Plant breeding as integral discipline must consider various aspects of genetic interactions and physiological plant responses to growing conditions. When creating new varieties purposefully as genetically protected from unfavourable abiotic and biotic factors, the analysis of hydrothermal conditions of vegetation and determination of their impact on plant growth, development, and productivity under specific environmental conditions are of great importance.

The research was aimed to determining the level of expression of spring barley yielding capacity depending on rainfed environments and thermal conditions of cropping season in the Central Forest-steppe of Ukraine.

Under environments of the V.M. Remeslo Myronivka Institute of Wheat of NAAS by long-term studies (2004-2016) with nine spring barley varieties it was ascertained significant changes in meteorological resources provision during vegetation. In particular, rainfall on average for 13 years from sowing date to maturation was 201.2 mm with a range of 123.7 mm in 2013 to 290.3 mm in 2014. Range of variation in mean daily air temperature by years in interphase periods was 4,6-8,0 °C. It was marked significant impact of weather conditions on the duration of both individual interphase periods and vegetation in general. Varying period from sowing date to maturation among years was 38 days, from emergence date to maturation was 32 days. As a result, it affected the level of expression of yielding capacity (from 2.52 to 6.92 t/ha) by year for the set of varieties studied.

Increase of mean daily air temperature have caused some reduction in the duration of individual periods and vegetation in general (r = -0.40 – -0.72). Instead of it, precipitation positively correlated with the extension of cropping season and its components (r = 0.45-0.81), except "heading-maturation" period (r = 0.07). This indicates that it is the air temperature that exercised stronger influence on the course of this period (r = -0.68).

Delay in emergence had a negative correlation with yielding capacity (r = -0.45). However, longer vegetation from emergence date to maturation positively correlated with yielding capacity. The correlation coefficient for "emergence-maturation" period was r = 0.57, for "sowing-maturation" period r = 0.34. The closest relationship was noted between yielding capacity and duration of "heading-maturation" period (r = 0.73). Thus the reduction of this period primarily as a result of high temperatures negatively affected the yielding capacity.

Moderate negative relationship between yielding capacity and mean daily air temperature in the interphase periods of vegetation was established (r = -0.32 – -0.49), except for the period from sowing date to emergence (r = 0.03).

Between yielding capacity and rainfall it was noted moderate positive relationship during periods "sowing-emergence"
(r = 0.41) and "emergence-heading" (r = 0.33). Weak relationship was observed for "heading-maturation" period (r = 0.26) and "emergence-maturation" period (r = 0.19). For "heading-maturation" period it was noted even weak negative relationship (r = -0.13). The character of correlation between rainfed environments and yielding capacity indicates the importance of not only the sum, but also the even distribution of rainfall during cropping season. Since limit of rainfed environments in the prior period, or on the contrary, following the drought can not be fully compensated by plants to produce yield. In addition, excessive rainfall (especially storm nature) from heading to maturity can provoke lodging barley crops, thus also leading to appreciable drop in yield of the crop.

Thus, uneven rainfall and increased air temperatures are the most characteristic expression of unfavourable abiotic factors during spring barley cropping season in the Central Forest-steppe of Ukraine, which cause varying duration of interphase periods and level of yielding capacity. The defined features of hydrothermal regime should be considered when elaborating breeding programs for adaptability.

Key words: spring barley, yielding capacity, cropping season, precipitation, air temperature, hydrothermal coefficient, sum of effective temperatures, correlation.