Abstract

The development of new pea (Pisum sativum L.) growth regulators that are safe for the environment and human health is a priority task for modern sustainable agriculture. This work is devoted to the screening of new synthetic low-molecular weight azaheterocyclic compounds, thioxopyrimidine derivatives, which have a stimulating effect on the growth and photosynthesis of pea (Pisum sativum L.) variety Pivnich during the vegetation phase. The regulatory effect of new synthetic azaheterocyclic compounds, thioxopyrimidine derivatives, used in a concentration of 10-6M for treating pea seeds, was compared with the regulatory effect of phytohormone auxin IAA (1H-indol-3-yl)acetic acid), as well as known synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), sodium and potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur, Kamethur), used in a similar concentration of 10-6M for treating pea seeds. Comparative analysis of average shoot and root length (mm), average number of roots (pcs), average biomass of 10 plants (g) and photosynthetic pigment content (mg/g FW) of pea plants showed that new synthetic azaheterocyclic compounds, thioxopyrimidine derivatives exhibit a similar or higher regulatory effects to the auxin IAA and the known synthetic azaheterocyclic compounds: Ivin, Methyur and Kamethur. The most physiologically active synthetic azaheterocyclic compounds, thioxopyrimidine derivatives were selected and the relationship between their regulatory effect and chemical structure was analyzed. A conclusion was made about the prospects of using the most active synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), sodium and potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur, Kamethur), as well as selected thioxopyrimidine derivatives as new environmentally friendly growth regulators of pea plants.

Keywords: Pisum sativum L.; IAA; Ivin; Methyur; Kamethur; Thioxopyrimidine Derivatives

Introduction

Pea (Pisum sativum L.) is one of the main agricultural crops belonging to the Fabaceae family, which includes wild and cultivated species grown in 84 countries worldwide [1-4]. Pea seeds, used as a source of human food and animal feed, contain bioactive substances such as protein (20 - 36 %), easily digestible starch (29 - 54 %), carbohydrates (4 - 10 %), essential fatty acids (0.7 - 1.6 %), omega-3 (alpha-linolenic acid, ALA) and omega-6 (linoleic acid, LA), vitamins (A, B1, B2, B3, B4, B6, B9, C, E, K, PP), macroelements and microelements (potassium, phosphorus, magnesium, calcium, iron, zinc, manganese, copper), carotenoids, flavonoids, lectins, phytates, phenolic acids, saponins, galactooligosaccharides, soluble and insoluble dietary fiber, which have nutritional and medicinal properties [5-12]. Pea microgreens, enriched with biologically active substances such as amino acids, chlorophyll, calcium, iron, magnesium, niacin, phosphorus, potassium, polysaccharides, polyphenols, vitamins A, B, C, E, K, lutein, which have anti-inflammatory, antiviral, antioxidant, immunomodulatory properties, are very popular in dietary nutrition and should be considered as a sprouted vegetable used for human health [13,14].

In line with the main objective of the European Green Deal to protect the environment, biodiversity and climate, and to reduce soil, water and air pollution, pea is one of the important crops for sustainable agriculture, which promotes the accumulation of organic nitrogen and carbon in the soil, which leads to increased soil fertility, reduces the use of mineral fertilizers and minimizes their accumulation in soil and water, and can also be used as an intercrop in grain crop rotations, as it is resistant to diseases and pests of cereal crops [1-4]. According to FAOSTAT, the world production of dry pea is estimated at 14.360.000 tons, among the varieties of vegetable peas, the most cultivated are green pea and shell pea with an annual production of more than 17 and 11 million tons worldwide [2,3].

Growing pea plants in field and greenhouse conditions depends on many factors that affect biomass production and nutritional quality of the crop, such as: soil fertility or type of growing medium, temperature, photoperiod, intensity and quality of light, air humidity, fertilization, post-harvest treatment, risk of bacterial or fungal contamination [14-19]. In agricultural practice, plant growth regulators, biostimulants and fertilizers are used to improve the growth of pea plants and increase their resistance to abiotic stresses such as water deficiency, heat, cold, drought, salinity, light and nutrient deficiency [20-25].

