Adaptability of moderm soybean varieties to environmental stresses

We present the results of research into drought and cold tolerance of 34 modern soybean varieties developed in the leading breeding institutions of Ukraine. The objectives on evaluation of tolerance to abiotic factors by ability of seeds to germinate at low above-zero temperatures and insufficient moisture were achieved. Varieties with consistently (throughout the years) high cold tolerance and average drought tolerance in combination with increased (in comparison with the check varoeties) yields were identified and classified as valuable sources on grounds of their adaptability to stressful environmental conditions.

1-2°С increase in the cold tolerance during the germination and seed ripening stages provided 10-20 additional days for the photosynthetic activity of soybean plants, accumulation of storage substances, augmentation in the yield and improvement of the seed quality [20].
Numerous studies have found that reduction in the ambient temperature during the vegetation period increases the protoplasm viscosity. The less cold tolerant the genotype is, the more viscous its protoplasm becomes.
The main criteria for prospective soybean varieties, suitable for super-early sowing, which will give stable high yields, should be increased cold tolerance, which will ensure early shoots, and maintenance of the plant habitus and the total length of the growing period on shortened spring photoperiods [21][22][23].
Determination of the drought resistance and cold tolerance of starting material used in breeding programs to develop soybean varieties requires that breeding centers should have climatic chambers, phytotron equipment and other newest devices improving the effectiveness of evaluation methods and creation of new intensive high-yielding varieties, which would be characterized by complex resistance to unfavorable conditions of growth and development [24].
Current studies are mainly focused on developing universal varieties with increased protein content in seeds, tolerance to unfavorable environmental conditions, ability to fix significant amounts of nitrogen from the air. At the same time, the numbers of recessive genes, which are responsible for important traits, such as yield, led to a significant reduction in the adaptive responses of modern varieties. Therefore, at present, the priority trend in soybean breeding is to increase the adaptability of starting material to stressful conditions of cultivation [7].
Researchers from different countries revealed differences in the cold tolerance and drought resistance between soybean genotypes at the early stages of ontogenesis, which assisted to transfer these valuable features to newly-developed varieties [25][26][27]19].
Therefore, in order to reduce losses caused by low above-zero temperatures and water deficit at the early stages of soybean ontogenesis, it is necessary to identify and implement coldtolerant and drought-resistant sources for the development of high-yielding varieties.
Purpose and Objectives. Our purpose was to evaluate drought resistance and cold tolerance of modern soybean varieties bred at the leading research institutions of Ukraine. Concurrently, a number of objectives were achieved: to identify varieties by ability of seeds to germinate at low above-zero temperatures and on water deficit as well as varieties combining high cold tolerance and drought resistance with consistently (throughout the years) increased (higher than those from the check varieties) yields. The study was conducted for three years (2015-2017) in the laboratory and field conditions at the Plant Production Institute named after VYa Yuriev NAAS. Seeds harvested in different years were used in compliance with BA Dospekhov"s methods of field experimentation [28].
Drought resistance was assessed in the laboratory by VG Yatsenko"s method (1988), according to which seeds (25 seeds of each variety in 4 replicates) were sprouted in 8,8% sucrose in Petri dishes for five days at 23,0°С. Concurrently, seeds of each accession were sprouted in distilled water (control). The percentage of seeds germinated in sucrose, i.e. under osmotic pressure, was calculated in relation to the number of control germinated seeds [24].
Cold tolerance was determined as described by T Chirkova, LA Lutova, et al., by the method known as "cold germination of seeds" [29]. The essence of this method is to sprout seeds in filter paper rolls (30 seeds in 4 replicates) at 10.0°C for seven days. The seed germinability was compared with the control, where seeds were sprouted in a thermostat under optimal for this crop conditions (22,0-24,0°C). Seeds were considered as germinable if they formed corcules of a minimum length (5 mm). The cold tolerance of the varieties under investigation was assessed by comparing the germinability at 10,0°C with that at optimal temperatures and expressed as a percentage.
The yield of plants was determined upon cultivation of the varieties under investigation in the field on small plots of 1,2 m 2 . To evaluate the yield, plants were harvested manually from the plots. We measured the pod number per plant, the seed number per pod, the seed weight per plant, and the 1000-seed weight [30]. The yield of plants was compared with the check varieties of the corresponding ripeness groups. The very early varieties were compared with variety Annushka; early ones -with variety Diona; mid-early ones -with Kyivska 98; mid-ripening oneswith variety Yatran; mid-late ones -with variety Vinnychanka; and late-ripening varieties -with check variety Melpomena.
The hydrothermal coefficient was determined by to AV Kuleshov, MO Bilyk and SV Dovhan"s method [31]. This revealed differences in seed setting depending on weather conditions  [32].
Results and Discussion. Analysis of the laboratory data on cold tolerance investigated for three years showed that the varieties could be categorized into five groups according to the values determined by the "cold germination" as T Chirkova, AA Lutova, et al described: susceptible, with a percentage of germinated seeds of 0,0-20,0 % related to the control, mildly tolerant (21,0-40,0 %), mid-tolerant (41,0-60,0 %), tolerant (61,0-80,0 %), and highly tolerant (81,0-100,0 %) ( Table 1). There were differences the ability of seeds to germinate at the low above-zero temperature between the varieties, indicating their levels of cold tolerance at the initial stages of development. No cold susceptible varieties bred in Ukraine were detected. The mildly tolerant group only comprised two soybean varieties, Sharm and Vilishanka, their percentages of germinated seeds were 32,4 % and 25,2 %, respectively, related to the corresponding controls. Three varieties (Raiduha, Perlyna and Pysanka) were mid-tolerant to cold, with the percentages of germinated of 58,3 %, 56,7 % and 53,6 %, respectively, related to the corresponding controls. Twelve varieties or 35,3 % were referred to tolerant ones (Baika, Diadema, Melodiia, Siverka, Suziria, Rusa, Hali, Vezha, Sprytna, Khvylia, Rizdviana, and Farvater). The highly-tolerant group comprised 17 soybean varieties, which is 50,0 % of the sample. These results attest to the high level of the domestic soybean breeding for cold resistance, as the percentages of germinated seeds of these varieties at 10 ℃ during seven days were within 80-100 % as compared with the corresponding controls.