Introduction

Soybean (Glycine max (L.) Merr.) is currently considered the world’s main source of plant-based protein. The increase in soybean production in Brazil is associated with scientific and technological advances in the agricultural sector, which highlight changes in the cultivation of this oilseed crop1. The demand for soybean production is linked to the wide range of applications of the grain, including protein supply for human and animal consumption, various food products, biofuel, and pharmaceutical products2.

For crop establishment, seeds are the main vehicle for transferring genetic advancements to the production field3. Therefore, the use of improved seeds with high quality and desirable agronomic traits forms the foundation for advancing soybean cultivation in tropical conditions, aiming to increase the global supply of this commodity4. Seed quality is a fundamental aspect for crop establishment and yield. Germination and vigor are intrinsic characteristics of the physiological potential of seed lots5. Seed germination begins with water absorption and involves the resumption of embryonic growth, driven by the activation of a series of physiological events that ultimately result in the emergence of the plant’s constituent parts under favorable environmental conditions3,6.

Seed vigor is acquired during seed development and is influenced by environmental conditions experienced by the mother plant7. It encompasses several traits related to physiological potential, such as germination speed, seedling growth rate, ability to germinate under suboptimal or supra-optimal temperatures, and other stress tolerance characteristics5,8. In agricultural settings, seeds available for sowing come in lots with distinct characteristics. Thus, seeds sharing the same genotype may still vary in terms of vigor and germination potential9. Rapid germination is a key component of seed vigor, as it often correlates with faster field emergence8,10. However, this trait can differ among lots, affecting uniform plant establishment, especially under current climate conditions characterized by irregular and unpredictable rainfall patterns.

In this context, chemical agents and methods have been considered to improve germination and seedling uniformity. One such method is seed priming, or physiological conditioning, which involves controlled seed hydration to trigger pre-germinative metabolism without transitioning to visible germination11,12. Various priming methods have been developed using water, plant growth regulators, osmotic solutions, and other chemical agents. These approaches aim to partially pre-hydrate the seeds and activate key metabolic processes—such as enzyme activity and damage repair—before radicle protrusion occurs13. However, their effectiveness depends on species-specific responses and the physiological status of the seeds14.

Halopriming is a method involving seed exposure to inorganic salt solutions such as sodium chloride (NaCl), calcium chloride (CaCl₂), potassium chloride (KCl), and potassium nitrate (KNO₃), which may exert direct or indirect nutritional effects on seeds15. Potassium (K) aids germination by promoting rapid water uptake and facilitating physiological processes like stomatal regulation. K salts—especially KCl and KNO₃—have been studied as catalysts to enhance seed germination and emergence rates16. The use of nitrates in priming is justified by their role in breaking dormancy and enhancing seed germination, as nitrates act as oxidants in the pentose phosphate metabolic pathway17. Positive effects have also been observed in early seedling growth due to nitrogen’s dual role as an essential nutrient and signaling molecule18, influencing the plant’s hormonal state19. KNO₃ has been reported to improve seed quality, germination, and seedling growth in crops such as sunflower20, black gram (Vigna mungo)21, and soybean22.

Considering the lack of data regarding KNO₃’s effects on soybean seeds with varying physiological statuses, this study was carried out under the hypothesis that priming with potassium nitrate (KNO₃) improves the performance of soybean seeds with different initial quality levels. The objective was to evaluate the germination and vigor of soybean seeds subjected to KNO₃ priming at different concentrations.

Methods

The experiment was conducted using three seed lots of soybean cultivar BMX FIBRA 64I61 RSF IPRO, produced during the 2022/2023 growing season in Ponta Porã, MS, Brazil. During the development and execution of the study, the seeds were stored in kraft paper bags and kept in a cold and dry chamber (15 °C, 45% RH) at the Seed Technology Laboratory of the Faculty of Agricultural Sciences, Federal University of Grande Dourados.

The seed lots were classified as high quality (germination and vigor ≥ 90%), medium quality (germination and vigor ≥ 85%), and low quality (acceptable germination and vigor ≤ 80%). Seeds were placed on filter paper moistened with potassium nitrate (KNO₃, molecular weight 101.10, Vetec Química Fina LTDA) solutions at concentrations of 1.0% and 1.5% for 12 h21,23 (Fig. 1). After treatment, seed moisture content was determined using the oven method24. Subsequently, seeds were rinsed in running water and dried at 25 °C for 24 h until reaching approximately 13% moisture content.

