Agronomía Colombiana

Location

The research was conducted at the Tunguavita Experimental Farm of the Pedagogical and Technological University of Colombia (UPTC) located in the municipality of Paipa (Boyacá, Colombia) in the Salitre district. The geographical coordinates are 5°45' N and 73°45' W at an altitude of 2470 m a.s.l. The site has an average annual temperature of 14.1°C, a bimodal rainfall pattern with an average annual precipitation of 966 mm, and a relative air humidity of 75%.

Plant material

The plant material used was seed-tubers of potato variety CIP 39, with the following morphological characteristics: predominant yellow skin color, white flesh, oblong tuber shape, white flowers, early to semi-early vegetative period (120 to 130 d), and industrial uses (French fries) (Instituto Nacional de Innovación Agraria, 2012).

The fertilization plan was adjusted according to the results of soil analysis (Tab. 1) and following the recommendations for potato cultivation by Guerrero-Riascos (1995). The nutrient requirements in terms of nitrogen, phosphorus, and potassium were adjusted using simple fertilizer sources such as urea, diammonium phosphate, and KCl. In addition, micronutrients were applied using B-Zn (8% boron + 4% zinc).

For the control of pest insects and diseases, protective and systemic action products were used. The most limiting disease was late blight caused by the fungus Phytophthora infestans, which was managed using protective and systemic fungicides such as chlorothalonil, dimethomorph, mancozeb, cymoxanil, and metalaxyl. For the control of the most limiting pest insect, the Guatemalan moth (Tecia solanivora), applications of thiamethoxam, cyantraniliprole, and imidacloprid were carried out. Irrigation was adjusted based on the percentage of allowed depletion according to the phenological state, following the methodology employed by Guerrero-Guio et al. (2019), and was supplied through a sprinkler system.

TABLE 1.: Physical and chemical properties of soil at the experimental site.

A completely randomized design (CRD) was employed, with four treatments corresponding to registered commercial biostimulants in Colombia, clearly labeled with the type of active compound. The treatments were as follows: T1: Hormonal (Hormonal), corresponding to the commercial product Stimulate® (Stoller Colombia S.A.), which contains a mixture of kinetin, indole-3-butyric acid, and gibberellic acid (GA3); T2: Seaweed extract (SWE), corresponding to the product Radifarm™ (Valagro) composed of proteins, amino acids, betaines, alginates, and polysaccharides extracted from Ascophyllum nodosum algae; T3: Carboxylic acids (Carboxylic A), corresponding to the product Radigrow® (Innovak Global, S.A.) composed of Carboxy acids® expressed as total oxidizable organic carbon; and T4: No application (water). Each treatment had three replicates, totaling 12 experimental units which corresponded to plots of 72 m² with a planting distance of 0.3 m between plants and 1 m between rows, with 240 plants per plot.

The treatments were applied by soil spraying at two times with a solution volume of 1.5 L per plot. The first application was performed at phenological stage 08, which corresponds to stems growing towards the soil surface (Hack et al., 1993). The second application was performed 15 d after the first application. The dosage for all treatments was 10 ml L⁻¹ of commercial product, following the technical recommendation for ground spraying. Water was used as the diluent for preparing the solutions.

Growth parameters

Growth parameters were evaluated 94 d after planting, when the plants were at phenological stage 69, corresponding to full bloom (Hack et al., 1993). At this stage, plants achieve their maximum fresh and dry mass gain as well as the largest leaf area.

To determine the leaf area index (LAI), leaf area was measured using an electronic leaf area meter (Area Meter CI-202, CID Bio-Science, Inc., USA). For this, two plants per experimental unit were selected, and all leaves from each plant were removed for measurement. Leaf area was expressed in square meters (m²), and the number of plants per 1 m² was also determined. The obtained values were used to calculate the LAI using Equation 1 proposed by Reis et al. (2013):

where LAI is in m² m⁻², LA is the average leaf area of two plants (m²), NP is the number of plants per m², and TA is the total area considered (1 m²).

Fresh weight gain was evaluated on four plants from the center of each experimental unit. These plants were placed in paper bags with a capacity of 10 kg, properly labeled according to treatment. The plants were then separated into root and above-ground parts (stems + leaves) and weighed using an Acculab VIC 612 electronic balance with a precision of 0.01 g. Subsequently, the samples were dried in a Memmert oven at 65°C until reaching a constant weight (approximately 96 h) to determine dry weight gain. The analyses were conducted at the Plant Physiology Laboratory of the Pedagogical and Technological University of Colombia.

