Impacts of Aminopyralid on Tomato Seedlings

Abstract

Effective aminopyralid herbicides are commonly used to control broadleaf weeds in cereals or pastures, but their residues in straw or manure may damage cultivated crops and reduce the yield. In our experiments, the response of tomato plants to aminopyralid at doses of 0.6, 1.5, 3, 7.5, and 15 g/ha was evaluated, and extracts from straw treated with the herbicide Mustang Forte were tested. As the concentration of aminopyralid increased, seed germination was delayed by 1 to 3 days, compared to the control, and all the germinating seeds were deformed already at the lowest concentration of 0.6 g/ha. With the increased concentration of aminopyralid, injury to tomato plants also increased, and at the highest applied dose of 15 g/ha, 93.75% of the tomato plants were damaged. The critical level of concentration of aminopyralid in the soil was determined between 3 and 7.5 g/ha. Treatment with aminopyralid influences plant height in the indeterminate cultivar from an aminopyralid concentration of 3 g/ha and in the determinate cultivar from a concentration of 7.5 g/ha, but not as significantly. Thus, this experiment suggests that in the indeterminate cultivar, aminopyralid has a greater effect on height than in the determinate cultivar. This varietal sensitivity should be subjected to further study.

1. Introduction

Herbicides with poor public perception, such as glyphosate, are little by little being restricted in the European Union, and these products are being replaced by phytohormone-based herbicides, such as auxin. These new herbicides are almost completely nontoxic to the environment and organisms, including humans [1]. However, their residues remain in the environment or are absorbed in plant tissues for a long time, with a half-life of 34.5 days for North American sites and 25 days for European sites [2,3,4], which causes problems during crop rotation or when using treated straw as mulch or compost. With the increasing consumption of these herbicides in the last ten years, there is also a higher incidence of damage to subsequent crops. While in 2011, only 243.236 kg of acid pyridine carboxyl herbicides were placed on the market in only 8 EU states, in 2020, it was already 1870.147 kg in 24 EU states [5].

Aminopyralid is a selective, systemic post- and pre-emergence herbicide [2] that is effective on many persistent and invasive broadleaf weed species in the Asteraceae, Fabaceae, and Solanaceae families [6]. It is mainly used to treat cereals, pastures, and grass. Aminopyralid is a synthetic auxin herbicide of the pyridine carboxylic acid family. Herbicides with aminopyralid bases, such as Mustang Forte or Hurricane, are widely used to control broadleaf weeds in cereals or grasslands. On the basis of studies, aminopyralid has potential to provide excellent control of mugwort (Artemisia vulgaris) [7] and canada thistle (Cirsium arvense) [6] at lower use rates compared to other auxin herbicides, such as clopyralid and picloram.

Their disadvantage is that the residues remain not only in the soil but also bind to the plant tissues of the treated plants. The acidic and hydrophilic nature of these herbicides can mimic the apoplast in plant tissue, leading to their ability to penetrate plasma membranes and accumulate in cells and phloem. [8]. Active substances of these herbicides also have a long half-life in the environment, and aminopyralid residues were shown to be stable for at least 16 months in dry matrices, such as hay and straw [1].

Symptoms of damage by auxin-based herbicides are manifested by twisting of leaves and stems, deformations of leaves, elongation and abnormal growth of leaves, stunting of roots, and formation of tumors [9]. Subsequently, necrosis and chlorosis of the top of the vegetation develop, and the plants wither [10].

Auxin is probably one of the most important plant hormones and is involved in almost all aspects of development, including specification of the apical–basal axis of the body, formation of lateral organs, organogenesis, tropisms, and environmental response [11]. Apical dominance allows the plant to concentrate resources in the main axis of growth, while activation of dormant buds allows recovery after damage or loss of the main shoot. Auxin inhibits axillary bud activation and thus shoot branching, and while cytokinin has the opposite effect, auxin is thus the main signal for apical dominance [12].

Herbicides based on the phytohormone auxin act as growth regulators. When taken up by the plant, they deregulate plant growth metabolic pathways, affecting the growth process by binding to auxin receptor sites, resulting in thickening, curved, and twisted stems and leaves, bulbous and wrinkled leaves, stem cracking, narrow leaves with callus tissue, stunted growth of stems, enlarged roots, and proliferated growth to the tip of the plant, leading to the death of the plant [2].