Fig. 1
figure 1

Priming of soybean seeds with KNO3 and subsequent evaluations performed.

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The experiment followed a completely randomized design in a 3 × 3 factorial arrangement, with factors being initial seed quality levels (low, medium, high) and KNO₃ priming concentrations (0% – control, 1.0%, and 1.5%), using four replicates per treatment. The effects of physiological conditioning on seed lots with different initial quality levels were evaluated using the following tests and measurements:

Germination test

Four replicates of 50 seeds were sown in rolls of filter paper moistened with distilled water equivalent to 2.5 times the paper’s dry weight. The rolls were placed in a B.O.D. germinator at 25 °C with constant white light for 8 days. Germination was evaluated on the 8th day based on the number of normal seedlings, following the Rules for Seed Testing (RAS)24. Results were expressed as the percentage of normal seedlings.

First germination count (FGC)

Conducted together with the germination test, this involved counting the number of normal seedlings on the fifth day after test initiation24.

Germination speed index (GSI)

Daily counts of normal seedlings were performed during the germination test. The GSI was calculated using the formula and criteria established25.

Primary root protrusion speed index

Also conducted during the germination test, this involved daily counts of seeds with root protrusion ≥ 1 cm. The index was calculated based on25 formula.

Cold test

Four replicates of 50 seeds from each lot were placed in filter paper rolls moistened with distilled water equivalent to 2.5 times the dry paper mass. The rolls were kept at 10 °C in a B.O.D. chamber for five days, then transferred to 25 °C for five more days. Normal seedlings were counted at the end26, and results were expressed as a percentage24.

Seedling length

Four replicates of 10 seeds were placed along a line on the upper third of filter paper moistened with distilled water (2.5× dry paper weight) and incubated in a B.O.D. chamber at 25 °C for five days. After incubation, the shoot length (from root insertion to the apical bud) and root length (from root insertion to primary root tip) of normal seedlings were measured using a millimeter ruler. Results were expressed in centimeters27.

Fresh and dry biomass of seedlings

After measuring seedling length, shoot and root parts were separated and weighed individually using a precision balance, with fresh weight expressed in grams. The samples were then dried in a forced-air oven at 40 °C for 48 h. After drying, samples were reweighed to determine dry biomass, expressed in grams27.

Data were subjected to analysis of variance (ANOVA) using the F test. When significant differences were found, means were compared by Tukey’s test at a 5% probability level, using the SISVAR statistical software28.

Results

Effects of seed priming on germination parameters

No significant differences were observed in the primary root protrusion speed index (RPI) among the factors studied: initial seed lot quality and KNO₃ priming (Fig. 2A). For the germination speed index (IVG), only the seeds from the medium-quality lot showed a lower germination speed when conditioned with 1.5% KNO₃ compared to the other treatments. Without priming, the low-quality seed lot had the lowest IVG compared to the other seed lots. However, with seed priming using 1.0% KNO₃, no significant differences in IVG were observed between this seed lot and the others with initially superior physiological attributes (Fig. 2B).

Similar results were observed for the first germination count (PC). Seeds from the low-quality lot treated with 1.0% KNO₃ did not show significant differences compared to the other lots with superior quality (Fig. 2C). Without priming, the low-quality lot showed the lowest result compared to the other seed lots. However, the medium-quality seed lot treated with 1.5% KNO₃ showed the lowest result compared to the control (Fig. 2C). Regarding final germination, beneficial effects of priming with 1.0% KNO₃ were evident in both medium- and low-quality seed lots (Fig. 2D). Additionally, the low-quality lot treated with 1.5% also performed better than the control and did not differ significantly from the 1.0% concentration (Fig. 2D). Figure 3 shows the evaluations of root protrusion and first germination count five days after the test was set up, illustrating the development behavior of soybean seedlings from seeds conditioned with KNO₃.

