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BY 4.0 license Open Access Published by De Gruyter Open Access December 31, 2022

New plant resistance inducers based on polyamines

  • Patrycja Czerwoniec EMAIL logo
From the journal Open Chemistry

Abstract

The novel and revolutionary approach to plant protection presented in this work, based on the preparation of bifunctional salts of a plant resistance inducer combined with a polyamine cation, may become a potential solution in the future for reducing the effects of abiotic and biotic stresses to which the plant is exposed. This study presents the synthesis, physical properties, phytotoxicity, and systemic acquired resistance (SAR) induction efficacy of new salts composed of the anion of plant resistance inducers and N,N,N,N′,N′,N′-hexamethylpropane-1,3-diammonium cation (5 salts), N,N,N,N,N,N-hexamethyl-butane-1,4-diammonium cation (5 salts), spermidine salicylate, and spermine salicylate. SAR induction efficiency tests were performed on tobacco, Nicotiana tabacum var. Xanthi, infected with the tobacco mosaic virus.

1 Introduction

Ionic liquids are a very common group of compounds in the modern chemistry that have a melting point below the decomposition temperature (usually below 100°C and even close to room temperature in some cases (room temperature ionic liquids, RTILs)) [1]. These compounds are composed of an organic cation and an anion, which may be organic or inorganic. The abundant possibility of combining positively charged organic compounds with various negatively charged molecules makes it possible to create a substance with desired properties that can find practical applications [2]. There are three generations of ionic liquids [3]. The first generation of ionic liquids includes compounds designed to have unique physical properties, such as high viscosity [4]. The second generation ionic liquids are designed for specific chemical and physical properties that will have specific application use, such as lubricants [3] or dehumidifiers [5]. The third generation of ionic liquids comprises compounds that are designed for specific biological properties and have specific physical and chemical properties. Compounds classified as the third generation typically have unique properties designed for specific applications, such as sweet antimicrobials [6], fungicidal herbicides [7], and resistance inducer compounds with bactericidal properties [8,9,10]. The designability of ionic liquids can be a solution to the problems of agriculture which is facing adverse factors such as weed infestation, pathogen attack, pests, and abiotic stress. The ability to design ionic liquids may be an opportunity to create a substance that effectively counteracts not just one, but several adverse environmental factors occurring simultaneously.

Plants to survive in hostile environments have evolved defense mechanisms against biotic stresses (pathogens) [11]. One of them is the systemic acquired resistance (SAR). This defense mechanism allows for a broad spectrum of resistance against bacteria, viruses, and fungi [12]. SAR is triggered immediately after a pathogen attack or as a result of the application of elicitors, substances that simulate a pathogen attack, to the plant. Activation of immune mechanisms leads to the accumulation of salicylic acid in the plant, which activates PR-1 gene expression and the synthesis of pathogenesis-related (PR) proteins [13,14,15]. As mentioned above, systemic acquired resistance aside from pathogen attack can also be activated by the external application of elicitors to plants [16]. It is worth pointing out that often synthetic elicitors (e.g., acibenzolar S-methyl ester (BTH) and their derivatives, [8,9,10,17,18,19,20,21,22,23] or 2,6-dichloroisonicotinic acid (INA) [24] and their derivatives [25,26]) exhibit higher biological activity than natural elicitors (e.g., salicylic acid, azelaic acid, and oxalic acid [27]). For instance, chemically modified BTH compounds used in low doses (20 mg L−1) reduced up to 100% of viral infections in a model of tobacco, Nicotiana tabacum var. Xanthi, infected with the tobacco mosaic virus (TMV) [9]. The simultaneous application of resistance inducers and pesticides can be an efficient method to reduce the effective dose of pesticides, thus leading to a reduction in pesticide use. In addition, resistance inducers do not interact directly with the pathogens, thus pathogens will not become resistant to the applied elicitors, and hence their effective dose will always be unchanged. The exploitation of the phenomenon of systemic plant resistance in plant protection may be a new method in the fight against pathogens in the future.

