Sunflower is a crop that is widespread in the southern regions of Europe and Ukraine in particular. In recent years, due to significant changes in weather conditions—increasing in air temperature, and a prolongation of the period with air temperatures above 8˚C - 10˚C in the northern regions, there has been an expansion of the geographical zone of sunflower cultivation. Establishing the features of sunflower cultivation and development of adaptive cultivation technologies is an important area of research for the northern regions of Europe and Ukraine [8].

In Ukraine, the main oil crop is a sunflower, which produces over 90% of vegetable fats. Gross sunflower production in Ukraine has increased to 12 million tons in recent years. Between 2010 and 2019, the area under sunflower in Ukraine increased by almost one and a half million hectares and reached 6.1 million hectares. Ukraine is a leader in sunflower production and the largest exporter of sunflower oil in the world—5, 6 million tons (57% of total exports in 2018). The gross harvest of sunflower seeds increase was mainly due to the expansion of sown areas. A feature of the soil-climatic zone of the Steppe of Ukraine, where sunflower is mainly grown, is an insufficient amount of precipitation with a significant potential of solar energy. Due to such natural features, almost every year there is an acute shortage of soil moisture, which prevents the planned level of yield [9]. With high plowing levels of soils and oversaturation of crop rotations by sunflower, specific diseases and pests are spread. All this necessitates research on the adaptability of sunflower hybrids in the Northern regions of Ukraine, where in recent decades there has been a significant change in weather conditions and which may be favorable for sunflower growing.

The task of creating and implementing in production hybrids with high yield potential with optimization of growing conditions is complicated by the problem of ecological adaptation: the potential of production conditions can’t grow adequately to the growth of genetically determined yield potential of varieties [10] [11] [12]. The ability to economically and efficiently use environmental factors is a highly adaptive genotype property [13]. The adaptability of hybrids is an extremely important feature of variable abiotic and biotic environmental factors [14] [15]. Sunflower is a culture of the temperate zone and is quite flexible to weather and soil conditions [16] [17]. Sunflower is a salt-tolerant crop [18]. Sunflower is characterized by high adaptability and yield [8]. Optimally formed plant population of sunflower influence the quality of seed for processing [19] [20].

The level of varieties’ and hybrids’ biological potential realization depends on both—growing technology and climatic conditions of a particular year and their interaction [21] [22] [23] [24]. Sunflower yield is determined by several factors, among which the role of hybrid and plants’ standing density are among the determinants [25] [26]. Agrocenosis formation with an optimal plant density and optimal nutrients supply provides an increase in its productivity [27] [28].

Morphological characteristics of the plant are varietal characteristics, but under the influence of environmental factors and cultivation technologies, they may change [29]. Under the influence of nitrogen fertilizers, a yield increase of sunflower hybrids, an increase in the anthode diameter was recorded [30] [31] [32]. The height of plants with plants standing density increasing usually increases, especially with using irrigation [33]. There is also a decrease in the weight of 1000 seeds and seeds number from the anthode [34]. However, with plant density increasing, competition for light between plants increases, which leads to a decrease in vegetative and reproductive plasticity of the plant [35], while lower plant density leads to reduced light absorption by plants what leads to a reduction in yield per unit area [36] [37] [38]. Theoretical studies and practical experience have shown that high yields per unit area are usually associated with low plant competitiveness due to reduced anthode diameter, plant biomass, weight of 1000 seeds, seed yield per plant, unsaturated fatty acids [39] [40].

The climate of the district is temperate-continental. The average temperature in July is +20˚C. The average January temperature is between −6˚C and −8˚C. The growing season lasts an average of 200 - 210 days. The period, when the average daily temperature exceeds +15˚C, is approximately 110 days. Annual sum of temperatures exceeding +10˚C is 2500 - 2600. Period without light frosts on the soil surface lasts about 135 - 140 days. Precipitation averages about 550 - 600 mm. The biggest amount of precipitation, about 75%, falls from April to October.

The analysis of average daily air temperatures was performed in the matching of long-term data to 2005-2015. The average daily temperature only in April steadily passes over +10˚С, but the dynamics of temperature rise over the years are quite different. The average daily air temperature is quite moderate and during the period of active sunflower’s vegetation varies from 14.0˚C to 21.6˚C. The sum of temperatures, both at a biological minimum temperature 10˚C and 5˚C, corresponds to the biological needs of sunflower plants and is not a limiting factor of cultivation. The sum of thermal units for the period April-October, on average for three years, is 3780. During the period of active sunflower vegetation (April-August) the sum of thermal units is 2868 - 3258, significantly exceeding the sum of active and effective temperatures at biologically active temperatures 5˚С and 10˚С.

In the research area, the maximum temperatures are quite comfortable—do not exceed 30˚C, with some exceptions (the second decade of August 2017—36˚C). Temperatures in the range up to 30˚C promote the formation and development of generative organs.

The average amount of precipitation according to long-term data is 556 mm, in the years of research it was: 849, 592, 516 mm, respectively, in 2016, 2017, 2018. However, during the period of sunflower’s active vegetation (May-September) precipitation was 390, 247, 211 mm, respectively, in 2016, 2017, 2018. Hybrids differ significantly by their response to moisture, maximum and minimum temperatures, and other factors.

Growing of sunflower hybrids in the Northern part of the Left-Bank Forest-Steppe of Ukraine produces a consistently high yield. During 2016-2018, yield of hybrids ranged: Ukrainskyi F1—2.16 - 3.11; P63LL06—2.58 - 3.52; NK Brio—3.20 - 4.12; NK Ferti—2.79 - 3.79 t/ha (Table 1). The productivity of sunflower is determined by genetically determined features, but within certain limits, they can change under the influence of environmental factors.