Fig. 2
figure 2

Germination parameters of soybean seed lots cv. BMX Potência FIBRA 64I61 RSF IPRO after physiological conditioning with KNO3. (A) Root protusion index (RPI), (B) germination speed index (GSI), (C) first germination count (PC, %), and (D) germination (G, %) of soybean seeds belonging to lots with low (≤ 80%), medium (≥ 85%) and high (≥ 90%) quality levels primed with potassium nitrate (KNO3) solutions. Capital letters compare the same KNO3 concentration in different soybean seed lots, Lowercase letters compare different KNO3 concentrations in the same soybean seed lot (Tukey test, p<0.05).

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Fig. 3
figure 3

Development of soybean seedlings belonging to seed lots with low (≤ 80%), medium (≥ 85%) and high (≥ 90%) qualities primed with potassium nitrate (KNO3) solutions.

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Effects of seed priming during low-temperature imbibition

Positive effects of KNO₃ seed priming were also observed in overcoming low-temperature stress during imbibition (Fig. 4). Specifically, for seeds with low initial quality, priming with KNO₃ led to improved germination compared to the control, with no significant differences between the two tested concentrations. With 1.0% KNO₃, the initially observed vigor discrepancies among seed lots were no longer statistically significant. It is worth noting that although still lower than medium- and high-quality lots, low-quality seeds showed improved physiological performance in the cold test. For medium- and high-quality lots, no significant differences were found between the control and KNO₃ treatments (Fig. 4).

Fig. 4
figure 4

Cold test (CT) of soybean seeds belonging to seed lots with low (≤ 80%), medium (≥ 85%) and high (≥ 90%) quality levels primed with potassium nitrate (KNO3) solutions. Capital letters compare the same KNO3 concentration in different soybean seed lots, Lowercase letters compare different KNO3 concentrations in the same soybean seed lot (Tukey test, p<0.05).

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Priming with KNO₃ on seedling growth

In the shoot length evaluation, no significant differences were observed between priming doses within the same seed quality level, except for medium-quality seeds treated with 1.5% KNO₃, which showed the lowest results compared to other treatments (Fig. 5A). The effects of KNO₃ priming were also evident in root length of seedlings from low-quality seeds, since priming treatments resulted in significantly greater root length than the control (Fig. 5B). Notably, primed low-quality seeds produced seedlings with longer roots than those from medium- and high-quality seeds (Fig. 5B).

Fig. 5
figure 5

(A) Shoot length (APL, cm-1 seedlings), (B) root length (RL, cm-1 seedlings) of soybean seedlings belonging to seed lots with low (≤ 80%), medium (≥ 85%) and high (≥ 90%) quality levels primed with potassium nitrate (KNO3) solutions. Capital letters compare the same KNO3 concentration in different soybean seed lots, Lowercase letters compare different KNO3 concentrations in the same soybean seed lot (Tukey test, p < 0.05).

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Fresh and dry shoot biomass results followed a similar pattern due to their correlation. Seedlings from low- and high-quality seeds primed with KNO₃ exhibited higher fresh and dry shoot biomass than the control (Fig. 6A and C). No significant differences were found between KNO₃ concentrations for the medium-quality lot, which had significantly lower values than the other seed lots (Fig. 6A and C).

As for fresh and dry root biomass, seedlings from the low-quality lot showed increases in fresh root mass with KNO₃ priming treatments. Additionally, priming with 1.0% KNO₃ yielded the highest fresh root biomass compared to the other seed quality levels (Fig. 6B). However, for dry root biomass, no significant differences were found between primed and unprimed treatments (Fig. 6D). In contrast, high-quality seeds primed with 1.5% KNO₃ exhibited the highest fresh and dry root biomass compared to other treatments (Fig. 6B and D).

Fig. 6
figure 6

(A) Fresh matter of the aerial part (g seedling-1), (B) Fresh matter of the root (g seedling-1), (C) Dry matter of the aerial part (g seedling-1) and (D) Dry matter of the root (g seedling-1) of soybean seedlings belonging to seed lots with low (≤ 80%), medium (≥ 85%) and high (≥ 90%) quality levels primed with potassium nitrate (KNO3) solutions. Capital letters compare the same KNO3 concentration in different soybean seed lots, Lowercase letters compare different KNO3 concentrations in the same soybean seed lot (Tukey test, p < 0.05).