Productivity parameters

For the productivity evaluation, all tubers were collected from each experimental unit and then placed in white fiber bags with a capacity of 50 kg. They were commercially classified into two categories according to the Colombian technical standard NTC 341 (ICONTEC, 1996). The category of first quality (Quality 1) corresponds to tubers with a diameter of 65 to 90 mm, and the category of second quality (Quality 2) corresponds to tubers with a diameter of 45 to 64 mm. The fresh weight data of tubers obtained from each experimental unit was extrapolated to obtain productivity expressed as yield in tons per ha (t ha⁻¹).

Statistical analysis

The obtained data were tested for normal distribution of residuals and homogeneity of variances using the Shapiro-Wilk test (P≥0.05) and Bartlett test (P≥0.05), respectively. Once the assumptions were confirmed, hypotheses were evaluated for each of the variables assessed through an analysis of variance (ANOVA). Finally, a multiple mean comparison test was conducted using the Tukey test (P≤0.05). The analyses were performed using the 'agricolae' package of the statistical software R Core Team (2022).

Growth variables

The variables of fresh and dry weight were significantly affected (P≤0.05) by the application of biostimulants (Fig. 1). In the Hormonal treatment, the fresh weight of the roots, the aerial part, and the total were 124.8±3.4 g, 675.7±17.6 g, and 800.5±18.7 g, respectively, with gains of 65%, 14%, and 16%, respectively, compared to the control treatment, which had values of 75.4±2.3 g, 595.2±12.6 g, and 690.1±8.3 g, respectively (Fig. 1A).

In the carboxylic acids treatment (Carboxylic A), the dry weight of the roots, the aerial part, and the total plant weight showed values of 35.8±0.9 g, 116.2±2.9 g, and 152.0±3.8 g, respectively, with gains of 2%, 41%, and 29%, respectively, compared to the control treatment, which had values of 35.3±1.6 g, 82.3±1.0 g, and 117.6±1.1 g, respectively (Fig. 1B).

Fresh weight is considered a good estimator of plant volume, as water is the main component of all organs and tissues, while dry weight is a good estimator of the total carbon content of the plant, allowing for the analysis of plant physiology (Di Benedetto & Tognetti, 2016). Increases in fresh weight may be related to changes occurring in the organization and cellular metabolism of plants grown under the influence of biologically active substances or products, as these substances regulate nutrient absorption and translocation and alter the phytohormone levels (Falcón Rodríguez et al., 2015).

FIGURE 1.: Fresh weight (A) and dry weight (B) of potato plants variety CIP 39 under the application of different types of biostimulants. Different letters indicate significant differences between treatments according to the Tukey mean test (P≤0.05). Vertical bars represent standard error (n=3). SWE -- seaweed extract, Carboxylic A -- carboxylic acids.

The results with the hormonal treatment are attributed to the ratio between kinetin, indole-3-butyric acid, and GA3 in the applied product, which preferentially stimulates growth of roots, leaves, and stems. This could be related to its action on hormonal balance (Asari et al., 2017). One of the most widespread uses of commercial biostimulants is as growth promoters. These morphological responses are frequently attributed to the activity of endogenous auxins and cytokinins in treated plants (Sharma et al., 2014; Wally et al., 2013) or exogenous ones present in the extracts (Vinoth et al., 2019), which often contain fractions of polysaccharides that could induce root growth similarly to synthetic auxins (Hernández-Herrera et al., 2016).

Biostimulants are primarily derived from a variety of organic materials, including humic, fulvic, and carboxylic acids, among others (Drobek et al., 2019; Zuzunaga-Rosas et al., 2023). The treatment with the application of carboxylic acids favored both root and shoot growth (Fig. 1). The weight gain obtained with the application of organic biostimulants may be attributed to their promotion of the absorption of macro and micronutrients, boosting metabolic activity. This makes root absorption more efficient and regulates the nutrient absorption activity of the rhizosphere by stimulating activity of H⁺ ATPases in the plasma membrane. These enzymes convert the free energy released by ATP hydrolysis into an electrochemical potential across the membrane, which is used for the uptake of nitrate and other nutrients by roots (Canellas et al., 2015; Drobek et al., 2019; du Jardin, 2015).

Leaf area index (LAI)

The application of a biostimulants resulted in significant differences (P≤0.05) compared to the control treatment (Fig. 2). The treatments with seaweed extract (SWE), carboxylic acids (Carboxylic A), and hormonal treatment resulted in mean values of 2.4±0.04 m² m⁻², 2.3±0.12 m² m⁻², and 2.2±0.06 m² m⁻², respectively. In contrast, the control treatment (water) presented the lowest LAI with a mean value of 1.6±0.02 m² m⁻².