Polyamines (PAs) are common naturally occurring biogenic aliphatic amines of polycationic nature, containing two or more amine groups (–NH2) [28,29]. At physiological pH, these compounds undergo protonation and can bond with anionic groups of polysaccharides, peptides, and proteins, or with phosphate groups in nucleic acids, phospholipids, or phosphoproteins. The effect of these interactions is a stabilization of the structure or a change in the conformation of macromolecules [30]. The most common PAs in living organisms are putrescine (1,4-diaminobutane), spermine, and spermidine [29]. PAs are involved in regulating plant growth and development. However, these compounds are not classified as phytohormones, [31] because they occur in higher concentrations than plant hormones, while they show activity in millimolar and not micromolar concentrations [30]. PAs have several functions in plant cells and also participate in many physiological and metabolic processes and in maintaining cell viability. It has been shown that these compounds can directly bind to potassium ion channels, leading to their inactivation [32]. PAs are also involved in replication, transcription, translation, cell division, [33,34] interactions between proteins and nucleic acids, [30] as well as in plant responses to stress factors [35].

During exposure of plants to various stress factors, both biotic and abiotic, PAs act as stress messengers in the plant’s response to stress signals [36]. PAs are also important in the plant’s defense response to several environmental stresses, as well as, drought, salinity, oxidative stress, heavy metal toxicity, and thermal stress (both low and high temperature) [32,37]. Changes in PA concentrations were observed in Brassica oleracea, Beta vulgaris, Spinacia oleracea, Cucumis melo, Lactuca sativa, Lycopersicon esculentum, and Capsicum annuum plants subjected to salinity stress. In most of the plants tested, putrescine level is reduced, meanwhile, spermidine and/or spermine levels are improved. Resilience to salinity stress may be attributable to an enhanced ratio of (spermidine + spermine)/putrescine. Various plants subjected to salinity stress produce a comparable response in the form of the production of PAs [38].

A huge number of studies show that the exogenous application of PAs results in improved stress tolerance in various plant species [39]. As an example, exogenous application of putrescine by foliar spraying enhanced the water status, chlorophyll, soluble sugars, and amino acid contents in water-stressed wheat plants, resulting in increased plant height, leaf area, and grain yield [40]. Bermudagrass plants treated with spermidine had greener leaf tissue and greater survivability after drought or salinity stress compared to control plants (untreated) [41]. Many physiological and proteomic analyses indicate that spermidine can trigger multiple pathways (including proteins involved in electron transport and energy, and antioxidant enzyme systems), and leads to osmolyte accumulation, which increases bermudagrass adaptation to drought and salinity stress. Accordingly, foliar application of spermidine or spermine to valerian plants similarly increased antioxidant enzyme activity, proline content, and photosynthetic pigments in response to drought stress [42]. The exogenous application of spermidine improves the salinity tolerance of ginseng and cucumber seedlings by activating antioxidant enzymes and increasing proline levels [43,44]. Similar relationships were observed for two Kentucky bluegrass cultivars, which also showed activation of multiple antioxidant enzymes and reduction in malondialdehyde levels [45]. Exogenous application of spermidine to chrysanthemum seedlings reduced Na+ uptake and improved osmotic and ionic balance, cell membrane stabilization, enzymatic ROS scavenging capacity, and photosynthetic capacity [46]. Likewise, in the case of temperature stress, the exogenous application of PAs to the plant increases the plant’s tolerance to stress conditions [47,48].

The novel and revolutionary approach to plant protection presented in this work, based on the preparation of bifunctional salts of a plant resistance inducer in combination with a PA cation, may become a potential solution in the future for reducing the effects of abiotic and biotic stresses to which the plant is exposed. Moreover, it is worth emphasizing that the plant resistance inducers through their action will not disturb the balance of the ecosystem, because they do not directly interact with pathogens, but only enable plants to become resistant to infections caused by them.

2 Materials and methods

2.1 Synthesis

2.1.1 Materials, method of synthesis, and spectra data

All reactions were conducted on commercially available pure solvents (abcr GmbH & Co. KG, Germany, POCH S.A., Poland, Chempur, Poland). Sodium salicylate, nicotinic acid, sodium Saccharinate, 2,6-dichloroisonicotinate, N,N,N′,N′-tetramethyl-1,3-propanediamine, N,N,N′,N′-tetramethyl-1,4-butanediamine, and iodomethane were purchased from Merck. BTH used for further derivatization was extracted from commercially available BION™ 50WG (Syngenta) plant protection agent. Method of BTH extraction and synthesis of compounds [K][BTHCOO] were previously reported [8,9]. Synthetic protocols and all spectra data of the obtained compounds are reported in Supplementary information (SI).