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Discussion

Improving seed performance—and, consequently, crop yield—under both normal and adverse environmental conditions is essential to meet the growing demand for soybean-derived products and ensure global food security. In this context, seed priming, or physiological conditioning, has emerged a technique to enhance germination and seedling uniformity. Our results indicate that, without priming, the germination speed parameters (GSI and FGC) and germination percentage of low-quality seeds were significantly lower compared to the other seed quality levels (Fig. 2). However, low-quality seeds primed with 1.0% KNO₃, did not differ statistically from that of medium- and high-quality seeds, despite their initially lower physiological attributes (Fig. 2). These results indicate that 1.0% KNO₃ priming mitigated the physiological quality differences among seed lots, as shown by germination parameters (Fig. 2B and C, and 2D).

According to the literature, KNO₃ concentrations above a certain threshold may not favor improved germination29, as higher concentrations require more time for seeds to progress through phases I and II of germination30. This increase in germination may be attributed to KNO₃’s modulation of abscisic acid (ABA) metabolism in seeds31,32. Moreover, exogenous KNO₃ application increases gibberellin (GA) levels and reduces ABA levels, resulting in a lower ABA/GA ratio33. In the present study, the results indicate that KNO₃ priming did not influence the performance of seed lots with high physiological quality. However, particularly with the 1.0% KNO₃ treatment, priming was effective in increasing germination speed and inducing tolerance to low temperatures during soybean seed imbibition (Figs. 2 and 4).

The positive results of priming with KNO₃ are consistent with the observed increases in germination, seedling emergence, plant height, and leaf area of different soybean cultivars22. During seed imbibition, many metabolic processes are activated, such as the breaking of dormancy, rapid respiration rates, and the repair of membranes damaged during dormancy and aging. Immersing the seeds in a KNO₃ solution facilitated water uptake and the processes that precede seed germination34.

Our results also indicate that KNO₃ priming reduced sensitivity to low temperatures during the imbibition phase of soybean seeds. This effect may be linked to the activation of enzymes and the antioxidant metabolism in seeds35. In this sense, conditioning seeds with inorganic salts may overcome stress-induced dormancy and promote germination29. Additionally, exogenous KNO₃ application may stimulate germination under biotic and abiotic stress by releasing nitric oxide (NO)36. In higher plants, NO acts as a signaling molecule involved in growth and development, pathogen defense, and stress responses37.

KNO₃ priming was effective in promoting root growth in soybean seedlings, even in those derived from initially low-quality seeds. This may be attributed to priming’s influence on seedling development via metabolic activity modulation, resulting in improved early growth performance3839. also reported positive effects of KNO₃ priming on early soybean seedling development, with increased root length in seedlings from primed seeds. In this study, seeds of soybean cv. BMX FIBRA 64I61 with low initial quality showed increased seedling biomass after KNO₃ priming. For low-quality seeds, 1.0% KNO₃ priming resulted in greater fresh root mass compared to other seed quality levels (Fig. 6B). These results suggest that KNO₃ priming effectively enhanced reserve translocation from the embryonic axis to the seedling shoot. In high-quality seeds, the positive priming effect was observed mainly at the higher KNO₃ concentration (1.5%). However, medium-quality seeds previously treated with KNO3 at 1.0% did show significant improvement in seed germination, but no improvements were observed in the other growth parameters.

Considering that soybean seed lots with acceptable germination for commercialization are often discarded due to low vigor, the results of this study are useful for soybean seed technology. KNO₃ priming is a low-cost, easy-to-implement pre-germinative treatment. According to our findings, this technique provides satisfactory improvements in germination, seedling growth and biomass, and tolerance to low-temperature stress during imbibition. These results demonstrate the technique’s potential as a sustainable practice for soybean cultivation and rational use of natural resources.

Conclusion

Seed priming with potassium nitrate promotes increases in germination and vigor attributes of soybean seeds (cv. BMX FIBRA 64I61). The physiological performance benefits were most evident in seeds with low initial quality (acceptable germination and vigor ≤ 80%), treated with 1.0% and 1.5% KNO₃, resulting in improved germination and vigor, as determined by the cold test. Priming high-quality seeds (germination and vigor ≥ 90%) with 1.5% KNO₃ increased the shoot and root biomass of the seedlings.