Studies on the evaluation of 20 accessions of fiber flax of domestic and foreign selection according to flax fiber yield and adaptability parameters were conducted in 2018-2020 in the conditions of the Pskov region on sod-weakly podzolic light loamy soil. The Dobrynya variety (Russia), the yield of which depending on the year of testing was 215...273 g/m2, was used as standard. The most favorable conditions for fiber flax were in 2018 (Ij = +24.3), and stressful conditions caused by hot and dry weather during the critical period of plant growth and development (GTK = 0.83) - in 2020 (Ij = -30.8). The samples from China V 51267, 97192-79, y 7S12-13, Vuan 2009, M0269-1, wsh2-5-4 were characterized by the highest average fiber yield over the years of testing (259...275 g/m2) and yield under contrasting conditions (254...279 g/m2), with an average varietal of 232 and 230 g/m2, respectively. The varieties Hon Jku 350, Honkei 28 (Japan), L-1 Soglasie х Alfa (Russia), V 51267 (China) showed the lowest yield variability (7.0...10.8 %). The same samples were distinguished by higher stress resistance. According to homeostaticity (Hom), Hon Jku 350 (Japan), L-1 Soglasie х Alfa (Russia), V 51267 (China) were distinguished: 106.9; 98.9 and 52.1, respectively. Highly responsive to growth conditions (bi>1) were samples 97192-79 (1.93), 97192-79-8 (1.88) from China and L-3 Orshansky-2 x Tvertsa (1.74), Nadezhda (1.56) from Russia; plastic - Heiya 8(1.06), 92199-6-5(1.08) (China) and st. Dobrynya (1.01) (Russia), weakly responsive (bi < 1) -L-1 Soglasie х Alfa (Russia), Hon Jku 350, Honkei 28 (Japan). The best in stability (Gd2) samples were Heiya 8 (13) – China; L-3 Orshansky-2 x Tvertsa (26), L-1 Soglasie х Alfa (179), L-2 Voskhod х Zaryanka (182) - Russia; Hon Jku 350 (186) – Japan. According to the yield of flax fiber and the complex of adaptability parameters, based on the ranking, the cultivars L-1 Soglasie х Alfa (Russia), V 51267, wsh2-5-4, 97192-79 (China) with the greatest adaptive potential were identified. They can be used as sources of adaptability when creating new varieties of flax.

The article presents the results of screening of 20 varieties of fiber flax (Linum usitatissimum L.) according to the yield of flax fiber and adaptability parameters in the conditions of the Northwestern region. The variety Dobrynya (Russia) zoned in the region was taken as standard. On average, over the years of research (2015-2017), the yield of flax fiber in varie-ty numbers varied in the range of 1.22-2.67 t/ha. According to the calculations of the environmental conditions index, the most favorable conditions for growth and development of fiber flax varieties were in 2016-2017 (Ij = +0.07-0.73), the worst in 2015 (Ij = -0.79). The highest yield of fiber (t/ha) was observed in varieties Y 51269 (2.67) (China), A-236 (2.66), Dobrynya (2.49), M-249 (2.48) and L 280-02 (2.38) (Russia), Glazur (2.48) (Ukraine), which exceeded the average among the varieties by 9.7-2.3 %. Among them, the varieties Y 51269 (China) and A-236 (Russia) exceeded the standard by 0.17-0.18 t/ha, in two years of three the growth was reliable. The studied varieties did not fully realize their relatively high yield potential and were characterized by a strong degree of variability. The best indicators of stress resistance (0.79-1.16) were observed in Vesnichka, L-2 (Russia), B-226 (Lithuania), Tyy (China) the varieties and lines Dobrynya, M-249, A-236, L-6, L-4, L-5, L 280-02 (Russia), Glazur, Vruchiy (2.34-2.32) (Ukraine) and Tyy (2.33) (China) were genetically flexible genotypes (2.56-2.23). Samples B-226 (Lithuania), Glazur (Ukraina), L-2-1, Vesnichka, and L 280-02 (Russia) had high homeostatic values and low coefficient of variation. Highly responsive to environmental conditions were samples L-5, L-3, L-4, A-236, M-249, Dobrynya, (Russia), Sxy20, Sxy, Y 51269 (China), Vruchiy (Ukraine); plastic ˗- varieties Glazur (Ukraine), L 280-02, L-5-1, 85-58-26-20 (Russia); weakly responsive − Lu-1, Tyy (China), L-2-1, L-205, Vesnichka (Russia), B-226 (Lithuania). According to the yield of flax fiber and the complex of adaptability parameters, the cultivars B-226 (Lithuania), Y 51269 , Tyy, Lu 1 (China), L 280-02, L-6, A-236, Dobrynya (Russia), Vruchiy and Glazur (Ukraine) were distinguished. They had high stress resistance, genetic flexibility, plasticity and stability. They can be used as sources of adaptability when creating new varieties of fiber flax.