Straw from the treated cereals, which enters the soil in the form of organic fertilizer (manure, postharvest residues, and compost) or mulching (it is washed away by rain and irrigation that flow through the straw), in which sensitive crops, such as tomato, pepper and eggplant [13], will be grown, can be problematic. The absence of treatment of cereals with herbicides based on the active substance aminopyralid, from which straw or manure comes, cannot always be guaranteed and may damage the crop of cultivated vegetables and reduce economic yield. It can be assumed that if the plants are damaged by the herbicide at this early stage of the plant’s development, it will also affect its further development, including flowering and fruiting, which will lead to an undesired reduction in the yield of the cultivated crop, as has already been proven in the case of experiments with potato [14], pepper [10,15], tobacco [15], eggplant [12,16], cucumber [16], and tomato [13,15,16].

The aim of this study was, therefore, to evaluate the effect of aminopyralid on the germination and juvenile phases of tomato plants; on their damage, height, and phytomass weight; and to try to determine the critical concentration level of aminopyralid for germinating seeds and for damaging young tomato plants. Whether there was any varietal difference in tolerance to aminopyralid damage between indeterminate and determinate tomato varieties was also investigated.

2. Materials and Methods

2.1. Materials

The study evaluated the response of tomato seedlings to the selected herbicide Mustang Forte. Two tomato varieties, ‘Start’ S F1, an indeterminate cultivar (later in the text as well as ‘Start’) and ‘Šejk’, a determinate cultivar, were tested. The seeds of both varieties were bought from SEMO a.s. Two pot experiments with tomato plants and seeds were performed. In the first experiment, the susceptibility of seedlings and germinating seeds to substrate treatment with aminopyralid was evaluated. This experiment was to verify whether increasing the concentration of the active herbicidal substance in the soil would also result in increasing damage to tomato plants. The second experiment evaluated the reaction of tomato plants to straw extracts that were obtained from different farms with different herbicide treatment intensities. This experiment was to simulate a real situation in an agroecosystem, which includes growing tomatoes in various forms of straw, either as mulch or in the manure used as organic fertilizer.

Containers with PR33 fine-grained river sand (Provodínské písky a.s.) with a granularity of 0.1–0.5 mm were used for seed germination tests; 210 g of sand was weighed into each container. The experiment took place in a Q-Cell incubator, which maintained a constant temperature of 20 °C and darkness.

The peat substrate PROFIMIX 2 RS II (AGRO CS a. s.) was used for cultivation in both tests with plants. This substrate is made of a mixture of high-quality white peat (80%) and black peat (20%) with the addition of clay (30 kg/m³) and is also suitable for seedlings. The plants were planted in 9 cm × 9 cm plastic pots.

Our studies with plants were conducted in 2021 in an experimental greenhouse at 25 °C during the day, 19 °C at night, and in natural light mode (14 h light, 10 h dark). During cultivation, the plants were regularly irrigated with demineralized water as needed.

2.2. Bioassay with the Use of Herbicidal Substances of Various Concentrations

The application dose of aminopyralid was determined based on similar studies. Fast et al. [13] used 1.4–44.8 g/ha, but they planted the vegetable plants only after three weeks when the calculated half-life of aminopyralid was 34.5 days. Jeffries at al. [15] applied 12 g/ha and found that this dose, in the case of tomato plants, led to the death of 100% of the plants. In a preliminary experiment carried out with tomato and strawberry plants using a dose of 2, 5, 10, 25, and 50 g/ha, we found that at a concentration of 2 g/ha, visible injury to the plant, manifested by the twisting of the leaves and their drying, became evident. At concentrations of 25 and 50 g/ha, all plants died completely after 10 days from the application of the herbicide to the soil. So, the application dose was therefore determined for this study in such a way that, in the case of the lowest dose, it was possible to observe an effect on the plants, but also so that the highest doses did not have a completely lethal effect.