The plant is essentially a capturer of solar energy, which is stored in the form of carbohydrates. This process takes place in leaves, from which carbohydrates are then mobilized towards the tubers (storage organs). The LAI is closely related to the plant's ability to intercept solar radiation, directly associated with the processes of photosynthesis and transpiration. These processes are directly linked to biomass accumulation and productivity (Hernández-Hernández et al., 2011). LAI is a fundamental parameter for evaluating crop growth and development, as it can be used to estimate water and nutrient requirements as well as bioenergetic efficiency (Reis et al., 2013). Therefore, LAI is a useful variable for quantifying crop growth and agronomic performance (Mendoza-Pérez et al., 2017).

FIGURE 2.: Leaf area index (LAI) of potato plants variety CIP 39 under the application of biostimulants. Different letters indicate significant differences according to the Tukey test (P≤0.05). Vertical bars represent standard error (n=3). SWE -- seaweed extract, Carboxylic A -- carboxylic acids.

Dry matter production is commonly related to the plant's capacity to increase its leaf area; therefore, a larger leaf area will result in greater dry matter accumulation. However, this is not always the case, as the optimal leaf area index is the one that maximizes the dry matter accumulation rate. This is achieved when the crop intercepts nearly all available photosynthetically active radiation (PAR) and, consequently, the lower leaf layers are still capable of maintaining a positive carbon balance (Bergamaschi et al., 2010; Hunt, 2016).

Polysaccharides extracted from macroalgae cell walls and their derived oligosaccharides can enhance growth (Zou et al., 2019). This is because seaweeds affect plant metabolism and physiology, as their extracts possess growth inducers and/or trigger differential expression of genes involved in the synthesis of endogenous phytohormones and other primary metabolism pathways (Ghaderiardakani et al., 2019).

According to Santos et al. (2010), who evaluated four potato cultivars, the highest LAI values were registered during flowering, with values of 2.8 m² m⁻². The application of SWA resulted in an LAI of 2.4±0.04 m² m⁻², which generated a better balance between the plant's ability to intercept light and photosynthesis processes, leading to increased crop productivity (Figs. 3 and 4).

Productivity variables

Yield by commercial grades showed statistical differences between the treatments (P≤0.05). For Quality 1 tubers, with diameters of 65 to 90 mm, the SWE treatment exhibited a 17.6% increase with a mean value of 40.7±0.36 t ha⁻¹, compared to the control which had a value of 34.6±0.52 t ha⁻¹ (Fig. 3). For Quality 2 tubers, with a diameter of 45 to 64 mm, the treatment based on carboxylic acids showed an 80% increase with a mean value of 7.4±0.6 t ha⁻¹, compared to the control, which had a value of 4.07±0.6 t ha⁻¹ (Fig. 3).

FIGURE 3.: Yield by grades in potato plants variety CIP 39 subjected to different types of biostimulants. Identical letters among treatments indicate no significant differences according to the Tukey mean test (P≤0.05). Vertical bars represent standard error (n=3). Quality 1: tubers with a diameter of 65 to 90 mm; Quality 2: tubers with a diameter of 45 to 64 mm (ICONTEC, 1996). SWE -- seaweed extract, Carboxylic A -- carboxylic acids.

Total yield analysis had a significant increase (P≤0.05) compared to the control treatment. SWE treatment had an 18.2% rise in total yield, averaging 46.7±0.75 t ha⁻¹, compared to the control yield of 39.4±1.6 t ha⁻¹ (Fig. 4).

FIGURE 4.: Total yield in potato plants (Solanum tuberosum L.) variety CIP 39 subjected to different types of biostimulants. Identical letters among treatments indicate no significant differences according to the Tukey mean test (P≤0.05). Vertical bars represent standard error (n=3). SWE -- seaweed extract, Carboxylic A -- carboxylic acids.

The application of biostimulants at different stages of crop growth can improve yield per plant by triggering a series of physiological and biochemical events in plants that result in increased production (Martínez et al., 2017).

Extract-based biostimulants promote many physiological processes in the plants, including photosynthesis. These biostimulants contain amino acids such as alanine and glycine, which enhance photosynthesis and also play a role in the synthesis of porphyrins, structural pillars of chlorophyll and cytochromes. This enhances plant activity, increasing reserve substances that are translocated to different parts of the plants, such as storage organs (Díaz et al., 2020; Ertani et al., 2018).

Various studies confirm the positive effects of seaweed-based biostimulants on crop performance and post-harvest quality. According to Abbas et al. (2020), foliar applications significantly increased bulb and neck diameter as well as yield per hectare in four onion cultivars and improved the contents of total soluble solids, ascorbic acid, nitrogen, potassium, and phosphorus. Similarly, Yao et al. (2020) indicate that seaweed-based products significantly increased the net yield of Solanum lycopersicum L. by 6.9% compared to the control and positively affected fruit firmness and soluble sugar content.

However, the effects of biostimulants are not always consistent, as they depend on the plant species, the sensitivity thresholds to one or more bioactive molecules, as well as the different extraction procedures that ensure the purity and quality of the bioactive compounds in the products.