Currently, the priority task of modern sustainable agriculture is the development of new environmentally friendly growth regulators to increase the productivity of pea plants. Today, synthetic low-molecular weight azaheterocyclic compounds, which have demonstrated significant biological activity as therapeutic agents against various human diseases [26-28], should also be considered as an environmentally friendly alternative to traditional plant growth regulators [29,30]. Among the various classes of synthetic azaheterocyclic compounds, the most biologically active compounds used in medicine as antiviral, antibacterial, antifungal, anti-inflammatory, anticancer, antituberculosis, antidiabetic, antihypertensive, antimalarial and anthelmintic drugs [31-38], as well as in agriculture as plant growth regulators, herbicides, and insecticides, are pyrimidine derivatives [39-45].

Over the last decade, screening of new synthetic analogues of phytohormones auxins and cytokinins has been carried out among new synthetic azaheterocyclic compounds, pyrimidine derivatives, synthesized at the V.P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine [29,30]. Numerous studies conducted in field and laboratory conditions have shown a high regulatory effect of these classes of synthetic azaheterocyclic compounds, pyrimidine derivatives, used in low concentrations that are non-toxic to the environment and human and animal health in the range from 10- 5M to 10-8M, on accelerating plant growth during the growing season, increasing crop yields and increasing plant resistance to abiotic stress [46-53]. Due to the broad specificity of the regulatory action on different plant species and varieties and the absence of toxic effects on the environment and human health, these synthetic azaheterocyclic compounds, pyrimidine derivatives, can be considered as effective and environmentally friendly regulators of growth and development of pea plants.

The aim of this work is to study the regulatory effect of new synthetic low-molecular weight azaheterocyclic compounds, thioxopyrimidine derivatives, on the growth and photosynthesis of pea (Pisum sativum L.) variety Pivnich during the vegetation phase.

Materials and Methods

Chemical structures of auxin and synthetic compounds

The phytohormone auxin IAA (1H-indol-3-yl)acetic acid) was produced by Sigma-Aldrich, USA. Synthetic low-molecular weight azaheterocyclic compounds, derivatives of N-oxide-2,6- dimethylpyridine (Ivin), sodium and potassium salts of 6-methyl- 2-mercapto-4-hydroxypyrimidine (Methyur and Kamethur) were synthesized at the Department for Chemistry of Bioactive Nitrogen-Containing Heterocyclic Compounds, V.P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry of the National Academy of Sciences of Ukraine. The chemical structures of auxin IAA and synthetic compounds Ivin, Methyur, Kamethur are shown in Figure 1.

Synthetic low-molecular weight azaheterocyclic compounds, thioxopyrimidine derivatives were also synthesized at the Department for Chemistry of Bioactive Nitrogen-Containing Heterocyclic Compounds, V.P. Kukhar Institute of Bioorganic Chemistry and Petrochemistry of the National Academy of Sciences of Ukraine; their chemical structures are presented in Table 1.

Treatment of seeds with auxin and synthetic compounds. The seeds of pea (Pisum sativum L.) variety Pivnich were sterilized with 1 % KMnO4 solution for 15 min, then treated with 96 % ethanol solution for 1 min, after which they were washed three times with sterile distilled water. After this procedure, seeds were placed in the plastic cuvettes (each containing 15-20 seeds) on the perlite moistened with distilled water (control sample) or water solutions of auxin IAA (1H-indol-3-yl)acetic acid or synthetic low-molecular weight azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), sodium and potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur and Kamethur), or thioxopyrimidine derivatives (compounds № 1 – 11) in a concentration of 10-6M (experimental samples). Then seeds were placed in the thermostat for germination in darkness at the temperature 20 - 22 °C during 48 h.

Plant cultivation conditions

The germinated seeds were placed in a climatic chamber, in which the seedlings were grown for 4 weeks under a light/dark regime of 16/8 h, a temperature of 22 - 23 °C, a light intensity of 3000 lux, and an air humidity of 60 - 80 %. A comparative analysis of the morphometric parameters of seedlings (average length of the shoots (mm), average length of the roots (mm), average number of the roots (pcs) and average biomass of 10 plants (g)) was carried out at the end of a 4-week period according to the methodological manual [54]. Morphometric parameters determined on experimental seedlings, in comparison with similar parameters of control seedlings, were expressed as %.