2.2 Biological properties evaluation

2.2.1 Phytotoxicity assessment

Phytotoxicity tests were performed on two plant models: N. tabacum var. Xanthi and Agrimonia eupatoria. In the first variant of the experiment, the N. tabacum var. Xanthi (planted in 0.4 L pots filled with soil) plants were watered with a 120 mL water solution of obtained substances in water at a concentration of 250 mg L−1 (or 20 mg L−1 in the case of the salt with the benzo[1,2,3]thiadiazole-7-carboxylate anion (1.2 and 2.2)). Seven days after treatment, the visual effects of the phytotoxicity (necrotic spots, yellowing, or browning of the leaves) on the leaves, which could be caused by the application of the test substances to the plant, were analyzed. In the second variant of the phytotoxicity experiment, the Agrimonia eupatoria seeds were grown on Petri dishes in a water solution of tested substances at concentrations of 125, 250, and 500 mg L−1 (or 20 and 125 mg L−1 in the case of salt with the benzo[1,2,3]thiadiazole-7-carboxylate anion (1.2 and 2.2)). Reference samples were grown on Petri dishes in water. After 5 days, sprouts were weighed. Phytotoxicity was expressed as the percentage of sprout weight reduction in comparison to untreated sprouts. Data were collected in three replicates and statistically subjected to the one-way analysis of variance.

2.2.2 SAR induction activity

Plants of N. tabacum var Xanthi (at the stage of three-developed leaves) were sprayed with 20 mL solutions of analyzed salts in water at a concentration of 250 mg L−1 (1, 2, 3, 4, 1.1, 2.1, 1.3, 2.3, 1.4, 2.4, 3.1, 4.1) or 20 mg L−1 (the salt with the benzo[1,2,3]thiadiazole-7-carboxylate anion (1.2 and 2.2)). The control plants were sprayed with distilled water. After 7 days, the tested plants were evenly dusted with carborundum and gently rubbed with fingers dipped in TMV inoculum. After inoculation, plants were washed out with a light stream of water to remove additional inoculum and carborundum. Inoculated plants were kept at 25°C in the greenhouse. Five days later, local necrotic spots on leaves (the result of a viral infection) were counted and compared to control leaves. Reduction in the area of necrotic spots on the leaves treated with salts, in comparison with the control, exhibits inhibition of pathogen infection by induction of plant resistance. The necrotic areas on leaves were examined using ImageJ software [49]. The surface of the leaf or area with necrotic spots was represented by a set of pixels on the picture which were counted by the program. Data were collected in three replicates and statistically subjected to the one-way analysis of variance. More details are reported in SI.

2.2.3 Drought tests

The tests were started by weighing empty pots and pouring the same amount of soil into them – the weight of a pot filled with soil was to be 160 g. Then, young tobacco plants, Nicotiana Tabacum var. Xanthi, at an equal growth stage were planted in the prepared pots. For 1 week, the plants were watered regularly every 2 days. One week after planting, 4 plants from one test sample were sprayed to fully wet the leaves (about 10 mL of solution per plant) with an aqueous solution of the selected substance (1, 1.1, 1.2, 2, 2.1, 2.2, 3, 3.1, 4, 4.1) at a concentration of 250 mg L−1 (or at a concentration of 20 mg L−1 in the case of the salt with the benzo[1,2,3]thiadiazole-7-carboxylate anion (2, 1.2, 2.2)). The plants continued to be watered every 2 days. Then, 1 week after treating the plants, the plants were watered generously (200 mL per pot), waited 15 min for the excess water to pour out through the holes at the bottom of the pot, and all the pots were weighed. The resulting weights were considered 100% ppw. Plants were not watered and pots were weighed every 2 days checking if 30% of the initial pot weight was reached. Positive control plants continued to be watered every 2 days. When the pot reached 30% ppw, the plants were again watered generously and weighed again to determine the new 100% ppw. On the third occasion that the plants reached 30% ppw, they were watered and weighed the next day (the above-ground part of the plant). The plants were then dried to determine the dry weight of the plants. The results obtained are shown in Table 3. Data were collected in three replicates and statistically subjected to the one-way analysis of variance. The % ppw value was calculated from the following formula:

PPW = ( ( A D G ) / O ) × 100 % ,

where A is the mass of water + soil, D is the mass of an empty pot, G is the mass of dry soil (soil dried at 105°C for 24 h), O is the mass of water and soil at the moment of reaching full saturation with water in the pot (100% ppw) minus dry mass of soil and mass of pot.