Aminopyralid (Shanghai Tianfu Chemical Ltd., Hong Kong) was applied at corresponding doses of 0.6, 1.5, 3, 7.5, and 15 g/ha. These application rates are less than the labelled application rate of 60 g/ha, which is permitted in the European Union [1] and were used to create soil concentrations of aminopyralid that would be representative of the concentrations present in field sites where aminopyralid had been applied in the past. For aminopyralid’s soil half-life, we relied primarily on the study of Fast et al. [13]. Aminopyralid was used as a pure substance in this experiment because the herbicide Mustang Forte also contains other active substances, such as 2,4-D and florasulam. The test was based on a modified methodology by Chhokar et al. [17].

In the seed germination tests, herbicide was applied in the above-mentioned concentrations in the amount of 40 mL in each container. Four replicates were performed for each concentration with fifty seeds. Control variants were flooded with the same amount of distilled water. Then, seeds were placed on the surface of the sand. The containers prepared in this way were fitted with a lid and placed in the Q-Cell. Counting of germinated seeds started on day 4 after sowing and then continued every 24 h until the 10th day.

In the test with tomato seedlings, herbicide was applied on the soil before planting after diluting 0.243 mL of herbicide at each concentration in 50 mL of distilled water. This dose was calculated based on the determination of the dry matter in the substrate (43%) and the optimal spray rate (300 L/ha) according to Jursík et al. [10]. As a control, the same amount of distilled water was used. Then, plants with their the second true leaves, the 12th phase according to BBCH identification, were transplanted into the soil, ten plants for each concentration.

After 2 months of cultivation in the greenhouse (phase 21 BBCH), the following parameters were evaluated for the plants: the leaf injury, the height of the plant (cm), the root collar diameter (mm), and the weight of the aboveground part (g). A point scale was used to assess leaf injury: 0—without injury; 1—slight bending of leaves; 2—light twisting of leaves 10–20%; 3—slight twisting of leaves 20–30%; 4—slight moderate injury, twisted leaves, and chlorosis 30–40%; 5—moderate injury, twisted leaves, and chlorosis 40–50%; 6—significant twisting of leaves and chlorosis, drying of leaf tips over 50%; 7—severe twisting of leaves and their drying 75% or more; 8—very severe injury to the plant. The illustrative images from the experiment are in Supplementary Materials Figure S6.

2.3. Bioassay with Treated Straw Extracts

In the second experiment, treated straw was used. Wheat was treated during growing by Mustang Forte (Dow Chemical Company) herbicide, which contains the active substances 2,4-D (180 g/L), aminopyralid (10 g/L), and florasulam (5 g/L). Three different straws were obtained: straw from the organic farm Rýzner at Kojátky without treatment of herbicide, straw from the farm Libodřice treated with Mustang Forte herbicide at a corresponding dose of 10 g/ha of active substance aminopyralid, and straw from the school farm Červený Újezd treated with herbicide Mustang Forte at a corresponding dose of 20 g/ha of active substance aminopyralid.

Extracts from all types of straw were prepared in the same way according to the modified methodology by Hyväkkö et al. [18] and Zain et al. [19]. Straw (2.5 kg) was soaked in 25 L of distilled water for 48 h, and after straining, 2 L of leachate was used on ten plants. Clean leachate as an undiluted solution and solutions diluted by distilled water of 50% and 25% from each kind of leachate were used on plants. As a control, 2 L of distilled water was used. The solutions were applied evenly to the soil surface in the planter using a 2 L jug for ten plants with their second true leaves, phase 12 according to BBCH identification, ten plants for each concentration.

After 2 months (phase 21 BBCH) of cultivation in the greenhouse, the following parameters were evaluated: the height of the plant (cm), the root collar diameter (mm), and the weight of the aboveground part (g).

During the seed germination tests, extracts from straw were applied in the above-mentioned dilutions in the amount of 40 mL in each container. Four replicates were performed for each concentration. Control pots were provided with the same amount of distilled water. Fifty seeds were placed on the surface of the sand for each concentration. The containers thus prepared were fitted with a lid and placed in the Q-Cell. Counting of germinated seeds began on day 4 after sowing and then continued after 24 h until day 10.

The measured values were evaluated by analysis of variance (ANOVA) and Fisher’s LSD method in TIBCO Statistica Ultimate Academic software (version 13.5; StatSoft (Europe) GmbH, Hamburg, Germany).

