Abstract

The regulatory effect of new synthetic azaheterocyclic compounds, furopyrimidine derivatives on the vegetative growth and adaptation of maize (Zea mays L.) variety Mas 24.C to abiotic stresses such as heat and drought was studied. A comparative analysis of the regulatory effect of these synthetic azaheterocyclic compounds at a concentration of 10^-6M with the regulatory effect of phytohormones: auxin IAA (1H-indol-3-yl)acetic acid) and cytokinin Kinetin (N-(2-Furylmethyl)-7H-purin-6-amine), as well as known synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), sodium and potassium salts of methyl-2-mercapto-4-hydroxypyrimidine (Methyur and Kamethur) at a similar concentration on maize growth and adaptation to heat and drought stresses was also carried out. After 3 weeks of maize plant growth, morphological parameters such as average shoot length (mm), average root length (mm), average number of the roots (pcs), average biomass of 10 plants (g), as well as biochemical parameters such as chlorophyll and carotenoid content (mg/g fresh weight) and total soluble protein content (g/100 g FW) of maize plants were measured. The most active synthetic azaheterocyclic compounds, furopyrimidine derivatives that showed auxin- and cytokinin-like regulatory effects on improving maize growth and its adaptation to heat and drought stresses, were selected and their chemical structures were analyzed. The practical application of the most physiologically active synthetic azaheterocyclic compounds, furopyrimidine derivatives, for regulating the vegetative growth of maize and its adaptation to heat and drought stresses is proposed.

Keywords:Zea mays L; Auxin IAA; Cytokinin kinetin; Ivin; Methyur;Kamethur; furopyrimidine derivatives; heat and drought stresses

Introductıon

Maize (Zea mays L.) is one of the most important grain and oilseed crops used in human nutrition due to its high content of nutrients such as proteins, lipids, carbohydrates, dietary fiber, vitamins A and E from grains, as well as a raw material for the biofuel industry and the production of feed for livestock and poultry [1]. Maize can play a role in carbon sequestration by capturing atmospheric carbon dioxide and reducing greenhouse gas emissions, biomass production, and soil conservation in crop rotations [2-5].

Currently, maize production tends to decline due to abiotic stress factors such as rising global average air temperature, falling relative humidity, water deficit, uneven rainfall distribution, increased solar radiation, as well as soil salinization and depletion [6]. Among the abiotic stresses, heat and drought are the most negative factors that reduce plant productivity by disrupting the processes of respiration, photosynthesis and metabolism, reducing the uptake of nutrients from the soil through the root system, declining chlorophyll content and water accumulation in plant leaves, and reducing the reproductive capacity of plants [7-11]. According to FAO and a recent comprehensive study conducted in framework of Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) and Agricultural Model Intercomparison and Improvement Project (AgMIP) on the impacts of world climate change on agriculture, if current climate change continues, maize yields will decline sharply by 20–45% by 2100 [12].

As is known, the root system plays a key role in the growth and development of above-ground plant organs: shoots, leaves, fruits and seeds in the vegetative and reproductive phases, providing plants with water, micro- and macroelements, as well as organic matter from the soil, and enhances plant adaptation to abiotic stress [13-14]. Phytohormones auxins and cytokinins play a central role in the formation, organization, and maintenance of root and shoot meristems, starting from embryogenesis and continuing with postembryonic development [15-19]. Since homeostasis and metabolism of endogenous auxins and cytokinins change in different phases of growth and development, as well as under the influence of stress factors [20-22], synthetic analogues of these phytohormones, capable of exerting a direct regulatory effect on plant growth or manipulating the biosynthesis and metabolism of endogenous phytohormones, are widely used for exogenous treatment of agricultural crops in order to improve their growth and increase resistance to stress factors [23-30]. Currently, maize cultivation is also based on the use of phytohormones auxins and cytokinins to improve plant growth and enhance photosynthesis in leaves, increase plant productivity and its resistance to biotic and abiotic stresses, among which heat and drought are the most negative stress factors for maize yields [31-35].