The article presents the results of scientific research for 2014-2019 on the reaction of early-maturing fiber flax varieties (Linum usitatissimum L.) of the Pskov selection Peresvet, Kvartet, Voskhod and Dobrynya to growing conditions: sowing time, seeding rates (16, 18, 20, 22, 24 million pieces of germinating seeds per 1 ha), doses of nitrogen fertilizers (0, 15, 30, 45 kg of active ingredient (a. i.) per hectare. The hydrothermal coefficient (GTC) for the growing season varied from 0.98 to 1.94. The optimal seeding rate for fiber flax of the Peresvet variety is 22 million pcs /ha, and for the Kvartet variety − 20 million pcs /ha. With these seeding rates the yield of flax fiber was 21.9-20.6 hundred kilograms (centners) per hectare, respectively, that significantly exceeded the yield of fiber with the minimum seeding rate of 1,0-1.7 hkg/ha, respectively, with LSD05 = 0.7-0.9 hkg/ha. The maximum yield of seeds in both varieties was obtained at the minimum seeding rate. The optimal sowing period for the Peresvet variety was the third period (May 8-17th, 2014-2016) and the second period for the Kvartet variety (May 11-20th, 2017-2019). The yield of flax fiber for these varieties significantly exceeded the 1stsowing period by 0.7-2.3 hkg/ha, respectively (LSD05 = 0.6-0.8 hkg/ha ) and amounted to 23.0-21.1 hkg/ha , that of seeds – 9.8-9.1 hkg/ha. The optimal nitrogen doses for R40K60 background are for Voskhod and Kvartet varieties 15 kg a. i./ha, and for the Dobrynya and Peresvet varieties − 30 kg a. i./ha. The yield of flax fiber for them exceeded the control by 1.5-1.3 hkg/ha, respectively (LSD05 = 0.6-0.8 hkg/ha) and 2.7-2.5 hkg/ha (LSD05 = 1,0-0.9 hkg/ha). The payback of 1 kg of nitrogen by flax fiber increase in all varieties was the highest with the minimum dose of N15. Among the varieties, it ranged from 8 to 12.7 kg/kg. The specific removal of food elements per 1 ton of flax straw and flax fiber recommended for use in determining the doses of mineral fertilizers for the planned yield of flax fiber, has been determined.

The purpose of the research is a comprehensive assessment of promising lines of fiber flax according to the main agronomic traits and adaptability based on the results of breeding variety testing in the conditions of the North-Western region of the Russian Federation. The research was carried out in the conditions of Pskov region in 2016-2019 on sod-weakly podzolic light loamy well-cultivated soil. The hydrothermal coefficient (HTC) for the growing season was in 2016 – 1.98, in 2017 – 1.60, in 2018 – 0.95 and in 2019 – 1.44. Abiotic conditions of the year had the greatest significant impact at 5 % significance level on the formation of plant height (79.2 %), the yield of flax seeds (83.6 %), flax straw (78,0 %) and flax fiber (68.5 %), the duration of the growing season (58.3 %). The share of the genotype prevailed in the formation of the fiber content in the stem (75.0 %). Variability was greatest in the yield of flax seeds (CV = 26.4...34.1 %); mainly average ‒ in the yield of flax straw and flax fiber (CV = 11.3…21.5 %) and weak ‒ in other characteristics (CV = 1.5...10.5 %). Based on the results of the research according to the complex of agronomic traits, three best lines of practical interest have been identified: P-4382-3-2 f-3, P-4453 c-6 and P-4629 sh-1. All of them belong to early ripening (the growing season is 77...79 days), are resistant to lodging (4.1...4.7 points) and fusarium wilting (86.5...93.3 %), tall (78...81 cm), have the yield of flax straw (50.2...51 c/ha) and flax seeds (5.9...6.9 c/ha) at the level of standard variety Voskhod. They are characterized by high fiber content (35.9...39.6 %) and flax fiber yield (18.4...19.9 c/ha), exceeded the standard by 2.5...5.9 % (abs.) and 7.6...17 %, respectively. Based on a comprehensive assessment of the breeding material according to productivity and adaptability parameters, using the ranking principle, the same genotypes are the most adapted to the soil and climatic conditions of the NorthWestern region, as having scored the lowest amounts of ranks. Their use in breeding programs will contribute to the development of new varieties that ensure high and stable yields of flax products of good quality.

Studies on the evaluation of 20 fiber flax accessions of Russian, Chinese, Japanese and Polish breeding from the collection of the Federal Research Center "All-Russian Institute of Plant Genetic Resources named after N. I. Vavilov" (VIR) were conducted in 2018-2020 in order to identify the sources of agronomic valuable characters that are adaptive to the conditions of the North-West of the Russian Federation. The HTC for the growing season in 2018 was 0.95, in 2019 – 1.53 and in 2020-1.1. Abiotic conditions had a greater influence on the formation of plant height (59.6 %), seed yield (60 %) and fiber content in plants (55.1 %), but the influence of the genotype on these indicators was also significant. The effect size of the genotype prevailed in the formation of fiber yield (67.2 %) and the duration of the growing season (52.8 %). The variability of characters was the greatest according to the seed yield (V = 6.7-33.9 %), disease resistance (V = 3.1-26.5 %); the lowest one was noted according to the duration of the growing season (V = 4.4-9.7 %). For the rest characters, it was at the level of up to 20 %. A valuable and plastic source material was identified for practical breeding aimed at creating new varieties with the following characters: early maturity – K-8680 (Russia), K-8782, K-8783 (Japan), which matured 3 days earlier than the Dobrynya standard and had a growing period of 67 days; productivity – K-8651, K-8760, K-8795, K-8797, K-8874, K-8877 (China), which significantly exceeded the fiber yield of the Dobrynya variety (241 g/m2) by 8-14 %. There were identified genotypes significantly exceeding the standard: according to the plant height (82 cm) ‒ K-8681 (Russia), K-8751, K-8794, K-8795, K-8754 (China) ‒ by 12-25 %; yield of flax straw (241 g/m2) – K-8681 (Russia), K-8751, K-8794, K-8795, K-8874, K-8880 (China) – by 25-40 %; seed yield (149 g/m2) ‒ K-8680 (Russia), K-8772 (Poland), K-8782 (Japan) – by 10-23 %; resistance to lodging (9 samples) and diseases (7 samples). Use of the identified gene sources in breeding practice should accelerate the creation of varieties that provide high and stable yields.