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Currently, the issue of developing adaptive varieties of crops based on evolutionary principles is becoming urgent. The need to develop adaptive varieties for Russia is determined by the contrast of its natural and climatic conditions, global climate changes, and the increasing unpredictability of the weather. This task is highly relevant to the valuable food crop buckwheat, the yields of which are still low due to insufficient resistance of modern varieties to extreme environmental factors. Vegetation experiments to study the genotypic reaction of light and dark phases of photosynthesis of leaves on changes in soil moisture availability were carried out. During the period of buckwheat breeding from local samples to modern varieties the plants showed a significant trend in increasing the activity of photosynthesis and transpiration of plant leaves with a certain decrease in their resistance to moisture limitation. It was established that a decrease of soil moisture content from 70 to 30% of total moisture capacity caused, in modern cultivars, a mean decrease of fluorescence quantum yield by 52.5%, of electron-transport chain activity by 53.7%, of photosynthesis intensity by 66.5%, of transpiration intensity by 82.1%, whereas in local cultivars it decreased by 40.0%, 52.1%, 51.0%, and 76.1%, respectively.

The article is devoted to the results of research on species and genotypic features of photosynthetic activity in spring wheat plants in connection with the creation of varieties that form a high, stable, high-quality and environmentally safe crop due to the effective use of renewable natural energy of the sun by photosynthesis. It was found that the intensity of leaf photosynthesis in a culture significantly depends on the hereditary characteristics of plants, the growth phase, location, time of day, and lighting conditions. The range of genotypic changes in the trait in 2017 was 9.65-16.35, in 2018 – 9.55 – 21.42, in 2019 – 8.73 – 17.15 ?mol CO2/m2s. In the phase of tillering and milk ripeness in crop varieties, the most significant manifestation of CO2 gas exchange activity by leaves is noted. In the tillering phase, varieties with high photosynthesis intensity were 30% superior, and in the milk ripeness phase, varieties with low photosynthesis intensity were 31% superior. The most active is the absorption of CO2 molecules from the air by the flag leaf of plants. During the period of grain formation, the intensity of photosynthesis exceeded the pre – flagged leaf by an average of 1.6 times, and the lower-lying leaf by 2.8 times. This shows a high dependence of the intensity of leaf photosynthesis on the light regime. When the light intensity increased from 300 to 1000 ?mol/m2s, the photosynthesis intensity of the flag leaf of spring wheat plants increased by an average of 102 %, and above 1000 ?mol/m2sby 14 %. Of the studied varieties, the most responsive to the conditions of insolation is Volnodonskaya, Lyubava, Chernozem-Uralskaya, in which the full light saturation of photosynthesis was observed at 1800 ?mol/m2s. The correlation coefficient between the intensity of photosynthesis and illumination was 0.93.