Today, the development of new environmentally friendly plant growth regulators based on synthetic low-molecular-weight azaheterocyclic compounds, pyridine and pyrimidine derivatives capable of exerting a regulatory effect on plant growth similar to the phytohormones auxins and cytokinins is a very promising task for modern agriculture [36-41]. Among these classes of synthetic compounds, the most attractive as plant growth regulators are the well-known synthetic 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 new pyrimidine derivatives that exhibit physiological activity at low non-toxic concentrations and are capable of reducing the toxic effects of pesticides, which is of great importance for the environmental ecology and human health [40, 42-52]. The main advantage of using pyridine and pyrimidine derivatives in agriculture is their regulatory effect, similar to the phytohormones auxins and cytokinins, on agricultural crops of various species and varieties at the vegetation stage, starting with seed germination, subsequent formation and growth of roots, shoots, leaves, and reproductive organs of plants, as well as their effect on increasing the adaptation of plants to abiotic stress factors [42-62].This is why the development of new maize plant growth regulators based on pyridine and pyrimidine derivatives has the greatest potential for modern agriculture [42-44].

The main objective of this work is to study the regulatory effect of new synthetic low-molecular-weight azaheterocyclic compounds, furopyrimidine derivatives, in comparison with the regulatory effect of the phytohormones auxin IAA and cytokinin Kinetin, or known synthetic azaheterocyclic compounds such as Ivin, Methyur and Kamethur, on the vegetative growth of maize (Z. mays L.) variety Mas 24.C and its adaptation to heat and drought.

Materials and Methods

Chemical structures of phytohormones and synthetic compounds

Phytohormones auxin IAA and cytokinin Kinetin were produced by Sigma-Aldrich, USA. Synthetic low-molecular-weight azaheterocyclic compounds: Ivin, Methyur, Kamethur, as well as furopyrimidine derivatives (compounds № 1-12) 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.

Chemical structures of phytohormones: auxin IAA (1H-indol- 3-yl) acetic acid) and cytokinin Kinetin (N-(2-Furylmethyl)- 7H-purin-6-amine), and 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 furopyrimidine derivatives (compounds № 1 – 12) are described in (Table 1).

Seed treatment and plant growing conditions

The seeds of maize (Z. mays L.) variety Mas 24.C were sterilized with 1 % KMnO₄ solution for 10-15 min., then treated with 96 % ethanol solution for 1 min, after which they were washed three times with sterile distilled water. The sterilized seeds were then placed in the plastic cuvettes each containing 15-20 seeds on the perlite moistened with solutions of phytohormones: auxin IAA (1H-indol-3-yl)acetic acid) or cytokinin Kinetin (N-(2-Furylmethyl)- 7H-purin-6-amine), or synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), or sodium and potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur, Kamethur), or furopyrimidine derivatives (compounds № 1 – 12) at a concentration of 10-6M.

Seed germination was carried out in a thermostat in the dark at a temperature of 20-22 °C for 48 hours. Seedling cultivation was carried out in a climate chamber, in which the plants were grown for 3 weeks under a light intensity of 3000 lux, a light/dark regime of 16/8 h., and under conditions of abiotic stresses: heat (at an increased temperature to 35 °C) and drought (with reduced watering by 50%). Control maize plants were germinated from seeds moistened with distilled water and grown under similar conditions of abiotic stress factors: heat and drought.

Comparative analysis of average plant growth parameters (length of the shoots (mm), length of the roots (mm), number of the roots (pcs), and biomass of 10 plants (g)) was performed according to the methodical manual [63]. Plant growth parameters determined at the end of the 3-week period on experimental plants, compared with similar parameters of control plants, were expressed in %.

Determination of chlorophyll and carotenoid content

To extract photosynthetic pigments (chlorophylls and carot enoids) from plant leaves, we homogenized the sample (500 mg) of leaves 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 (mg/g fresh weight) in plant leaves was calculated in accordance with formula [64-65]:
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 1g of fresh weight 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 ratio of chlorophyll and carotenoid content between experimental and control plants was expressed in %.

