The specificity of viticulture industry development is determined by its close connection with the weather and climatic conditions. Due to global climate change, there is a need to assess the response of grape plants to possible changes in agro-climatic growing conditions to use the resources of a particular region more efficiently and increase gross harvest and crop quality (Kovalyshyna et al., 2020a).

Grapes are in demand in many countries around the world. The share of fertile grape plantations in the world is over 3.1%, which is about 12.5 thousand hectares. Most of these plantations are concentrated in the Middle East and European countries (Zheplinska et al., 2019). Ukraine ranks in the world in terms of grape production. According to the FAO, the world grape market is underfunded. According to scientifically sound standards, the annual consumption of grapes per capita should be 2 (Zheplinska et al., 2021). World grape production should reach 14 million tons to ensure such volumes.

The weather has a significant impact on the formation of the grape harvest. Weather conditions change from year to year, which determines the variability of crop values (Kovalyshyna et al., 2020b; Kolyanovska et al., 2019). The fundamental process, during which the formation of the crop takes place, is photosynthesis (Smetanska et al., 2021). The intensity of photosynthesis in fruit and grape plants is most often investigated by assessing the functional state of their photosynthetic apparatus using the method of induction of chlorophyll fluorescence (ICF). This method determines the sensitivity of the process of photosynthesis to various stress factors, such as temperature, light intensity, the action of toxins, heavy metals, air pollution, and changes in vertical zonation, precipitation, etc. (Mamonova et al., 2018). Studies using the method of ICF and correlation analysis make it possible to determine the potential productivity of grape varieties and establish the level of influence of weather conditions on them. Therefore, this work aimed to determine the level of functional activity of leaves of different grape varieties during the growing season and to establish the influence of weather conditions on the processes of photosynthesis in plants grown in vineyards in the northern part of Ukraine.

According to agro-climatic zoning, the study area according to the average long-term data belongs to the northern part of Forest-Steppe (HTI 1.0 – 1.3, the sum of active temperatures of 10 °С and more is 2700 °С). Several scientists from Ukraine and Europe were engaged in similar scientific researches, they carried out the scientific work, both in the open and in the closed ground in various climatic zones (Conradie et al., 2019; Malu, Sharma and Pearce, 2017; Schwerz et al., 2017; Alameldin, 2017). Over the years of the field experiment during the growing season, various weather conditions developed, which allowed assessing the degree of their influence on the functional activity of the leaves of grape plants in certain phases of development of the latter. In 2019, during the growing season, the hydrothermal coefficient fluctuated in the range of 0.8 – 1.2, the lowest being at the end of summer. The sum of active temperatures of 10 °С and more (Σ act t ≥10 °С) during the growing season was 3317 °С. The weather of 2020 differed from 2019 by less heat accumulation in the spring and summer and more precipitation (HTI 1.6 – 3.6). According to Σ act t ≥10 °C the terms of the study were almost the same (2020 – 3370 °С), which is on average 644 °С more than the climatic norm.

In 2019, from the beginning of the growing season to the phase ‟the inflorescence is visible” Σ act t ≥10 °C was 577 °C, in 2020 – 522 °C, but its accumulation occurred 12 days later, compared with 2019. In the phase ‟the beginning of ovary growth” Σ act t ≥10 °C was 1300 °C, which in 2019 was recorded 11 days earlier compared to 2020 (1308 °C). In 2019, from the beginning of the vegetation to the phase “the beginning of berry ripening” Σ act t ≥10 °C was equal to 1906 °C and occurred 12 days earlier than in 2020 (1944 °C).

A comparative analysis of precipitation in the years of the study showed that in 2020 in all three terms of measuring the amount of precipitation was higher by 64, 53, and 48%, respectively. In May 2020, precipitation was 3.2 times more than in 2019 and 3.1 times more than the climatic norm. June precipitation of 2019 and 2020 accounted for only 62% of the long-term norm, for July the situation with humidity was also different: in 2020 precipitation was almost the norm – 92%, and in 2019 only half of the climatic norm. In August, the amount of precipitation in 2019 was 75%, in 2020 – 45% of long-term values.

After the onset of light influence on the previously adapted to the dark leaf of the plant, the fluorescence intensity of chlorophyll changes significantly during the measurement period. The time dependence of ICF has the characteristic appearance of a curve with several maxima. The shape of this curve reflects the changes that occur in the photosynthetic apparatus of the plant during its adaptation to different environmental conditions.

According to Kornyeyev (2002), the level of chlorophyll fluorescence emitted by PS II complexes with “open” reaction centers depends on the loss of excitation energy during its migration through the pigment matrix of light-harvesting complexes.