Analysis of chlorophyll and carotenoid content in seedlings

The content of photosynthetic pigments such as chlorophylls and carotenoids (mg/g FW) in seedlings was analyzed according to methodological recommendations [55,56]. To perform the extraction of photosynthetic pigments, we homogenized a sample (500 mg) of leaves separated from seedlings in the porcelain mortar in a cooled at the temperature 10 °С 96 % ethanol at the ratio of 1: 10 (weight:volume) with addition of 0,1-0,2 g CaCO3 (to neutralize the plant acids). The 1 ml of obtained homogenate was centrifuged at 8000 g in a refrigerated centrifuge K24D (MLW, Engelsdorf, Germany) during 5 min at the temperature 4 °С. The obtained precipitate was washed three times, with 1 ml 96 % ethanol and centrifuged at above mentioned conditions. After this procedure, the optical density of chlorophyll a, chlorophyll b and carotenoid in the obtained extract was measured using spectrophotometer Specord M-40 (Carl Zeiss, Germany).

The content of chlorophyll a, chlorophyll b, and carotenoids in leaves was calculated in accordance with formula [55,56]:
Cchl a = 13.36×A664.2 – 5.19×A648.6,
Cchl b = 27.43×A648.6 – 8.12A×664.2,
Cchl (a + b) = 5.24×A664.2 + 22.24×A648.6,
Ccar = (1000×A470 – 2.13×Cchl a – 97.64×Cchlb)/209,

Where, Cchl – concentration of chlorophylls (μg/ml), Cchl a - concentration of chlorophyll a (μg/ml), Cchl b - concentration of chlorophyll b (μg/ml), Ccar - concentration of carotenoids (μg/ ml), А - absorbance value at a proper wavelength in nm.

The chlorophyll and carotenoids content per 1 g of fresh weight (FW) of extracted from leaves was calculated by the following formula (separately for chlorophyll a, chlorophyll b and carotenoids):
A₁=(C×V)/(1000×a₁),
Where, A₁ - content of chlorophyll a, chlorophyll b, or carotenoids (mg/g FW), C - concentration of pigments (μg/ml), V - volume of extract (ml), a₁ - sample of leaves (g).

The content of photosynthetic pigments determined in the leaves of experimental seedlings in relation to control seedlings was expressed as %.

Statistical data analysis

Each experiment was performed three times. Statistical processing of the experimental data was carried out using Student’s t-test with a significance level of P≤0.05; mean values ± standard deviation (± SD) [57].

Results and Discussion

The effect of auxin IAA and synthetic azaheterocyclic compounds on pea growth parameters. The regulatory effect of auxin IAA, known synthetic azaheterocyclic compounds such as N-oxide-2,6-dimethylpyridine (Ivin), sodium and potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur and Kamethur), previously studied on various cereal, leguminous and industrial crops [46- 53,58-70], as well as new synthetic azaheterocyclic compounds, thioxopyrimidine derivatives (№ 1 - 11), previously studied on wheat and barley plants [66-70], on the growth of pea (Pisum sativum L.) variety Pivnich was studied. The obtained experimental and control seedlings are presented in Figure 2.

Comparative analysis of morphometric parameters of pea seedlings (average length of the shoots (mm), average length of the roots (mm), average number of the roots (pcs) and average biomass of 10 plants (g)) showed that synthetic azaheterocyclic compounds, thioxopyrimidine derivatives, exhibit a similar or higher regulatory effect on the growth of shoots and roots of pea seedlings compared to the effect of auxin IAA or known synthetic azaheterocyclic compounds such as Ivin, Methyur, and Kamethur./p>

The highest average length of the shoots (mm) was observed in pea seedlings grown for 4 weeks after seed treatment with Methyur, Kamethur, and thioxopyrimidine derivatives № 1, 2, 6, 7, 8, 9 and 10 compared to control shoots. These synthetic azaheterocyclic compounds increased the average length of the shoots (mm) as follows: Methyur - by 45,71 %, Kamethur - by 42,39 %, thioxopyrimidine derivatives № 1, 2, 6, 7, 8, 9 and 10 - by 41,44 - 67,7 % compared to control shoots (Figure 3)

The lower average length of the shoots (mm) was observed in pea seedlings grown for 4 weeks after seed treatment with auxin IAA, Ivin, and thioxopyrimidine derivatives № 3, 4, 5 and 11 compared to control shoots. It was shown that auxin IAA and these synthetic azaheterocyclic compounds increased the average length of the shoots (mm) as follows: IAA – by 22,58 %, Ivin – by 29,5 %, thioxopyrimidine derivatives № 3, 4, 5 and 11 – by 11,18 – 21,24 % compared to control shoots (Figure 3).