Determination of total soluble protein content

The content of total soluble protein (g/100 g FW) in plant leaves was determined by the Bradford technique [66]. To make the plant extracts, a sample (100 mg) of plant leaves was homogenized in a porcelain mortar in a 0.1 M sodium phosphate buffer (pH 6.0–8.0) at a weight-to-volume ratio of 1:5 at 4 °C for 1 h. The resulting homogenates were centrifuged at 8000 g in a refrigerated centrifuge K24D (manufactured by MLW, Engelsdorf, Germany) at 4°C for 15 min. A volume of 1.5 mL of distilled water and 1.5 mL of Coomassie Brilliant Blue G-250 reagent (manufactured by Bio-Rad, 500-0006) were added to 50 mL of the obtained supernatant. The resulting mixture was stirred for 10 min. The optical density (OD) of total soluble protein was then determined using spectrophotometer Specord M-40 at a wavelength of 595 nm. The total soluble protein content (g of protein per 100 g of fresh weight (FW) of plant material) in the sample was quantified using a calibration curve based on the optical density (OD) measurements of standard samples containing 1.5 mL of bovine serum albumin (BSA) solution and 1.5 mL of Coomassie Brilliant Blue G-250 reagent (manufactured by Bio-Rad, 500-0006). The total soluble protein content in the leaves of experimental plants was calculated relative to that of the control plants and expressed in %.

Statistical processing of the experimental data

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) [67].

Results and Discussion

Regulatory effect of furopyrimidine derivatives on morphometric parameters of maize plants

The vegetative growth of maize (Z. mays L.) variety Mas 24.C under heat and drought conditions was studied. A comparative analysis of the regulatory effect of phytohormones auxin IAA and cytokinin Kinetin, known and well-studied in various agricultural crops synthetic azaheterocyclic compounds, derivatives of N-oxide- 2,6-dimethylpyridine (Ivin), or sodium and potassium salts of 6-methyl-2-mercapto-4-hydroxypyrimidine (Methyur, Kamethur) [42-51, 53-62], as well as new synthetic azaheterocyclic compounds, furopyrimidine derivatives (compounds № 1-12), applied at a concentration of 10-6M, on the formation and development of roots and shoots of maize plants in the vegetative phase under conditions of heat and drought stress was carried out (Figure 1).

Treatment of maize plants with the synthetic azaheterocyclic compounds such as Ivin, Methyur, Kamethur and furopyrimidine derivatives had a positive effect on the growth and development of roots and shoots, as also increased the resistance of plants to heat and drought stresses, compared to control maize plants. The regulatory effect of synthetic azaheterocyclic compounds on the growth and development of roots and shoots of maize plants under heat and drought stresses was similar to or higher than the effects of phytohormones auxin IAA and cytokinin Kinetin (Figure 1).

Statistical analysis of the morphometric parameters of 3-week-old maize plants, grown under heat and drought stresses, showed that known synthetic azaheterocyclic compounds, derivatives of sodium and potassium salts of 6-methyl-2-mercapto- 4-hydroxypyrimidine (Methyur, Kamethur), as well as new synthetic azaheterocyclic compounds, furopyrimidine derivatives (compounds № 2, 3, 4, 5, 6, 7, 8, 9, 11, 12) revealed the highest regulatory effect, which was similar to or exceeded the effects of auxin IAA and cytokinin Kinetin. Synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin) and furopyrimidine derivatives (compounds № 1 and 10) revealed a lower regulatory effect, which was similar to or lower than the effect of auxin IAA and cytokinin Kinetin.

According to the length of the shoots of maize plants, grown under heat and drought stresses, the highest regulatory effect was demonstrated by synthetic compounds, derivatives of 6-methyl- 2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur) and furopyrimidine derivatives (compounds № 4, 5, 7, 8, 9 and 10), their parameters increased: by 43,92 % – under effect of Methyur, by 38,17 % – under effect of Kamethur, by 39,75–63,25 % – under effect of compounds № 4, 5, 7, 8, 9 and 10, compared to control maize plants (Figure 2)

A lower regulatory effect according to the length of the shoots of maize plants, grown under heat and drought stresses, was demonstrated by synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin) and furopyrimidine (compounds № 1, 2, 3, 6, 11 and 12), under the effect of which these parameters increased: by 19,08 % - under effect of Ivin, by 22,08–34,28 %- under effect of compounds № 1, 2, 3, 6, 11 and 12, compared to control maize plants (Figure 2).

Auxin IAA and cytokinin Kinetin showed similar regulatory effects according to the length of the shoots of maize plants, grown under heat and drought stresses, under the effect of which these parameters increased: by 44,52% - under effect of IAA, by 31,1% - under effect of Kinetin, compared to control maize plants (Figure 2).