In active photosynthesis, when all reaction centers are in the open operating state, in low light conditions, about 3% of the absorbed light energy is not used in the process of photosynthesis. But the authors of the following scientific papers argue that in active photosynthesis when all reaction centers are in normal light, about 10% of absorbed light energy is not used in photosynthesis, which can have a significant impact on the yield and quality of grapes (Palamarchuk et al., 2020; Drenjančević et al., 2017). As a rule, in conditions of cultivation favorable for the plant, the value of F₀ is insignificant which indicates the active use of energy of absorbed light by cells. At different phases of development of grape bushes, the background level of chlorophyll fluorescence in leaves varied in the range of 235 – 286, 238 – 318, and 226 – 274 с. u. (Table 1).

The more efficient use of quantum energy of absorbed light by F₀ in the leaves of most varieties was noted in the phase “the beginning of berry ripening”. During the three periods of the study, the varieties Aromatnyj, Zagadka, Ilichevskij rannij, and Kardishah showed a lower content of chlorophyll molecules, which do not have a functional connection with the reaction centers (RC) (Pascale et al., 2017; Martínez et al., 2018).

Adaptive changes in the structure of the pigment complex to the lighting conditions because the F_p index is the most variable. The values of the maximum value of fluorescence depend on the intensity of the excited radiation and they are proportional to the total number of chlorophylls and inversely proportional to the density of the reaction centers. It is believed that at the point F_p at the maximum level of chlorophyll fluorescence photosynthesis is at a minimum one (Kytaiev et al., 2005). Thus, the highest intensity of photosynthesis on average for all varieties was observed in the phase “the beginning of ovary growth”, the lowest – “the inflorescence is visible”. Analyzing in detail the F_p index by varieties, it was found that the intensity of photosynthesis was significantly higher in the second measurement period compared to the first, namely: for the variety Aromatnyj – by 34%; Zagadka – 23%; Ilichevskij rannij – 27%; Kardishah – 25%; Kishmish tairovskij – 18%; Kometa – 26%; Muskat odesskij – 25%; Persej – 26%; Shkoda – 19%; Yarilo – 26%, which in our opinion is due to insufficient insolation in the first measurement period. The process of photosynthesis under such conditions is limited by the intensity of light entering the plant (Lukatkin et al., 2011).

According to long-term studies of the Laboratory of Plant Physiology of the IH NAAS, the optimal level of background fluorescence is not more than 20 – 25% of the maximum value of fluorescence (F_p). We found that for all studied varieties of grapes pigment complex functions actively in all phases of bush development (Figure 2).

In the phases “the inflorescence is visible” and “the beginning of berry ripening” the index F₀/F_p fluctuates on the average on grades in the range – 16 – 17%, in a phase “the beginning of ovary growth” – 22%. We believe that this is due to the large amount of energy expended on the intensive growth of the fruit (ovary).

An effective way to monitor the impact of environmental stress on the plant is a calculated indicator – the fluorescence induction coefficient of chlorophyll (K_i), which shows the share of chlorophylls involved in photosynthesis, their total number and determines the efficiency of the light phase of photosynthesis. According to the obtained data (Table 2), the K_i index in the leaves of grape plants in the first and third measurement periods ranged in the same range of 0.822 – 0.845 and 0.823 – 0.842 с. u. respectively, in the second – 0.740 – 0.792 с. u. Studies performed by Lukatkin et al., (2011), indicate a high level of efficiency of photophysical processes near the reaction. Several scientists (Hu et al., 2019; Conde et al., 2018; Kernaghan, Mayerhofer and Griffin, 2017) have been engaged in similar scientific researches and found that the K_i index in the leaves of grape plants in the first and third measurement periods ranged in the same range of 0.810 – 0.838 and 0.813 – 0.822 s. u. respectively, in the second – 0.720 – 0.762 s. u.

The value of K_i is associated with changes in the efficiency of the process of photochemical quenching of chlorophyll fluorescence. This index of dark-adapted plants reflects the potential quantum efficiency of PS II, used as an indicator of photosynthesis productivity, the optimal value of which for most plant species, under saturating intensity of exciting light, does not exceed 0.83 (Ivanova et al., 2021).

An increase of this index diagnoses the phenomenon of photoinhibition and testifies to the influence of stress factors on plants (He et al., 1996; Sukhenko et al., 2019). At the phase “the inflorescence is visible” for grape leaves Ilichevskij rannij, Kishmish tairovskij, Kometa, Muskat odesskij indicator K_i was higher than the optimal value, but among varieties as to the value of K_i a significant difference was not found. At the phase of “the beginning of ovary growth,” variety Aromatnyj as to this index differed significantly from most varieties. In the third measurement period, a significant difference in the value of K_i was among varieties Muskat odesskij, Kishmish tairovskij, Yarilo, Kardishah compared to the variety Ilichevskij rannij.