The highest average length of the roots (mm) was observed in pea seedlings grown for 4 weeks after seed treatment with Methyur, Kamethur, and thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11 compared to control roots. These synthetic azaheterocyclic compounds increased the average length of the roots (mm) as follows: Methyur - by 173,33 %, Kamethur - by 275 %, thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11 - by 160,42 – 328,33 % compared to control roots (Figure 4).

The lower average length of the roots (mm) was observed in pea seedlings grown for 4 weeks after seed treatment with auxin IAA, Ivin, and thioxopyrimidine derivatives № 3, 4 and 9 compared to control roots. It was shown that auxin IAA and these synthetic azaheterocyclic compounds increased the average length of the roots (mm) as follows: IAA – by 98,21 %, Ivin – by 93,33 %, thioxopyrimidine derivatives № 3, 4 and 9 – by 15 – 40 % compared to control roots (Figure 4).

The highest average number of the roots (pcs) was observed in pea seedlings grown for 4 weeks after seed treatment with auxin IAA, Ivin, Methyur, Kamethur, and thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 9, 10 and 11 compared to control roots. It was shown that auxin IAA and these synthetic azaheterocyclic compounds increased the average number of the roots (pcs) as follows: IAA – by 112,34 %, Ivin – by 88,97 %, Methyur - by 107,41 %, Kamethur - by 129,3 %, thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 9, 10 and 11 - by 72,84 – 172,22 % compared to control roots (Figure 5).

The lower average number of the roots (pcs) was observed in pea seedlings grown for 4 weeks after seed treatment with thioxopyrimidine derivatives № 3 and 4 compared to control roots. It was shown that these synthetic azaheterocyclic compounds increased the average number of the roots (pcs) by 17,53 – 25,93 % compared to control roots (Figure 5).

The highest average biomass of 10 plants (g) was observed in pea seedlings grown for 4 weeks after seed treatment with auxin IAA, Ivin, Methyur, Kamethur, and thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8 and 10 compared to control plants. It was shown that auxin IAA and these synthetic azaheterocyclic compounds increased the average biomass of 10 plants (g) as follows: IAA – by 73,24 %, Ivin – by 91,98 %, Methyur - by 102,26 %, Kamethur - by 140,92 %, thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8 and 10 - by 72,76 – 153,15 % compared to control plants (Figure 6).

The lower average biomass of 10 plants (g) was observed in pea seedlings grown for 4 weeks after seed treatment with thioxopyrimidine derivatives № 3, 4, 9 and 11 compared to control plants. It was shown that these synthetic azaheterocyclic compounds increased the average biomass of 10 plants (g) by 41,22 – 54,9 % compared to control plants (Figure 6).

Summarizing the results obtained, it should be noted that the highest regulatory effect on the parameters of average shoot length (mm), average root length (mm), average number of the roots and average biomass (g) of 10 plants (pcs) of pea (Pisum sativum L.) variety Pivnich was exhibited by synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11. The regulatory effect of these synthetic compounds was similar to or exceeded the regulatory effect of auxin IAA or known synthetic azaheterocyclic compounds such as Ivin, Methyur and Kamethur. Synthetic compounds, thioxopyrimidine derivatives № 3, 4 and 9, exhibited a lower regulatory effect on the growth parameters of pea plants.

The effect of auxin IAA and synthetic azaheterocyclic compounds on the content of chlorophyll and carotenoids in pea plants

A comparative analysis of the regulatory effect of auxin IAA, known synthetic azaheterocyclic compounds such as Ivin, Methyur, Kamethur, and new synthetic azaheterocyclic compounds, thioxopyrimidine derivatives (compounds № 1 - 11) on the content of chlorophylls and carotenoids (mg/g FW) in seedlings of pea (Pisum sativum L.) variety Pivnich was conducted.