According to the length of the roots of maize plants, grown under heat and drought stresses, the highest regulatory effect was demonstrated by synthetic compounds, derivatives of 6-methyl- 2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur) and furopyrimidine derivatives (compounds № 2, 3, 4, 6, 8 and 9), under the effect of which these parameters increased: by 253,05 %- under effect of Methyur, by 200% - under effect of Kamethur, by 45,12–254,88% – under effect of compounds № 2, 3, 4, 6, 8 and 9, compared to control maize plants (Figure 3).

A lower regulatory effect according to the length of the roots of maize plants, grown under heat and drought stresses, was demonstrated by synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin) and furopyrimidine (compounds № 1, 5, 7, 10, 11 and 12), under the effect of which these parameters increased: by 73,17% - under effect of Ivin, by 85,37-135,37% - under effect of compounds № 1, 5, 7, 10, 11 and 12, compared to control maize plants (Figure 3).

Auxin IAA and cytokinin Kinetin showed similar regulatory effects according to the length of the roots of maize plants, grown under heat and drought stresses, under the effect of which these parameters increased: by 119,51% - under effect of IAA, by 112,2 % - under effect of Kinetin, compared to control maize plants (Figure 3).

According to the number of the roots of maize plants, grown under heat and drought stresses, the highest regulatory effect was demonstrated by synthetic compounds, derivatives of 6-methyl- 2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur) and furopyrimidine derivatives (compounds № 2, 4, 5, 6, 7, 8, 9, 11 and 12), under the effect of which these parameters increased: by 86,67% – under effect of Methyur, by 175,56% – under effect of Kamethur, by 40–96,67% – under effect of compounds № 2, 4, 5, 6, 7, 8, 9, 11 and 12, compared to control maize plants (Figure 4).

A lower regulatory effect according to the number of the roots of maize plants, grown under heat and drought stresses, was demonstrated by synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin) and furopyrimidine (compounds № 1, 3 and 10), under the effect of which these parameters increased: by 16,67% – under effect of Ivin, by 23,33– 38,89% – under effect of compounds № 1, 3 and 10, compared to control maize plants (Figure 4).

Auxin IAA and cytokinin Kinetin showed similar regulatory effects according to the number of the roots of maize, grown under heat and drought stresses, under the effect of which these parameters increased: by 33,33% – under effect of IAA, by 26,67% – under effect of Kinetin, compared to control maize plants (Figure 4).

According to the biomass of 10 maize plants, grown under heat and drought stresses, the highest regulatory effect was demonstrated by synthetic compounds, derivatives of 6-methyl- 2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur) and furopyrimidine derivatives (compounds № 2, 3, 4, 5 та 6), under the effect of which these parameters increased: by 79,6% – under effect of Methyur, by 69,4% – under effect of Kamethur, by 48,76–71,01% – under effect of compounds № 2, 3, 4, 5 та 6, compared to control maize plants (Figure 5).

A lower regulatory effect according to the biomass of 10 maize plants, grown under heat and drought stresses, was demonstrated by synthetic azaheterocyclic compounds, derivatives of N-oxide- 2,6-dimethylpyridine (Ivin) and furopyrimidine (compounds № 1, 7, 8, 9, 10, 11 and 12), under the effect of which these parameters increased: by 45,59% – under effect of Ivin, by 27,28– 39,96% – under effect of compounds № 1, 3 and 10, compared to control maize plants (Figure 5).

Auxin IAA and cytokinin Kinetin showed similar regulatory effects according to the biomass of 10 maize plants, grown under heat and drought stresses, under the effect of which these parameters increased: by 74,96% – under effect of IAA, by 59,64% – under effect of Kinetin, compared to control maize plants (Figure 5).

Thus, the results obtained confirmed the positive regulatory auxin-like and cytokinin-like effect of known synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), 6-methyl-2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur), and new furopyrimidine derivatives № 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, on increasing the morphometric parameters of 3-week-old maize (Z. mays L.) variety Mas 24.C, grown under heat and drought stresses. Obviously, this fact can be explained by the auxin-like and cytokinin-like regulatory effect of synthetic azaheterocyclic compounds on enhancing the division, elongation and differentiation of root and shoot meristem cells during the growth and development of maize in the vegetative phase, and protecting plants from damage caused by heat and drought stress, as well as preventing wilting and death of plants [15–19].