3 Results

Successfully, the new salts composed of plant resistance inducer anion and N,N,N,N′,N′,N′-hexamethylpropane-1,3-diammonium (POL1) cation (5 salts), N,N,N,N′,N′,N′-hexamethylbutane-1,4-diammonium (POL2) cation (5 salts), spermidine salicylate, and spermine salicylate (Figure 1) were synthesized and their physical and biological properties were reported. Details of the synthesis procedures and spectral data are given in SI.

Figure 1 
               Synthesized PA-based salts.
Figure 1

Synthesized PA-based salts.

For all of the obtained salts, thermogravimetric analysis was performed. The results show the diversity of thermal stability of resulting salts, ranging from 220.6°C (2.1) to 297.9°C (1). The exchange of the iodide anion (inorganic) for an organic anion resulted in a significant decrease in T 5%onset values for the obtained compounds, suggesting that the thermal stability of salts with N,N,N,N′,N′,N′-hexamethylpropane-1,3-diammonium and N,N,N,N′,N′,N′-hexamethylbutane-1,4-diammonium cations depends on the counterion used. In addition, it was noted that the values of the T 5%onset also depend on the structure of the cation. Salts with N,N,N,N′,N′,N′-hexamethylpropane-1,3-diammonium cation have higher T 5%onset values than salts with the same anions and N,N,N,N′,N′,N′-hexamethylbutane-1,4-diammonium cation. Of all the organic anion salts obtained, the highest values of the T 5%onset were observed for the salts with the nicotinate anion: 277.3 and 246.7°C (1.5 and 2.5), while the lowest values were observed for the salts with the salicylate anion: 256.1 and 220.6°C (1.1 and 2.1). All salts were characterized by a one-step decomposition profile.

The melting point and glass transition of new salts were determined using Differential Scanning Calorimetry (Table 1). In general, novel compounds obtained during the presented research had melting points between 76.0°C (2.4), and 103.2°C (1.2). Thus, the exchange of the inorganic (iodide) anion for an organic anion resulted in lower melting points for the newly obtained salts. Moreover, the salts with iodide anion melted with simultaneous decomposition. For salts 3.1 and 4.1, which were liquid at room temperature, the melting point was not observed. Moreover, only spermidine salicylate (3.1) and spermine salicylate (4.1) had glass transition of −16.1 and −36.0°C, respectively. For the other salts, the glass transition temperature was not observed.

Table 1

Results of thermal stabilities and melting points

No. Compound Thermal stability T 5%onset (°C) Thermal stability T onset (°C) Melting point T peak (°C) Glass transition T peak (°C)
1 [POL1][I] 297.9 315.3 297.0*
1.1 [POL1][Sal] 256.1 288.7 97.7
1.2 [POL1][BTHCOO] 276.9 290.5 103.2
1.3 [POL1][Sacc] 269.2 291.3 91.5
1.4 [POL1][Ina] 274.3 302.8 82.2
1.5 [POL1][Nic] 277.3 312.0 97.0
2 [POL2][I] 287.9 313.0 284.4*
2.1 [POL2][Sal] 220.6 253.5 89.6
2.2 [POL2][BTHCOO] 239.0 272.7 94.1
2.3 [POL2][Sacc] 229.3 268.4 78.9
2.4 [POL2][Ina] 245.3 260.6 76.0
2.5 [POL2][Nic] 246.7 276.4 91.7
3.1 Spermidine salicylate 254.3 271.3 (a) –16.1
4.1 Spermine salicylate 274.9 289.7 (a) –36.0

(a)The tested salts were in liquid form. No melting point temperature was observed in the tested temperature range (up to –80°C); * – the melting point with simultaneous decay.

