesting was performed at the stage of maximum vegetative development. The following agronomic traits were evaluated: stem length (SL, in cm), measured using a millimeter ruler; head diameter (HD, in cm), measured with a ruler in centimeters by measuring the plant transversely; fresh leaf mass (FLM, in g/plant), obtained by weighing all the leaves from the usable area plants on a digital scale; dry leaf mass (DLM, in g/plant), obtained by drying the leaves in a forced-air oven at 70°C until reaching constant weight and then weighing them on a digital scale; number of marketable leaves (NL), manually counted from the base to the apex, disregarding leaves smaller than three centimeters and senescent leaves; head volume (HV, in cm³), determined using a graduated container with a known volume capacity of 15 liters; 12 liters of water were added to the container, and the volume of water displaced by each plant was used to estimate the lettuce head volume; yield (YIELD, in t/ha), calculated by weighing the usable area plants and converting the value to t/ha.

Analysis of variance was performed for each response variable of each cultivar, using the following statistical model:

${\mathrm{y}}_{\mathrm{i}\mathrm{j}\mathrm{k}}=\mathrm{\mu }+{\mathrm{\alpha }}_{\mathrm{i}}+{\mathrm{\beta }}_{\mathrm{j}}+{\left(\mathrm{\alpha }\mathrm{\beta }\right)}_{\mathrm{i}\mathrm{j}}+{\mathrm{\epsilon }}_{\mathrm{i}\mathrm{j}\mathrm{k}},$

Where Y_ijk is the observed value of the trait, µ is the overall mean effect, α_i is the effect of the i^th season (i=1,2), β_j is the effect of the j^th treatment (j=1,2,3,…,12), (αβ)_ij is the interaction effect between the i^th season and the j^th treatment, and ɛ_ijk is the random error effect. When the interaction was not significant (p>0.05), the main effects were analyzed using the Scott-Knott test (1974). All statistical analyses were performed in R (version 4.4.3, R Core Team, 2024).

RESULTS AND DISCUSSION

The interaction effect between season and treatments was not significant for any of the traits in either cultivar. Regarding the main effects, the evaluated treatments differed (P<0.05) for the following traits: stem length (SL), head volume (HV), and yield (YLD) in the cultivar SVR 2005; and stem length (SL), head diameter (HD), number of leaves (NL), and yield (YLD) in the cultivar Rubinela. These results indicate that morphometric parameters are influenced by the treatments to which both cultivars were subjected. In lettuce cultivation, stem length is closely linked to sensitivity to premature bolting and the local climatic conditions. When lettuce plants are exposed to high temperatures, they tend to shorten their cycle and enter the reproductive phase earlier, exhibiting characteristics that are unfavorable for marketing, such as smaller plant size, elongated stems, bitter taste, and latex in the leaves.

In iceberg lettuce cultivars, shorter stem length is desirable, especially for processing purposes, as it helps to reduce losses. Conversely, excessively long stems result in less compact lettuce heads, which negatively affects both processing and the quality of the final product (Resende et al., 2005).

For the Rubinela cultivar, the treatments produced results similar to those observed for ‘SVR 2005’, with the control treatment (no application) presenting the lowest mean stem length (5.67 cm). The SH1 treatment resulted in a mean stem length of 6.06 cm (Figure 2A), which was higher than the other treatments.

Figure 2
Means from two planting seasons for the agronomic traits of the cultivar Rubinela, as affected by foliar application of Li and Zn sources combined with humic substances. Gurupi, UFT, 2023.

‘SVR 2005’ under the control treatment (no application) showed the lowest mean stem length, at 4.69 cm. In contrast, the treatment with SH2 application resulted in the highest mean for this trait, with 6.68 cm (Figure 3).

For curly lettuce, shorter stem lengths are desirable (Santi et al., 2013). According to Yuri et al. (2004), stems up to 6.0 cm are considered commercially suitable, lengths up to 9.0 cm are acceptable, and lengths above this threshold are unacceptable. Thus, the values obtained in the present study, regardless of cultivar or treatment applied, fall within the range considered acceptable for lettuce commercialization.

Evaluating the effect of foliar zinc fertilization, Mariano et al. (2021) reported mean stem lengths ranging from 3.74 to 5.86 cm, which are lower than or similar to those obtained in the present study, regardless of the fertilizer dose applied. Given that their experiment was conducted in the southern region of Minas Gerais, where temperatures are milder compared to the southern region of Tocantins, the influence of temperature on this trait becomes evident.

Head diameter is a particularly important trait for the end consumer, who tends to prefer larger and more robust lettuce heads (Trentini & Hojo, 2019). Plant spacing is closely related to the development of lettuce heads, as smaller spacing increases competition among plants for space, nutrients, water, light, and root development, consequently affecting overall plant growth (Campos et al., 2022).

The adoption of a 20 cm × 20 cm spacing between and within rows promotes good lettuce development, enabling the plants to efficiently absorb nutrients from both soil and foliar sources, as well as light and water (Koefender et al., 2016; Vasconcelos et al., 2017).