Regulatory effect of furopyrimidine derivatives on chlorophyll and carotenoid content in maize plants

A comparative analysis of the regulatory effect of auxin IAA, cytokinin Kinetin and synthetic azaheterocyclic compounds: Ivin, Methyur, Kamethur and furopyrimidine derivatives (№ 1-12) at a concentration of 10-6M on chlorophyll and carotenoid content (mg/g fresh weight), which are the main indicators of plant productivity and photoprotection [64, 65, 68-70], in the leaves of 3-week-old maize (Z. mays L.) variety Mas 24.C, grown under heat and drought stresses, was carried out.

The conducted studies showed that the highest regulatory effect on chlorophyll and carotenoid content in maize leaves was found by synthetic compounds, derivatives of 6-methyl-2-mercapto- 4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur), and furopyrimidine derivatives № 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 and 12, compared to control maize plants (Figure 6). The content of chlorophylls a, b, a+b and carotenoids in the leaves of maize, grown under heat and drought stresses, increased under the effect of these compounds as follows: chlorophyll a increased: by 39,58% - under the effect of Methyur, by 28,81% - under the effect of Kamethur, by 22,34– 47,33% - under the effect of furopyrimidine derivatives № 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 and 12; chlorophyll b increased: by 7,7% - under the effect of Methyur, by 7,1% - under the effect of Kamethur, by 6,6–61,78% - under the effect of furopyrimidine derivatives № 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 and 12; chlorophylls a+b increased: by 30,35% - under the effect of Methyur, by 22,52% - under the effect of Kamethur, by 10,73– 51,51% - under the effect of furopyrimidine derivatives № 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 and 12; carotenoids increased: by 51,37% - under the effect of Methyur, by 17,58% - under the effect of Kamethur, by 3,31–43,33% - under the effect of furopyrimidine derivatives № 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 and 12, compared to control maize plants (Figure 6).

Auxin IAA and cytokinin Kinetin had similar regulatory effects on the content of chlorophylls a, b, a+b and carotenoids in the leaves of maize, grown under heat and drought stresses; these indicators increased under the effect of these compounds as follows: chlorophyll a increased: by 26,79% - under the effect of IAA, by 34,67% - under the effect of Kinetin; chlorophyll b increased: by 8,34 % - under the effect of Kinetin; chlorophylls a+b increased: by 13,75% - under the effect of IAA, by 27,05% - under the effect of Kinetin; carotenoids increased: by 37,31% - under the effect of IAA, by 39,91% - under the effect of Kinetin, compared to control maize plants (Figure 6).

A lower regulatory effect on the content of chlorophylls a, b, a+b and carotenoids in the leaves of maize, grown for 3 weeks under heat and drought stresses, was found by synthetic compound, a derivative of N-oxide-2,6-dimethylpyridine (Ivin) and furopyrimidine derivative № 5, compared to control maize plants (Figure 6). The content of chlorophylls a, b, a+b and carotenoids increased under the effect of these compounds as follows: chlorophyll a increased: by 19,26% - under the effect of Ivin, by 12,35% - under the effect of furopyrimidine derivative № 5; chlorophylls a+b increased: by 11,81% - under the effect of Ivin, by 6,34% - under the effect of furopyrimidine derivative № 5; carotenoids increased: by 27,66% - under the effect of Ivin, by 15,13% - under the effect of furopyrimidine derivative № 5, compared to control maize plants (Figure 6). At the same time, the synthetic compound Ivin and furopyrimidine derivative № 5 did not show a regulatory effect on the content of chlorophyll b, which was not statistically significantly different or was slightly lower than in control maize plants (Figure 6).