Phytotoxicity tests were performed on Agrimonia eupatoria. For 5 days the seeds were grown in a Petri dish in a water solution of the tested PA-based salts at two concentrations of 20 and 125 mg L−1 (for salts 1.2 and 2.2) and 125, 250, and 500 mg L−1 (other salts). The percentage reduction in the weight of treated sprouts in comparison to untreated sprouts is shown in Table 2. The presented results are the average of 3 times replicates. The data were subjected to a one-way analysis of variance. The obtained results show that phytotoxicity enhanced with increase in the concentration. The most phytotoxic effects at 125 mg L−1 were exhibited by compounds 1.2 and 2.2, which reduced sprout weight by 65.8 and 63.6%, respectively. Also, high phytotoxicity was observed for salts with salicylate anion and 2,6-dichloroisonicotinate used at concentrations of 250 and 500 mg L−1, which reduced from 38.3 to 63.5%, while salt 1.1 used at a concentration of 500 mg L−1 completely inhibited the growth of Agrimonia eupatoria seeds. The least phytotoxic were the salts with saccharinate anion (1.4 and 2.4) and nicotinate anion (1.6 and 2.6), which at concentrations of 125 and 250 mg L−1 did not affect the germination of seeds. Thus, the phytotoxicity of the salts obtained depended on the anion introduced and increased in the order: [Nik] < [I] < [Sacc] < [Sal] < [Ina] < [BTHCOO].

Table 2

Phytotoxicity tests of the obtained salts (14.1) were performed on two plant models: N. tabacum var. Xanthi and Agrimonia eupatoria

No. Compound Percentage of sprouts mass reduction of Agrimonia eupatoria Phytotoxicity to tobacco
20 mg L−1 125 mg L−1 250 mg L−1 500 mg L−1
1.1 [POL1][Sal] 28.4h 53.7 –*
2.1 [POL2][Sal] 25.6hi 38.3e 51.5
3.1 Spermidine salicylate 12.9 20.1 38.4
4.1 Spermine salicylate 21.9i 35.5e –*
1.2 [POL1][BTHCOO]** 43.6b 65.8a
2.2 [POL2][BTHCOO]** 43.6b 63.6a
1.3 [POL1][Sacc] 0.0 0.0 7.1m
2.3 [POL2][Sacc] 0.0 0.0 5.2m
1.4 [POL1][Ina] 23.1 48.1d 63.5
2.4 [POL2][Ina] 30.5 48.2d 58.2
1.5 [POL1][Nik] 0.0 0.0 0.0
2.5 [POL2][Nik] 0.0 0.0 0.0
1 [POL1][I] 0.0 5.2 20.7i
2 [POL2][I] 9.4 18.8 22.5i
3 Spermidine 0.0n 7.8 21.5
4 Spermine 0.0 15.8 25.2
C Control 0.0 0.0 0.0 0.0

* – no seed growth was observed, ** – substance was tested at the concentration of 20 mg L−1, “–” – does not cause phytotoxic effects on plants. Means with the same letter (a, b, c, etc.) in a column are not statistically different at α = 0.05.

The obtained salts were also tested for causing harmful symptoms such as the yellowing of leaves, the appearance of necroses on leaves, and the stunting of tobacco plants. Nicotiana tabacum var. Xanthi plants were sprayed with solutions of the tested substances at a concentration of 250 mg L−1 and then after 7 days, it was checked whether phytotoxicity symptoms appeared on the plants. None of the tested PA-based salts induced phytotoxic symptoms on tobacco plants at the tested concentrations.

For the purpose of testing the SAR-inducing activity of the presented PA-based salts, tobacco plants were sprayed with an aqueous solution of the test substance at a concentration of 250 mg L−1 (or 20 mg L−1 in the case of the salts 1.2 and 2.2), and after 7 days, the plants were mechanically infected with TMV. Next the percentage of the total area of necrotic spots caused by virus infection on the leaves was estimated 7 days after infection and compared to untreated plants (control). The reduction in the necrotic area on leaves was determined by using ImageJ software [49]. Results are presented in Figures 2 and 3 and Figure S1 and Table S1. All tested salts showed plant resistance induction activity, which has been expressed by the decreased number of necrotic spots from 49% (1) to 99% (1.3) in comparison to the control. Salts 1.1, 2.1, 1.3, 2.3, 1.4, 2.4, 1.5, 2.5, and 4.1 had statistically significant higher biological activity compared to neutral counterparts of these resistance inducers. In the case of [POL1][BTHCOO] (1.2) and [POL2][BTHCOO] (2.2) salts, the introduction of N,N,N,N′,N′,N′-hexamethylpropane-1,3-diammonium cation and N,N,N,N′,N′,N′-hexamethylbutane-1,4-diammonium cation did not alter the activity of the inducer in ionic form compared to its neutral form. Also, N,N,N,N′,N′,N′-hexamethylpropane-1,3-diammonium and N,N,N,N′,N′,N′-hexamethylbutane-1,4-diamine iodides showed the ability to reduce the necrotic spots by 49 and 62%, respectively. This is probably related to the ability of PAs to induce resistance in plants.