3. Results

3.1. Bioassay with Herbicidal Substances of Various Concentrations

3.1.1. Effect of Herbicide Treatment on Seed Germination

All herbicide-treated seeds and the untreated control began to germinate on day 4 after sowing. However, with increasing concentrations of aminopyralid, germination was delayed. Of the cultivar ‘Start’, only 30.5% of seeds germinated on average on day 5 after sowing with the highest applied dose of 15 g/ha of aminopyralid, compared to the control with distilled water, where 98.5% of seeds germinated on the fifth day. Of the cultivar ‘Šejk’, an average of 19% of seeds germinated on day 5 at the highest dose of aminopyralid, and 75% of seeds germinated at the lowest dose of 0.6 g/ha. Germination was in all cases significantly less than in the control treatment of distilled water, where 88.5% of seeds germinated. Seed germination was delayed by 1 to 3 days compared to the control, more significantly in the highest aminopyralid treatment, as can be seen in the Supplementary Materials in Figure S1 for the cultivar ‘Start’ and Figure S2 for cultivar ‘Šejk’ and in Table S1. Germination delay was similar for both choice varieties. However, the effect on the total number of germinated seeds in 9 days was not statistically significant compared to the untreated control with distilled water.

A very important finding was that when aminopyralid is used, even in the lowest doses, deformation of germinating seeds occurs (see Figure 1). The deformation was manifested in the sprouts by a very noticeable shortening of the primary root and thickening of the hypocotyl. This damage did not appear in any seed from the control group. Seed deformation was similar in both cultivars.

3.1.2. Herbicide Injury to the Plants

Herbicide damage to the plants was evaluated continuously throughout the experiment. There were deformations of the leaves, twisting of the stems and leaves, and loss of chlorophyll. For this visual evaluation, a damage scale from 1 to 8 was set, as indicated in the methodology.

In the case of aminopyralid treatment, both tomato cultivars experienced a significant injury in the form of twisting of the leaves and stems. This injury increased with higher concentrations, as shown in Figure 2. There was a statistically significant difference between the untreated control variant and all herbicide-treated concentrations. The choice of cultivar had almost no effect on the severity of the damage.

3.1.3. Effect of Herbicide Treatment on Plant Height

The height of the aerial part of the plant was measured from the root collar to the top of the tomato plant. A decreasing trend was evident in both cultivars, as expected, and the lowest average height of the aerial part of the plant was measured at the highest concentration of 15 g/ha in the cultivar ‘Start’, but in the cultivar ‘Šejk’, the lowest average plant height was measured at a concentration of 7.5 g/ha.

In the cultivar ‘Start’ for treatments from a concentration of 3 g/ha and higher, the shorter plant height was statistically significant compared to the control variant. It can be stated for this cultivar that with the increasing concentration of aminopyralid above the concentration of 3 g/ha, the height of the plant was significantly stunted.

After treatment with aminopyralid at different concentrations, the cultivar ‘Šejk’ showed an irregularly decreasing and increasing trend of effect on plant height. A statistically significant difference between the height of the treated plants compared to the control variant without treatment was evaluated from a concentration of 7.5 g/ha (as can be seen in Figure 3).

It was therefore demonstrably established that treatment with aminopyralid influenced the plant height of the indeterminate cultivar from an aminopyralid concentration of 3 g/ha and of the determinate cultivar from a concentration of 7.5 g/ha, but not as markedly.

3.1.4. Effect of Herbicide Treatment on the Root Collar

In the aminopyralid treatment, the root collar diameter decreased in both tested tomato cultivars, with increasing concentrations of herbicide (Figure 4).

However, this difference was statistically significant from a concentration of 7.5 g/ha. At the highest-used concentration of 15 g/ha, the root collar diameter was 42% of the cultivar ‘Start’, and the root collar diameter of the cultivar ‘Šejk’ was 46.5% compared to the control.

3.1.5. Effect of Herbicide Treatment on the Weight of the Phytomass of the Plants

Based on our measurements, with the cultivar ‘Start’, increasing concentrations of aminopyralid led to a reduction in the formation of aboveground phytomass, and in the treatment with the highest concentration (15 g/ha), the average weight of the aboveground part of the plant was the lowest (22.1 g), while the average weight of the control plants was 48.4 g. The differences were statistically significant from the 1.5 g/ha concentration.

With the cultivar ‘Šejk’, such a significant decrease in green biomass was not observed, and it was statistically significant only from a concentration of 7.5 g/ha, as can be seen in Figure 5.