It was found that the highest content of chlorophyll a, chlorophyll b, chlorophylls a+b, and carotenoids was observed in pea seedlings grown for 4 weeks after seed treatment with Ivin, Methyur, Kamethur, and thioxopyrimidine derivatives № 3, 4, 7 and 10 compared to control seedlings. The content of chlorophyll a increased in pea seedlings as follows: by 49,56 % - under the effect of Ivin, by 53,58 % - under the effect of Methyur, by 43,26 % - under the effect of Kamethur, by 33,99 - 42,25 % - under the effect of thioxopyrimidine derivatives № 3, 4, 7 and 10 compared to control plants (Figure 7). The content of chlorophyll b increased in pea seedlings as follows: by 45,76 % - under the effect of Ivin, by 45,29 % - under the effect of Methyur, by 41,18 % - under the effect of Kamethur, by 13 - 43,72 % - under the effect of thioxopyrimidine derivatives № 3, 4, 7 and 10 compared to control plants (Figure 7). The content of chlorophylls a+b increased in pea seedlings as follows: by 48,36 % - under the effect of Ivin, by 50,96 % - under the effect of Methyur, by 42,6 % - under the effect of Kamethur, by 28,7 - 37,29 % - under the effect of thioxopyrimidine derivatives № 3, 4, 7 and 10 compared to control plants (Figure 7). The content of carotenoids increased in pea seedlings as follows: by 32,24 % - under the effect of Ivin, by 37,84 % - under the effect of Methyur, by 42,26 % - under the effect of Kamethur, by 13,78 - 83,61 % - under the effect of thioxopyrimidine derivatives № 3, 4, 7 and 10 compared to control plants (Figure 7).

Pea seedlings grown for 4 weeks after seed treatment with auxin IAA and thioxopyrimidine derivatives № 1, 2, 5, 6, 8, 9 and 11 had somewhat lower content of chlorophyll a, chlorophyll b, chlorophylls a+b, and carotenoids compared to control seedlings. The content of chlorophyll a increased in pea seedlings as follows: by 15,83 % - under the effect of IAA, by 11,53 - 26,16 % - under the effect of thioxopyrimidine derivatives № 1, 2, 5, 6, 8, 9 and 11 compared to control plants (Figure 7). The content of chlorophyll b increased in pea seedlings as follows: by 21,79 % - under the effect of IAA, by 4,98 - 29,4 % - under the effect of thioxopyrimidine derivatives № 1, 2, 5, 9 and 11 compared to control plants (Figure 7). The content of chlorophylls a+b increased in pea seedlings as follows: by 17,71 % - under the effect of IAA, by 1,39 - 25,36 % - under the effect of thioxopyrimidine derivatives № 1, 2, 5, 6, 8, 9 and 11 compared to control plants (Figure 7). The content of carotenoids increased in pea seedlings as follows: by 17,8 - 64 % - under the effect of thioxopyrimidine derivatives № 1, 2, 5, 6, 8, 9 and 11 compared to control plants (Figure 7).

Summarizing the results obtained, we can conclude that synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 3, 4, 7 and 10 showed a highest regulatory effect on the content of chlorophylls and carotenoids in the leaves of pea (Pisum sativum L.) variety Pivnich. The regulatory effect of these synthetic azaheterocyclic compounds was similar to the regulatory effect of known synthetic azaheterocyclic compounds such as Ivin, Methyur and Kamethur. Auxin IAA and synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 1, 2, 5, 6, 8, 9 and 11 showed a somewhat lower regulatory effect on the content of chlorophylls and carotenoids in the leaves of pea plants.

Comparing the regulatory effect of synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 1-11 on plant growth parameters and chlorophyll and carotenoid content, it should be noted that these compounds exhibit a selective regulatory effect. Synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11, showed the highest stimulating effect on the growth of shoots and roots of pea plants, and also caused an increase in the content of chlorophyll and carotenoids in the leaves of pea plants. Synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 3, 4, 7 and 10, showed the highest stimulating effect on the biosynthesis of chlorophylls and carotenoids in the leaves of pea plants, but at the same time their effect on the growth of shoots and roots of pea plants was lower.

It is obvious that synthetic azaheterocyclic compounds, derivatives of thioxopyrimidine № 1, 2, 5, 6, 7, 8, 10 and 11, exhibit more specific phytohormonal auxin- and cytokinin-like regulatory effects on seed germination, growth and development of root and shoot meristems of plants [71-75]. It can also be assumed that all synthetic azaheterocyclic compounds, derivatives of thioxopyrimidine № 1 - 11, also exhibit a specific phytohormonal cytokinin-like regulatory effect on enhancing the synthesis and slowing down the degradation of chlorophyll a, b and carotenoids in plant cells, which play a key role in photosynthesis and plant productivity [76,77].