Figure 2 
               The plant resistance induction activity test of the obtained salts (1–4.1). The tested substances were applied to plants as aqueous solutions at concentrations of 20 mg L−1 (salts based on benzo[1,2,3]thiadoazole-7-carboxylate anion) and 250 mg L−1 (other salts) by spraying the plants. The necrotic area on leaves was determined using ImageJ software. The presented results are the average of 3-time replicates. The data were subjected to a one-way analysis of variance. Means with the same letter (a, b, c, etc.) in a column are not statistically different at α = 0.05.
Figure 2

The plant resistance induction activity test of the obtained salts (1–4.1). The tested substances were applied to plants as aqueous solutions at concentrations of 20 mg L−1 (salts based on benzo[1,2,3]thiadoazole-7-carboxylate anion) and 250 mg L−1 (other salts) by spraying the plants. The necrotic area on leaves was determined using ImageJ software. The presented results are the average of 3-time replicates. The data were subjected to a one-way analysis of variance. Means with the same letter (a, b, c, etc.) in a column are not statistically different at α = 0.05.

Figure 3 
               Reduction in the necrotic area caused by a viral infection on plants treated with a solution of the best-tested compounds 1.1, 1.2, 1.4, 2.1, 2.2, and 2.4 in comparison with the control plants (C).
Figure 3

Reduction in the necrotic area caused by a viral infection on plants treated with a solution of the best-tested compounds 1.1, 1.2, 1.4, 2.1, 2.2, and 2.4 in comparison with the control plants (C).

The properties of PAs allow the plant to survive drought stress. In the presented study, our goal was to examine the ability of the obtained compounds to protect the plant from drought stress. For this research, salts composed of a natural resistance anion in plants (the salicylic anion) and a synthetic resistance anion in plants (the benzo[1,2,3]thiadiazole-7-carboxylate anion) were selected. Tobacco plants were sprayed with aqueous solutions of the following compounds: 1, 1.1, 2, 2.1, 3, 3.1, 4, and 4.1 at a concentration of 250 mg L−1, or a concentration of 20 mg L−1 in the case of the salt with the benzo[1,2,3]thiadiazole-7-carboxylate anion (2, 1.2, and 2.2). The experiments consisted of treating tobacco plants with drought stress 3 times (soil moisture of 30% sustained for 24 h was taken as drought stress). After drought stress was achieved, the plant was then watered (until soil moisture reached 100%) and not watered again. More detailed information is provided in the SI. After the third drought stress, plants were watered and weighed after 3 days to determine plant fresh matter. The plants were then dried (24 h at 60°C and 24 h at 103°C) to determine plant dry matter. The experiment included two control trials: a positive control (watered throughout the experiment) and negative control (not treated with the studied substances but exposed to the same drought stress as the other plants treated with the obtained compounds). Table 3 shows the average of fresh matter, the average of dry matter, and the ratio of fresh matter to dry matter of the test tobacco plants treated with the tested compounds exposed to drought stress. The presented results are the average of 3-times replicates. The data were subjected to a one-way analysis of variance. The highest values of fresh matter and dry matter were obtained for control plants that were watered regularly throughout the experiment. The fresh matter of almost all plants treated with the tested compounds did not significantly differ from control (untreated) plants exposed to drought stress.

Table 3

Average fresh matter, average dry matter, and average ratio of the dry-to-fresh matter of plants treated with presented salts based on salicylic and benzo[1,2,3]thiadoazole-7-carboxylate anion