3.2. Bioassay with the Use of Herbicidal Substances of Various Concentrations

3.2.1. Effect of Straw Extracts on the Seed Germination

In general, it can be stated that the dilution of the leachate extracts of straw to 50% and 25% had a significant effect on seed germination, as can be seen in Supplementary Materials Figure S5. Germination was delayed by 1 to 3 days in all extract-treated straw compared to the control, as was the case with pure herbicides. However, the use of straw from an organic farm also showed a delay in germination. The difference between cultivars was statistically significant. In the graphs in the Supplementary Materials, in the case of the cultivar ‘Šejk’ (Figure S4), significantly fewer seeds germinated in all treatments than in the case of the cultivar ‘Start’ (Figure S3).

3.2.2. Effect of Straw Extracts on Plant Height

As with the seed germination, a positive effect of the dilution of leachate extracts of straw was also observed at the height of the tomato plants, especially with the cultivar ‘Start’. With this cultivar ‘Start’, however, all plants in the treated groups were shorter than in the control group with distilled water. For the straw extract treatment with Mustang Forte at a dose corresponding to 10 g/ha diluted to 25%, this lower height was statistically significant.

When comparing the individual leachate extracts of straw, a double dose (20 g/ha) of the herbicide Mustang Forte led to a reduction in the height of the plant in all dilutions of the cultivar ‘Start’, but to an increase in the cultivar ‘Šejk’.

One interesting finding is that even in the case of using straw from an organic farm without aminopyralid treatment, there was a statistically significant retardation effect on plant height of the cultivar ‘Start’, while this effect was not detected in the cultivar ‘Šejk’ as can be seen in Table S3 in the Supplementary Materials.

3.2.3. Effect of Straw Extracts on the Root Collar

There was no clear trend in the evaluation of root collar width, either in the differences between the different straw treatments or of the individual tomato cultivars.

3.2.4. Effect of Straw Extracts on the Weight of the Phytomass

For all the used straw extracts, including the untreated variant from the organic farm, it can be stated that diluting the extract with distilled water to 50% and 25% had a positive effect on phytomass formation of both tomato cultivars. In the case of the use of straw treated with the herbicide Mustang Forte in both concentrations diluted to 25%, the weight of the aboveground part of the plants slightly exceeded the control group treated only with distilled water. However, these findings are not statistically significant as can be seen in Table S3 in the Supplementary Materials.