Analyzing the relationship between the regulatory effect and the chemical structure of new synthetic azaheterocyclic compounds, thioxopyrimidine derivatives, we can conclude that the high regulatory effect of thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11, on the growth parameters of pea plants, is associated with the presence of substituents in their chemical structure: compound № 1 contains a benzenesulfonyl group in position 5, an ethyl group in position 3 of the 2-thioxo-2,3-dihydro- 1H-pyrimidin-4-one ring; compound № 2 contains an allyl substituent in position 3, a phenylsulfonyl group in position 5 of the 2-thioxo-2,3-dihydro-1H-pyrimidin-4-one ring; compound № 5 contains a p-tolyl group in position 6, a cyano group in position 5 of the 4-oxo-2-thioxo-1,2,3,4-tetrahydropyrimidine ring; compound № 6 contains a phenyl group in position 6, a cyano group in position 5 of the 4-oxo-2-thioxo-1,2,3,4-tetrahydropyrimidine ring; compound № 7 contains a methylsulfanyl group in position 2, a p-tolyl group in position 4, and a cyano group in position 5 of the 6-oxo-1,6-dihydropyrimidine ring; compound № 8 contains a methyl group in position 6, a phenyl group in position 4, and an ethoxycarbonyl group in position 5 of the 2-thioxo-1,2,3,4- tetrahydropyrimidine ring; compound № 10 contains a methyl group in position 6, a 4-methoxycarbonylphenyl group in position 4, and an ethoxycarbonyl group in position 5 of the 2-thioxo- 1,2,3,4-tetrahydropyrimidine ring; compound № 11 contains a methyl group in position 6, a 4-hydroxyphenyl group in position 4, and an ethoxycarbonyl group in position 5 of the 2-thioxo-1,2,3,4- tetrahydropyrimidine ring (Table 1).

The decrease in the regulatory effect of synthetic compounds, thioxopyrimidine derivatives № 3, 4 and 9 on the growth parameters of pea plants can be explained by the presence of substituents in their chemical structures: compound № 3 contains a benzyl substituent in position 5, a methyl group in position 6 of the 2-thioxo-2,3-dihydro-1H-pyrimidin-4-one ring; compound № 4 contains a phenyl group in position 3, a benzenesulfonyl group in position 5 of the 2-thioxo-2,3-dihydro-1H-pyrimidin-4- one ring; compound № 9 contains a methyl group in position 6, a 4-methoxyphenyl group in position 4, and an ethoxycarbonyl group in position 5 of the 2-thioxo-1,2,3,4-tetrahydropyrimidine ring (Table 1).

Our previously published works [66-70] devoted to the study of the regulatory effect of synthetic azaheterocyclic compounds, derivatives of thioxopyrimidine № 1-11, on wheat and barley plants, are consistent with the data obtained in the present work. These studies have shown that synthetic compounds, derivatives of thioxopyrimidine № 1-11, exhibit a regulatory effect similar to phytohomones auxins and cytokinins and synthetic compounds Ivin, Methyur and Kamethur on the growth of roots and shoots, and also increase the content of chlorophylls and carotenoids in wheat and barley leaves. We also established a species- and variety-specific relationship between the chemical structure and regulatory activity of these compounds in wheat and barley plants.

It should also be noted that known synthetic azaheterocyclic compounds such as N-oxide-2,6-dimethylpyridine (Ivin), sodium and potassium salts of 6-methyl-2-mercapto-4- hydroxypyrimidine (Methyur and Kamethur), have previously been studied on various cereal, leguminous and industrial crops [46-53,58-70]. These synthetic compounds exhibit a high growthregulating effect on plants, equivalent to or more pronounced than the effect of phytohormones auxins and cytokinins, improve seed germination, the formation and growth of plant shoots and roots, enhance photosynthesis in plant leaves, and increase plant productivity and adaptation to abiotic stresses, which is of great practical, economic, and environmental importance.

Conclusion

The study of the regulatory effect of new synthetic azaheterocyclic compounds, derivatives of thioxopyrimidine № 1-11, on the growth and photosynthesis of pea (Pisum sativum L.) variety Pivnich was conducted. The most active synthetic compounds were selected – thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11, which at a concentration of 10-6M exhibit a regulatory effect similar to the phytohormone auxin IAA (1H-indol- 3-yl)acetic acid) or known synthetic azaheterocyclic compounds such as N-oxide-2,6-dimethylpyridine (Ivin), sodium or potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur, Kamethur), used at the same concentration of 10-6M. The results of this work indicate the prospects for the practical application of the known synthetic azaheterocyclic compounds such as Ivin, Methyur, Kamethur, as well as selected most active synthetic azaheterocyclic compounds, thioxopyrimidine derivatives № 1, 2, 5, 6, 7, 8, 10 and 11, as new effective regulators of growth and photosynthesis of pea plants.

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