Calculated coefficients of plasticity and yield stability indicate favorable conditions for yields of sunflower hybrids formation.

Stability coefficient for hybrid “Ukrainskyi F1” was 1.68 - 2.30; “P63LL06”—2.51 - 3.14; “NK Brio”—3.15 - 4.63; “NK Ferti”—2.70 - 3.75.

The early-ripening hybrid “Ukrainskyi F1” is approaching its maximum in terms of the level of yield potential realization, in the conditions of research. Coefficients of plasticity among the studied hybrids are the lowest—2.36 - 2.97 and with an average yield of 2.41 - 2.81 t/ha shows bigger stability—1.68 - 2.30 compared to other sunflower hybrids. Hybrid “NK Brio” is highly plastic and, as the results show, the yield potential of this hybrid in the Northern part of the Left-Bank Forest-Steppe has not been fully realized. The maximum yield of the hybrid was formed with a rate of 60 thousand plants/ha - 3.93 t/ha and the coefficient of plasticity increased to 4.84 with a stability coefficient 4.63.

Notes: ¹Yі—average yield; ²b—coefficient of plasticity;³σd²—coefficient of stability.

The yield of sunflower hybrids significantly depends on plants’ standing density and this is the predominant factor in yields’ formation. The individual yield of plant decreases due to crop thickening, but due to the increase of plants’ number in the area, the yield also increases.

Reaction to the sowings’ thickening was manifested through a change in the yield structure. The early-maturing hybrid “Ukrainskyi F1” formed a lower yield compared to other hybrids (Table 2).

Yield of early-ripening hybrid “Ukrainskyi F1” increases rectilinearly with plant density increasing (r = 0.977 - 0.998), while hybrids P63LL06 (r = 0.651 - 0.882); “NK Ferti” (r = 0.589 - 0.846) and “NK Brio” (r = 0.689 - 0.804) form a higher yield at standing densities up to 60 thousand plants per hectare, and with further thickening the yield decreases.

Height of sunflower plants is largely determined by the morphological features of a hybrid, varying slightly over the years: “Ukrainskyi F1”—173.5 - 177.8; “P63LL06”—184.1 - 188.4; “NK Brio”—167.5 - 171.5; “NK Ferti”—181.9 - 187.2 cm (Tables 2-5). With sowings thickening, the plants’ height of all hybrids increases (r = 0.97 - 0.99).

Hybrid “P63LL06” occupied an intermediate position between hybrids - forming a yield in a favorable for weather conditions year (2018) from 3.26 to 3.52 t/ha and in years with less favorable conditions—from 2.58 to 3.20 t/ha. The diameter of the anthode varies from 20.1 to 25.9 cm. The number of seeds in the anthode varies from 654 to 712 pieces, and the weight of seeds from the anthode is 44.5 - 54.8 grams. Plants of the hybrid have a stable height, which ranges from 184.1 to 187.8 cm (Table 3).

Hybrid “NK Brio” in all years of research and at different densities stood the highest yields (Table 4).

The hybrid formed anthodes with the largest diameter among the studied hybrids—from 24.0 to 27.7 cm, in which 748 - 831 achenes were differentiated, and their weight was 53.1 - 65.3 g. Quantitative reduction of yields structural components occurred with the thickening of sowings and less precipitation during the period of achenes’ differentiation. The plants of hybrid “NK Brio” were the lowest compare to other hybrids—167.5 - 171.5 cm, but the stem diameter was the largest—2.9 - 3.5 cm.

Hybrid “NK Ferti” yielded slightly relative to the yield of hybrid “NK Brio” (Table 5). According to the morphological features, plants of hybrid “NK Brio” differed from “NK Brio”. The diameter of the anthode varied from 18.5 to 23.4 cm. The number of achenes in the anthode 656 - 706, and the mass of achenes in the anthode—45.5 - 54.4 g. The mass of 1000 seeds was 69 - 77. Dependencies between yield and structure components show, that a close correlative positive relationship was established for yield and plants density in the sowings (r = 0.589 - 0.846); plant height (r = 0.616 - 0.883); leaf surface index (r = 0.633 - 0.851). However, these connections are not as close as in the hybrid “Ukrainskyi F1”.

Note: Significant differences (p < 0.05) between the values in the column are identified by different letters.

Note: Significant differences (p < 0.05) between the values in the column are identified by different letters.

Note: Significant differences (p < 0.05) between the values in the column are identified by different letters.

Note: Significant differences (p < 0.05) between the values in the column are identified by different letters.

With a positive correlation between yield and plant density, we found an inverse correlation of different strength relationship between yield and number of seeds in the anthode: Ukrainskyi F1—r = −0.957 - 0.987; P63LL06—r = −0.333 - 0.864; NK Brio—r = −0.262 - 0.552; NK Ferti—r = −0.164 - 0.623 in years with different weather conditions (Table 6).

Seed differentiation in anthodes has an inverse correlation to the density of plants’ standing (r = −0.935). With plant density increasing, the number of seeds in the anthodes decreases. This dependence was observed for all studied hybrids.

The yield of sunflower hybrids directly depends on the leaf surface index, but for hybrids this dependence is different. Correlation between these indicators for hybrid Ukrainskyi F1 is close (r = 0.965); NK Ferti—r = 0.757; P63LL06—r = 0.689; NK Brio—r = 0.680. So, it can be argued that the yield potential of hybrids P63LL06, NK Ferti, NK Brio depends on the increase of the leaf surface index. Sowings of hybrid Ukrainskyi F1 form the leaf surface area, which ensures the realization of the biological potential of the hybrid.

Note. Y—yield; D—density of plants’ standing; N—number of seeds in the anthode (on the plant), W—the weight of 1000 seeds; H—plants height; IL—leaf surface index. Actual at P < 0.01.