4. Discussion

Our data show that aminopyralid had a significant effect on the germination of tomato seeds with an applied dose of 0.6–15 g/ha of aminopyralid. These doses were used to simulate residuals in the soil when plants rotated or when straw treated with this herbicide was used as mulch. As the concentration increased, the degree of sprout damage increased, and germination decreased, as can be seen in the Supplementary Materials. Boobadila et al. [20] stated in their study that treatment with aminopyralid at a dose of 500 g/ha reduced the viability of Italian ryegrass seeds (Lolium perenne ssp. multiflorum) by more than 50%, and at the same time, aminopyralid slowed down the germination rate by 1 to 2 days. In addition, McManamen et al. [21] used a higher dose of 894.4 g/ha of aminopyralid and concluded that there was a decrease in the germination of 75% of the weed seeds in the case treated with aminopyralid compared to the control. At the same time, they also stated that over time, the negative effect of aminopyralid on seed germination reduces to 25% 11 months after application to the soil. However, in both cases, these are many times higher doses than in our study and higher than the 60 g/ha [1] allowed in the European Union. While both cited studies dealt with weed control, and apparently chose such a concentration of the active substance to achieve suppression of their growth, in the ideal case, it is 100% elimination of weed seeds. Our study, on the contrary, was about determining the sensitivity threshold of tomato to very low herbicide residues in the soil, so that the germinating plants are not damaged. This difference in the concentrations of the herbicide used ranged from ten to a hundred times, which also demonstrates how high the sensitivity of tomato plants is to the presence of aminopyralid compared to the weeds against which it is intended. In addition, Wagner and Nelson [22] submit that the application of aminopyralid at the manufacturer’s recommended dose (Milestone in Canada 361.2 g/ha of active ingredient) can damage monocotyledonous (Pseudoroegneria spicata, Festuca idahoensis, Elymus trachycaulus, Poa secunda, Koeleria macrantha, Agropyron cristatum, Bromus tectorum, Dactylis glomerata, Phleum pratense) and dicotyledonous (Erigeron speciosus, Geranium viscosissimum, Euphorbia esula, Centaurea stoebe, and Linaria dalmatica) plants, but when they used an application rate 0.01 times the recommended dose (corresponding to a 3.612 g/ha of aminopyralid dose), they did not observe any effect on these weeds. Compared to them, we found the effect of tomato seed damage and reduced germination even at the lowest dose of 0.6 g/ha. This supports our hypothesis that tomato plants can be much more sensitive to the presence of aminopyralid than the weed species used in the comparative study and that therefore even a small residual amount of this herbicide in the soil can lead to damage to subsequently grown vegetables, especially in their initial phase of growth. Plants from the Solanaceae family are generally more sensitive to various pollutants in the environment, not only to herbicides, but also to the presence of other substances in the soil [23,24,25,26]. This fact is also confirmed by a study by Jeffries et al. [15] carried out with several types of crops, including Solanaceae (pepper Capsicum annuum L., soybean Glycine max L., squash Cucurbita pepo L, tobacco Nicotiana tabacum L., and tomato Solanum lycopersicum L. cultivar ‘Homestead 24′), where tomato reacted most sensitively to aminopyralid in most of the evaluated parameters. At a dose of 12 g/ha of aminopyralid, there was 100% plant damage (complete plant death) both pre- and postapplication, while the least-sensitive plant in this study, squash, was damaged by only 24% preapplication and 54% postapplication. Similarly, in our study, tomato plant damage was 93.75% of both cultivars at the highest dose of 15 g/ha of aminopyralid. With the increasing dose of aminopyralid, plant damage increased in both tomato cultivars, even at a dose of 3 g/ha. The plants were damaged by more than 50%, and the determinate cultivar was damaged by at least 75%. Jeffries et al. [15] used the indeterminate cultivar ‘Homestead 24′, but from these results, it is not yet possible to determine whether any of the determinate/indeterminate cultivars of tomato were more sensitive to the presence of aminopyralid in the soil. This cultivar’s sensitivity would have to be subjected to a more extensive study.

A study from Alaska [14] showed that the uptake of aminopyralid by the plant and its damage depended on the time of application. In that study, 0, 8, 15, 31, 62, and 123 g/ha of aminopyralid were applied in three different locations, and then potatoes were planted there 1 day, 14 days, and 3 weeks after herbicide application. When potatoes were planted the day after the application of the herbicide, the potato damage was 68% at the lowest dose of 8 g/ha. When planting in the second location 14 days after the application, only 25% of the plants were damaged, and only at a dose of 15 g/ha of aminopyralid. The decrease in herbicide residues over time and decreasing damage confirm previous studies and our results, where similar damage to plants was found at similar doses. Even the above-mentioned study with weeds [21] showed that 11 months after application, the effect of aminopyralid residues in the soil was reduced to a third. However, it must be stated here that the presence of aminopyralid in the soil, even after such a long time, is a risk for plants from the Solanaceae family, which are much more sensitive, and this risk increases with the increasing dose of the herbicide. At a dose of 123 g/ha, damage to potato plants in all locations was almost 100% compared to the control [14], i.e., regardless of the time between herbicide application and tuber planting. Symptoms included cupped leaves and fiddle-necked stems at lower rates and the presence of only single stems with a few small leaves at the highest rates. In our study, we also noticed the twisting of leaves and stems and their drying and chlorosis, even at low doses. The thylakoid structure in the chloroplasts of developing leaves depends on the plant growth. Chloroplast thylakoids are dependent on the auxin level [27]. It is proposed that differences in concentrations of plant growth hormones may be responsible for the persistence of normal chloroplasts near the vascular tissue and leaf blade edges and that fluctuations in auxin levels could explain the bleaching leaves [28].