No. Test sample Average fresh matter (g) Average dry matter (g) Average ratio of dry matter-to-fresh matter
C Control 38.98a 3.90e 0.13hi
C D Control exposed to drought stress 28.78bc 2.54g 0.12hi
SA Salicylic acid 24.77cd 2.60g 0.11hi
1.1 [POL1][Sal] 24.71cd 2.57g 0.11hij
2.1 [POL2][Sal] 27.87bcd 3.08fg 0.11hij
3.1 Spermidine salicylate 29.44bc 3.03fg 0.10hij
4.1 Spermine salicylate 29.17bc 3.16ef 0.11hij
BTH BTHCOOH* 24.17cd 2.48g 0.11hij
1.2 [POL1][BTHCOO]* 21.41cd 2.19g 0.11hij
2.2 [POL2][BTHCOO]* 27.80bcd 2.76g 0.10ij
1 [POL1][I] 30.33bc 3.26ef 0.11hij
2 [POL2][I] 31.80bc 3.11fg 0.10ij
Spm Spermine 25.83bcd 2.56g 0.09j
Spd Spermidine 24.94cd 2.63g 0.09j

*Substance tested at the concentration of 20 mg L−1. Means with the same letter (a, b, c, etc.) in a column are not statistically different at α = 0.05.

The exception is the fresh matter of plants treated with compound 1.2, which was significantly lower than the dry matter obtained for plants from the negative control (untreated plants subjected to drought stress). Furthermore, it was observed that plants treated with compounds 4.1 and 1 had significantly higher dry matters compared to the dry matters obtained for the negative control plants (untreated plants subjected to drought stress). On the other hand, the dry matters obtained for plants treated with compounds 2.2, 3.1, and 2 were also higher than the dry matters of the negative control plants (untreated plants exposed to drought stress), but these values were not statistically significant. The dry-to-fresh matter ratio shows the plant’s ability to accumulate water during exposure to drought stress. The obtained values of the dry-to-fresh matter ratio of the tested plants show that the plants treated with spermine and spermidine accumulated the most water. The other values were not statistically different from one another.

4 Conclusion

In this study, we described 12 new salts, derivatives of SAR inducers with PA cation. All compounds were characterized in terms of their physicochemical and biological properties. The thermal stability of the obtained compounds was in the range of 220.6–297.9°C. The determining melting points for the studied compounds ranged from 76.0 to 103.2°C. Glass transition temperatures were determined for only two of the studied compounds. Phytotoxicity studies on turnip sprouts showed that most of the obtained substances affected the reduction in sprout weight. It was observed that phytotoxicity of the tested salts depended on the introduced anion and increased in the following order: nicotinate anion < iodide anion < saccharinate anion < salicylate anion < 2,6-dichloroisonicotinate anion < benzo[1,2,3]thiadiazole-7-carboxylate anion. In experiments testing plant resistance induction activity, it was shown that all the salts obtained reduced necrosis on tobacco leaves infected with TMV in the range of 49–99%. In addition, tobacco plants treated with most of the salts tested had significantly fewer necrotic spots compared to plants sprayed with unmodified inducers. In drought tests, it was shown that application of most of the obtained substances to tobacco plants increases the dry matter of plants exposed to drought stress. The substances presented in this article are designed to induce natural plant resistance against pathogens and to protect the plant against abiotic stresses such as drought. This approach to the designing of bifunctional ionic compounds allows the creation of new plant protection products that simultaneously prevent the occurrence of two adverse environmental factors that have a significant impact on reducing crop quality and yield.

Acknowledgements

Thanks to Maciej Nowak for his help with the graphics program. This article and the research behind it would not have been possible without the exceptional support of my supervisor, Prof. Marcin Śmiglak. His enthusiasm, knowledge, and exacting attention to detail have been an inspiration and kept my work on track from my first encounter with ionic liquids.

  1. Funding information: This work was supported by the National Science Centre (Poland), project PRELUDIUM (No. 2018/29/N/NZ9/01813) – “Examination of the impact of addition anti-stress agent on decreasing the phytotoxic effect caused by the application of SAR inducer to plant” co-financed by European Union. The work was supported by Grant no. POWR.03.02.00-00-I023/17 co-financed by the European Union through the European Social Fund under the Operational Program Knowledge Education Development.

  2. Author contributions: P.C. – conceptualization, methodology, analysis, investigation, writing – original draft and writing – review and editing. The author has carefully revised and approved the final version of the manuscript.

  3. Conflict of interest: There are no conflicts to declare.

  4. Ethical approval: The conducted research is not related to either human or animal use.

  5. Data availability statement: All data generated or analyzed during this study are included in this published article and its supplementary information files.

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Received: 2022-09-18
Revised: 2022-11-22
Accepted: 2022-11-28
Published Online: 2022-12-31

© 2022 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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