Thus, according to the fluorescence induction coefficient of chlorophyll (K_i), the potential productivity of grape plants of all varieties was at a very high level compared with data obtained by other analysis (Zelenianska, 2009; Havryliuk et al., 2019; He et al., 2018; Bober et al., 2020).

The coefficient of adaptability or “viability index” (Rfd) we used to assess the impact of growing conditions on the state of the plant. Reduction of the calculated Rfd to 1.47 – 1.91 c. u. indicates the negative impact of growth conditions or the possible effect of stress on the efficiency of the Calvin cycle (Lysenko et al., 2013; El Sayed et al., 2019; Shanina et al., 2019). In grape plants at the development phase, “the inflorescence is visible”, the average viability index among varieties was 2.49 c. u., at the phase of “the beginning of ovary growth” there was a decrease to 1.89; at the phase “the beginning of berry ripening” – up to 1.96 c. u. Plants were cultivated on a well-lit area, on a vertical trellis, the access of the sun to all parts of the grape bush is very good, so, in our opinion, the efficiency of photosynthetic processes is generally high.

The efficiency of photosynthetic processes by the coefficient of adaptability of the variety Muskat odesskij indicates a low intensity of photochemical processes in the second and third measurement period; the decrease in Rfd was 30.3 – 31.2%. In our opinion, it is associated with a negative effect of anthracnose on the plant; Muskat odesskij variety is more susceptible than other varieties. The smallest changes in the viability index (5.7 – 7.2%) were determined for plants of the variety Aromatnyj.

To establish the influence of weather conditions during the growing season on the processes of photosynthesis in grape leaves, we used the indices of hydrothermal coefficient, the sum of active temperatures, the amount of precipitation (K_i index), except for Kishmish tairovskij, Muskat odesskij and Persej, for which a greater influence (correlation coefficient r = 0.36 – 0.38, moderate influence) was noted in comparison with other grape varieties (Table 2).

In general, the potential productivity of grape plants (according to K_i index) was weakly and moderately dependent on weather conditions during the growing season. Among the meteorological elements, the greatest influence on the efficiency of the light phase of photosynthesis in the leaves of the studied grape plants was the amount of precipitation (moderate).

The influence of weather conditions on the value of the coefficient of adaptability (Rfd) was greater than the value of K_i. The high influence of HTI on the efficiency of dark photochemical processes in the leaves of the variety Kardishah, for most varieties of grapes it was noticeable (correlation coefficient r = 0.50 – 0.69), only for varieties Zagadka, Kishmish tairovskij, Kometa and Yarilo it was moderate (correlation coefficient r = 0.30 – 0.49). The efficiency of dark photochemical processes in the leaves of most grape varieties significantly (r = 0.50 – 0.69) depended on Σ act t ≥10 °C during the growing season, the least dependence on these factors was observed among the varieties Aromatnyj and Yarilo. The amount of precipitation had a weak effect on the quantum efficiency of photosynthesis in the leaves of most grape varieties, but it had a moderate effect on the efficiency of photosynthesis in the leaves of Aromatnyj and Kishmish tayirovskij varieties.

Analysis of changes in the induction of chlorophyll fluorescence (ICF) of leaves revealed the more efficient use of quantum energy of absorbed light by F₀ in the leaves of most varieties was noted in the phase “the beginning of berry ripening”. During the three periods of the study, the varieties Aromatnyj, Zagadka, Ilichevskij rannij, and Kardishah showed a lower content of chlorophyll molecules, which do not have a functional connection with the reaction centers (RC). According to the F_p index the highest intensity of photosynthesis on average for all varieties was observed in the phase “the beginning of ovary growth”, the lowest one was at “the inflorescence is visible” phase. The potential productivity of grape plants by the coefficient of ICF for all varieties was at a very high level and had a weak and moderate correlation with weather conditions during the growing season. The hydrothermal coefficient and Σ act t ≥10 °С had a weak effect on the efficiency of the light phase of photosynthesis and a noticeable one (correlation coefficient r = 0.50 – 0.69) on the efficiency of dark photochemical processes in the leaves of most grape varieties. Weather conditions during the vegetation in 2019 – 2020 differed from the average long-term data by a greater accumulation of heat (on average by 644 °C). The influence of meteorological elements of the spring-summer period on the onset of the vegetation phases, which in 2019 took place on average 12 days earlier than in 2020, was noted.

These data show that despite the differences in the onset of the vegetation phases and weather conditions during 2019 and 2020, the processes of photosynthesis in grape leaves are at a high level. That indicates a high potential for plant adaptation to the conditions of the northern part of the Forest-Steppe of Ukraine.

In conditions of local and global climate change, it is important to study the adaptive properties of plants to new soil and climatic conditions. In the future, it will allow expanding and changing the areas of cultivation of grapes plants. So that to provide with enough grapes, the population of Ukraine and the world.