Thus, the results obtained confirmed the positive regulatory cytokinin-like effect of known synthetic azaheterocyclic compounds, derivatives of 6-methyl-2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur), and new furopyrimidine derivatives № 1, 2, 3, 4, 6, 7, 8, 9, 10, 11 and 12, on increasing the content of photosynthetic pigments - chlorophylls and carotenoids in the leaves of 3-week-old maize (Z. mays L.) variety Mas 24.C. Obviously, this fact can be explained by the regulatory cytokinin-like effect of synthetic azaheterocyclic compounds on preventing leaf senescence by enhancing synthesis and slowing down degradation of chlorophyll a, b and carotenoids in plant cells [71-73], which plays a key role in photosynthesis of maize plants and ensure their productivity and photoprotection under conditions of heat and drought stresses [68-70].

Regulatory effect of furopyrimidine derivatives on total soluble protein content in maize plants

A comparative analysis of the regulatory effect of auxin IAA, cytokinin Kinetin and synthetic azaheterocyclic compounds: Ivin, Methyur, Kamethur and furopyrimidine derivatives (№ 1-12) at a concentration of 10-6M on total soluble protein content (g/100 g FW), which is the main indicator of plant growth and metabolism [66-74], in the leaves of 3-week-old maize (Z. mays L.) variety Mas 24.C, grown under heat and drought stresses, was carried out.

The conducted studies showed that the highest regulatory effect on total soluble protein content in maize leaves was found by synthetic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), 6-methyl-2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur), and furopyrimidine derivatives № 4, 5, 7, 8, 10, 11 and 12, compared to control maize plants (Figure 7). The total soluble protein content in the leaves of maize plants, grown under heat and drought stresses, increased under the effect of these compounds as follows: by 156,83% - under the effect of Ivin, by 124,75% – under effect of Methyur, by 163,51% – under effect of Kamethur, by 85,78–136,89% – under effect of compounds № 4, 5, 7, 8, 10, 11 and 12, compared to control maize plants (Figure 7).

Auxin IAA and cytokinin Kinetin had similar regulatory effects on total soluble protein content in the leaves of maize plants, grown under heat and drought stresses; this indicator increased under the effect of these phytohormones as follows: by 138,38% - under the effect of IAA, by 153,24 % - under the effect of Kinetin, compared to control maize plants (Figure 7).

A lower regulatory effect on total soluble protein content in the leaves of maize plants, grown under heat and drought stresses, was found by synthetic compounds, furopyrimidine derivatives № 6 and 9, compared to control maize plants (Figure 7). The total soluble protein content under the effect of these compounds increased by 4,88–13,84 %, compared to control maize plants (Figure 7).

At the same time, synthetic compounds, furopyrimidine derivatives № 1, 2 and 3 did not exert a regulatory effect on total soluble protein content in the leaves of maize plants, grown under heat and drought stresses; this indicator did not differ statistically significantly under the effect of these compounds or was lower than the indicator of control maize plants (Figure 7).

Thus, the results obtained confirmed the positive regulatory auxin-like and cytokinin-like effect of known synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), 6-methyl-2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur), and new furopyrimidine derivatives № 4, 5, 7, 8, 10, 11 and 12 on increasing the content of total soluble proteins in the leaves of maize, which play a key role in plant growth and metabolism [66-74]. Apparently, the increase in the biosynthesis of total soluble proteins may also be associated with the presence in their fraction of a large number of antioxidant enzymes and drought and heat stress-related proteins, which are involved in the processes of sensing, signaling and protection of maize plants in response to drought and heat stresses [75-79]. Thus, stimulation of the activity of antioxidant enzymatic system, as well as drought and heat stress-related proteins in plants using phytohormones auxins and cytokinins, or synthetic azaheterocyclic compounds may be one of the important defense mechanisms to avoid abiotic damage to plants.