Head diameter was not significantly affected by the treatments applied. However, the control treatment (no application) for ‘SVR 2005’exhibited the smallest head diameter (23.13 cm) compared to the other treatments (Figure 3B). The largest head diameter was observed with the application of SH1+Zn (27.47 cm), representing an increase of 18.72%.

For ‘Rubinela’, the control treatment (no application) also showed the lowest mean head diameter (21.23 cm), differing from the other treatments, which had means ranging from 24.29 to 26.63 cm for the Zn and SH2+Li treatments, respectively (Figure 2B).

The application of humic substances combined with foliar application of zinc or lithium for the SVR 2005 and Rubinela cultivars, respectively, resulted in larger head diameters due to the biostimulant effect of humic substances, which can modify plant metabolism and enhance nutrient uptake (Nardi et al., 2009). Consequently, this allowed for better utilization of the micronutrients zinc and lithium in lettuce physiological functions, since these elements have low mobility within the plant (Angelini et al., 2020).

Figure 3
Means from two planting seasons for the agronomic traits of the cultivar SVR 2005, as affected by foliar application of Li and Zn sources combined with humic substances. Gurupi, UFT, 2023.

The positive response to foliar zinc application combined with humic substances in the ‘SVR 2005’ may be attributed to its role in tryptophan production, which is a precursor of indole-3-acetic acid, a growth-promoting phytohormone (Moreira et al., 2010; Silveira et al., 2015). This, in turn, indirectly affects root production and growth, increasing water and nutrient uptake from the soil, especially in crops with a more superficial root system, such as lettuce (Cassán et al., 2014).

Evaluating the foliar application of zinc sulfate in curly lettuce cultivars, Graciano et al. (2019) reported head diameter values ranging from 27.90 to 28.43 cm, which are similar to those obtained in the present study, with humic substances serving as a supplement. Zinc application is important for the synthesis and repair of nucleic acids and proteins, and also influences biochemical and physiological processes related to growth, cell division and differentiation, development, and aging (Fukada et al., 2011).

For fresh leaf mass, the SVR 2005 cultivar under the control treatment (no application) had the lowest mean value (115.44 g). However, there were no significant differences between the control and the other treatments involving only humic substances and/or a source of Li or Zn. The highest mean values were observed in treatments combining humic substances with Li and Zn, with the SH1+Li+Zn treatment showing the greatest fresh leaf mass (201.90 g), an increase of 86 g compared to the control (Figure 3C).

The Rubinela cultivar showed a similar pattern to that of ‘SVR 2005’, with the lowest leaf fresh mass observed in the control treatment (no application), at 118.85 g (Figure 2C). The highest values were obtained with the combination of humic substances, particularly the SH2+Zn treatment, which resulted in a leaf fresh mass of 163.30 g, representing a 37% increase.

The leaves of lettuce plants are the primary commercial product; therefore, a succulent and visually appealing appearance is desirable to consumers. This leaf succulence results from the water content within the leaf tissues, as well as the amount of organic compounds.

Lettuce plants treated with zinc and lithium, in combination with humic substances, exhibited higher mean values. Mariano et al. (2021) observed a linear increase in plant weight with increasing doses of zinc compared to the treatment without zinc application, indicating that foliar application of this micronutrient leads to greater increases in plant weight.

According to studies conducted by Canellas et al. (2015), the use of humic substances can stimulate the activity of the plasma membrane ATPase enzyme, a typical effect of auxins. This, in turn, promotes the uptake of nutrients supplied through the root system and enhances both the development and quality of the final product.

Dry leaf mass is closely related to the plant's ability to accumulate organic compounds that remain after water is removed from the tissue. Most vegetables are composed of approximately 95% water (Morgan et al., 2011). After drying, only organic and mineral substances remain, with the organic fraction consisting of 92% carbon, hydrogen, and oxygen. Dry matter is considered more stable than fresh matter, as it is not affected by factors such as time of day, soil water availability, or temperature.

Lettuce plants are well known for the high water content of their tissues. For the SVR 2005 cultivar, the control treatment (no application) had the lowest mean dry mass (11.93 g). However, this value did not differ statistically from the SH1+Li+Zn treatment, which had the highest mean dry mass among the treatments, at 16.14 g (Figure 3D).

The Rubinela cultivar showed similar results, with a mean dry mass of 9.55 g when no Zn, Li, or humic substance sources were applied. A notably higher value was observed for the SH2+Zn treatment, which resulted in a dry mass of 13.64 g (Figure 2D).

The SVR 2005 cultivar showed no significant differences among treatments in the number of leaves, with mean values ranging from 20.33 to 21.58 leaves per plant (Figure 3E), whether humic substances were applied alone or in combination with the micronutrients Li or Zn.

In contrast, the Rubinela cultivar showed a significant effect of the treatments. The control treatment (no application) resulted in the lowest number of leaves (16.81), differing from the other treatments. The treatments combining humic substances SH1 and SH2 with micronutrients produced the highest mean values for this trait, with an average of 21.19 leaves per plant for the SH2 treatment and 21.64 for SH2+Zn.