Fast et al. [13] also reported that when applying aminopyralid at a dose of 1.4 to 44.8 g/ha, plants showed severe crop damage and reduced plant mass of pepper (Capsicum annuum L.), eggplant (Solanum melongena L.), and tomato (Lycopersicon lycopersicum L.). At six weeks after planting, crop injury was visually estimated on a scale of 0 (no injury) to 100 (plant death). Soil residues of aminopyralid caused severe crop injury in 90% of pepper, 90% of eggplant, and 89% tomato. The results in our study, when using similar doses to Fast et al., found similar plant damage at 93% at a dose of 15 g/ha. Plant height was reduced by 69% in bell pepper, 61% in eggplant, and 54% in tomato. However, Fast et al. [13] do not specify which cultivar of tomato they used for their experiment. In our study, it was established that treatment with aminopyralid influences plant height of the indeterminate cultivar from an aminopyralid concentration of 3 g/ha and of the determinate cultivar from a concentration of 7.5 g/ha. Thus, this experiment suggests that in the indeterminate cultivar, aminopyralid has a greater effect on height than on the determinate cultivar, which may be influenced by the effect of auxin on apical dominance [12].

As with plant height, varietal differences were also observed in the weight of the aboveground phytomass. While with the indeterminate cultivar, the increasing concentration had a growing negative effect on the formation of the aboveground phytomass, with the determinate cultivar, this trend was not as significant up to a concentration of 7.5 g/ha, and the average weight of the aboveground part of the plant was comparable to the control group. These findings were to be expected, given the nature of the substance used. Aminopyralid is a herbicide based on the synthetic plant growth hormone auxin. The application of auxin (2,4-D) increases the plant height and the number of branches in tomato plants [29], but only modestly. Pramanic et al. [30] stated that a high concentration of indoleacetic acid, on the contrary, reduces the height of plants.

In experiments with Mustang Forte herbicide-treated straw leachate, herbicide-treated straw leachate was found to harm seed germination, plant height, and aboveground plant weight compared to the distilled water control (as seen in the Supplementary Materials Figures S3 and S4). This may be affected by aminopyralid residues in treated straw tissues. In an experiment with Bahia grass (Paspalum notatum) [31], leaves treated with aminopyralid at a dose of 120 g/ha of aminopyralid added to soil were used, where the growth of tomato seedlings was reduced by 85%. These data indicate that aminopyralid is metabolized in plant residues and that it may be released into the soil and affect susceptible plant species. On the other hand, we found in our study that leachate from organic farming straw untreated with herbicide also harms tomato plants. Nakano et al. [32] observed that an extract from wheat (Triticum aestivum L.) inhibited the growth of lettuce, watercress, and rice. Their research shows that this is due to wheat’s ability to release inhibitory substances into its surroundings (allelopathy). At the same time, they proved that with a higher concentration of the leachate, its allelopathy effect increases, which corresponds to our findings.

5. Conclusions

Our data show that aminopyralid had a significant effect on the germination of tomato seeds with an applied dose of 0.6–15 g/ha of aminopyralid. As the concentration of aminopyralid increased, seed germination was delayed by 1 to 3 days, compared to the control, and all the germinating seeds were deformed already at the lowest concentration of 0.6 g/ha. With the increased concentration of aminopyralid, injury to tomato plants also increased, and at the highest-applied dose of 15 g/ha, 93.75% of tomato plants were damaged. In the case of negative effects on the root collar diameter and height and weight of the aboveground part of the plant, a critical level of concentration of aminopyralid in the soil was determined between 3 and 7.5 g/ha. Treatment with aminopyralid influences plant height of the indeterminate cultivar from an aminopyralid concentration of 3 g/ha and of the determinate cultivar from a concentration of 7.5 g/ha, but not as outstandingly. Thus, this experiment suggests that regarding the indeterminate cultivar, aminopyralid has a greater effect on its height than on that of the determinate cultivar. As with plant height, varietal differences were also observed in the weight of the aboveground phytomass, while with the indeterminate cultivar, the increasing concentration had a growing negative effect on the formation of the aboveground phytomass, and with the determinate cultivar, this trend was not as significant up to a concentration of 7.5 g/ha, and the average weight of the aboveground part of the plant was comparable to the control group. The results of this study bring new knowledge in relation to the studied herbicide substance, including the determination of the limit concentration for tomato plants, including varietal specificity and the reaction of tomato plants to leachate extracts of straw, which comes from strands of winter wheat with different intensities of herbicide treatments. This cultivar’s sensitivity should be subjected to further study.