Analyzing the relationship between the chemical structure and the highest regulatory auxin-like and cytokinin-like effect of synthetic azaheterocyclic compounds, furopyrimidine derivatives № 2, 3, 4, 5, 6, 7, 8, 9, 11 and 12, which increased the morphometric parameters of maize plants and increased the content of photosynthetic pigments, and some of these compounds № 4, 5, 7, 8, 11 and 12 also increased the content of total soluble proteins in plants, it is possible to suggest that their high activity similar to phytohormones auxins and cytokinins is associated with the presence of substituents in the chemical structures of the compounds: compound № 2 contains a 2-methoxyethyl substituent in position 3, a methyl group in position 6, and a carboxylic acid ethyl ester group in position 5 of the 4-oxofuro[ 2,3-d]pyrimidine ring; compound № 3 contains a furan-2-ylmethyl substituent in position 3, a methyl group in position 6, and a carboxyl group in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring; compound № 4 contains a cyclohexyl substituent in position 3, a methyl group in position 6, and a carboxyl group in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring; compound № 5 contains a cyclohexyl substituent in position 3, a methyl group in position 6, and a N-(naphthalen-1-yl)amide substituent in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring; compound № 6 contains a cyclohexyl substituent in position 3, a methyl group in position 6, and a N-(4-chlorophenyl)amide substituent in position 5 of the 4-oxofuro[ 2,3-d]pyrimidine ring; compound № 7 contains a benzyl substituent in position 3, a methyl group in position 6, and a methyl 4-amidobenzoate substituent in position 5 of the 4-oxofuro[2,3-d] pyrimidine ring; compound № 8 contains a benzyl substituent in position 3, a methyl group in position 6, and a N-(4-chlorophenyl) amide substituent in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring; compound № 9 contains a benzyl group in position 3, a methyl group in position 6, and a 4-ethylphenylamide group in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring; compound № 11 contains a benzyl group in position 3, a methyl group in position 6, and a 4-methylphenylamide group in position 5 of the 4-oxofuro[ 2,3-d]pyrimidine ring; compound № 12 contains a benzyl group in position 3, a methyl group in position 6, and a carboxyl group in position 5 of the 4oxofuro[2,3-d]pyrimidine ring. The decrease of the regulatory auxin-like and cytokinin-like effect of synthetic azaheterocyclic compounds, furopyrimidine derivatives № 1 and 10 can be explained by the presence of substituents in their chemical structures: compound № 1 contains a 2-methoxyethyl substituent in position 3, a methyl group in position 6, and a carboxyl group in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring; compound № 10 contains a benzyl group in position 3, a methyl group in position 6, and a N-(3,4-dimethylphenyl)amide group in position 5 of the 4-oxofuro[2,3-d]pyrimidine ring.

The results obtained in this work indicate the prospects for practical application in agriculture of the most active synthetic azaheterocyclic compounds: Ivin, Methyur, Kamethur and furopyrimidine derivatives № 2, 3, 4, 5, 6, 7, 8, 9, 11 and 12, which improve the growth of shoots and the roots of maize, increase biomass, enhance the biosynthesis of chlorophylls, carotenoids and proteins in maize, as well as improve plant adaptation to abiotic stress factors such as heat and drought.

Conclusion

The regulatory effect of new synthetic azaheterocyclic compounds, furopyrimidine derivatives № 1–12, on the vegetative grown and development of maize (Z. mays L.) variety Mas 24.C and its adaptation to heat and drought stresses was investigated. The conducted study showed that synthetic azaheterocyclic compounds, furopyrimidine derivatives exert an auxin-like and cytokinin-like regulatory effect on the improving morphometric parameters and enhancing the biosynthesis of chlorophylls, carotenoids, and total soluble proteins in 3-week-old maize (Z. mays L.) variety Mas 24. C, grown under heat and drought stresses. The regulatory effect of synthetic azaheterocyclic compounds applied at concentration of 10-6M was equal to or higher than the regulatory effect of auxin IAA and cytokinin Kinetin, or known synthetic azaheterocyclic compounds, derivatives of N-oxide-2,6-dimethylpyridine (Ivin), 6-methyl-2-mercapto-4-hydroxypyrimidine sodium and potassium salts (Methyur and Kamethur), applied at the same concentration of 10-6M. A correlation between the chemical structure and the selectivity of the regulatory effect of synthetic azaheterocyclic compounds, furopyrimidine derivatives, has been demonstrated. Based on the obtained results, a conclusion was made about the prospects for practical application in agriculture of the most active synthetic azaheterocyclic compounds: Ivin, Methyur, Kamethur and furopyrimidine derivatives № 2, 3, 4, 5, 6, 7, 8, 9, 11 and 12 to improve maize growth and increase adaptation to abiotic stress factors such as heat and drought.

Statement of conflict of interest

The authors are declared that they have no conflict with this research article.

• Research Article
• Abstract
• Introductıon
• Materials and Methods
• Results and Discussion
• Conclusion
• Statement of conflict of interest
• References