The use of humic substances may have created favorable conditions for nutrient absorption, which positively influenced the number of leaves. Faria et al. (2019) observed a quadratic increase in the number of leaves when applying lithium sulfate to the SVR 2005 cultivar, with increases up to 29 and 33 g/ha of lithium, followed by a decrease at higher doses. This occurred because lithium becomes toxic to plants at higher concentrations, consequently affecting agronomic traits of interest.

Although there is limited information regarding the functionality and essentiality of lithium in plants, it is known that, in small amounts, lithium exhibits hormonal effects (Kalinowska et al., 2013). Lithium also has an affinity for enzymes activated by Ca and/or Mg, and can substitute for both during metabolic processes in the plant, which indirectly affects plant development (Shahzad et al., 2016). In combination with humic substances, which are considered biostimulants, lithium contributed to positive results for the number of leaves.

The size of the lettuce head is one of the main features that attract consumers; however, the volume it occupies during transport is also crucial. Lettuce is typically transported in plastic crates to prevent physical damage, which can reduce its commercial value. Therefore, head volume is a key characteristic for ensuring the delivery of high-quality lettuce to the market (Queiroz et al., 2017).

For the trait head volume, a significant effect was observed. The lowest mean values were recorded for the control treatment (no application) in the SVR 2005 cultivar (138.37 cm³), which differed from the other treatments (Figure 3F). However, for the Rubinela cultivar, there was no significant effect for this trait, with similar results among treatments (Figure 2F).

The SVR 2005 cultivar showed the highest mean head volume with the SH1 treatment (246.46 cm³), while the highest value for ‘Rubinela’ (258.24 cm³) was observed with the SH2+Li treatment. Regardless of the lettuce cultivar, it was evident that the addition of 50 g/ha of the micronutrient Li and/or humic substances (100 mL/L SH1 and/or 120 g/L SH2) resulted in a substantial increase in head volume, as this trait is closely related to the number and size of leaves that form the lettuce head.

Foliar fertilization with zinc can enhance nitrogen metabolism. Nitrogen is directly related to the plant’s capacity to produce leaves for light interception, and consequently to achieve greater head volume (Benites et al., 2003; Barrameda-Medina et al., 2016).

Marketable yield reflects the weight of each lettuce plant that is produced and is a factor of crucial importance for growers of this crop. It is well known that greater plant weight results in higher productivity.

The SVR 2005 cultivar showed a statistically significant difference in yield between the control (no application) and the other treatments, with the control exhibiting the lowest yield (20.33 t/ha). A similar pattern was observed for the Rubinela cultivar; although the effect was not statistically significant; the control treatment still had the lowest mean yield (20.83 t/ha) compared to the other treatments.

The simultaneous or isolated use of humic substances with micronutrients led to increased yield in lettuce, with the highest means observed for the SVR 2005 cultivar with SH1+Li+Zn (35.25 t/ha) and for the Rubinela cultivar with SH2+Zn (32.89 t/ha). Similar results were reported by Lüdtke et al. (2021), who observed a satisfactory increase in yield by applying humic substances rich in humic and fulvic acids to iceberg lettuce grown in a greenhouse. Several other vegetables, such as cabbage (Benites et al., 2003), onion (Almendros et al., 2015), arugula (Reyes, 2017), and broccoli (White et al., 2018), among others, have also shown increased yields following the application of a zinc source during their development.

Foliar application of Zn in combination with humic substances may be a good alternative for growers, as it enhances productive traits in the crop. This is because zinc plays a key role in the synthesis and repair of DNA, RNA, and proteins, as well as in biochemical and physiological processes related to growth, cell division and differentiation, development, and aging (Fukada et al., 2011; Sousa et al., 2018).

Although the results obtained were favorable for the application of micronutrients, which are highly important for plant development, even in smaller quantities compared to macronutrients, their association with humic substances can further enhance nutrient uptake. Humic substances possess biostimulant properties and interact with the plant, improving the absorption of nutrients primarily through chelation. This process facilitates the uptake of these nutrients by the plant.

The response to the application of Li, Zn, and humic substances depends not only on the presence of these nutrients but also on their penetration and absorption through the leaf cuticle, as well as their subsequent transport via the plant’s vascular system.

The application of Zn and Li sulfates in combination with humic substances was able to elicit favorable responses in agronomic traits of interest in lettuce, such as the number of leaves, head diameter, head volume, and yield.

Foliar application of micronutrients and humic substances is an alternative to add value to product quality and increase yield.

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• Data availability:
  Data will be made available upon request to the corresponding author.

Edited by

• Responsible editor:
  Anderson F Wamser

Data availability

Data will be made available upon request to the corresponding author.

Publication Dates

• Publication in this collection
  24 Oct 2025
• Date of issue
  2025

History

• Received
  31 Jan 2025
• Accepted
  10 June 2025