Variability of the growing season period in F 2 -F 4 hybrids of cotton in different soil-climate zones of Uzbekistan

. One of the most crucial topics in cotton breeding is the identification of plant reactions to the environment, including its stressful circumstances and the amount of plant responsiveness to biotic and abiotic stimuli. Plants continuously interact with their environment as they grow and develop; this interaction results in the organism's ability to adapt, or adaptation. For the duration of the plant's existence, the process of adaptation never comes to a stop. The most crucial evolutionary component that enables the adaptability of species and populations to changing environmental conditions is variability, a feature of an organism that represents the processes of its interaction with the environment. Variability is the foundation of adaptation. An assessment was made of the variability of the length of the growing season in four hybrid combinations of medium staple cotton in 2018-2020 in the Tashkent, Fergana and Kashkadarya regions of the Republic of Uzbekistan. It has been established that the duration of the growing season depends mainly on the geographical latitude and genotype of the plant: in the more northern growing zone (Tashkent region), both the average length of the growing season and the amplitude of the trait variability increase compared to the indicators in the Fergana and Kashkadarya regions.


Introduction
Cotton is cultivated in 86 countries of the world, located in the tropical and subtropical zone, reaching 36 degrees south latitude and 45 degrees north longitude. It is sown with more than 40% of the world's acreage occupied by spinning crops [1,2].
Identification of plant reactions to the environment, its stressful conditions, determination of the level of plant responsiveness to biotic and abiotic factors is one of the most important areas in cotton breeding. In the process of growth and development, plants constantly interact with the environment, as a result, the process of adaptability of the organism or adaptation takes place [3,4]. The process of adaptation never ends and continues throughout the life of the plant. The basis of adaptation is variability, a property of an organism that reflects the mechanisms of its interaction with the environment; this is the most important evolutionary factor that ensures the adaptability of species and populations to changing environmental conditions [5,6].
According to Pivovarov and Dobrutskaya, the goal of breeding is to create genotypes with the desired rate of variability [7]. The growing season of a variety is a variable value, it varies both geographically and over the years [8].
The study of the population composition of varieties and ontogeny of plants, the observation of different morphobiotypes should be carried out against different environmental backgrounds.

Materials and methods
One of the objectives of the study was to study the variability of the length of the growing season in four hybrid combinations (F2-F4) of cotton in the conditions of Tashkent (Salar), Ferghana (Kuva), and Kashkadarya (Kasbi) regions.
Tashkent Province has three different climates and is dominated by Csa according to the Köppen-Geiger climate classification. The temperature here is on average for the year 14.6 °C and 429 mm average annual rainfall.
In Kuva (Fergana region) the steppe climate prevails. There is little rainfall throughout the year. This climate is considered BSk under the Köppen-Geiger climate classification. The average annual temperature is 14.1 °C. Approximately 220 mm of precipitation falls annually.
Kashkadarya Province has two different climates and is dominated by BSk. The Köppen-Geiger climate classification of Kasby is BSk. The average air temperature is 15.7 °C. About 212 mm of precipitation falls annually [9,10].
To analyze the variability of economically valuable traits, we used a graph used in descriptive statistics (box plot), which compactly depicts a one-dimensional probability distribution (variance) [9]. Several of these boxes can be drawn side by side to visually compare one distribution with another. In our experiments, we drew three boxes in one table -these are distributions obtained for one attribute in one year from three different regions. The left box is the distribution of the attribute in Tashkent region, the middle box is the distribution of the attribute in Ferghana region, and the rightmost box is the distribution of the attribute in Kashkadarya region. The volumes of the groups do not differ, so they are comparable.
The material of the study was complex and paired hybrids F2-F4 of introgressive forms with varieties with a high fiber yield of foreign selection:

Results and discussion
The length of the growing season is one of the main indicators for obtaining high yields. As can be seen from Figure 1  The figure also shows that the average indicators of this trait were approximately at the same level in the Fergana and Kashkadarya regions: 116.5 and 117 days, which is 12 days shorter than the duration of the growing season of the same hybrid in the Tashkent region. Approximately the same results for the above combination were observed in the experiments of 2019 and 2020 (Figures 2 and 3), with minor variations.   That is, the fluctuations in the duration of the growing season for this combination in all three regions amounted to 7-8 days. However, the average vegetation period in this combination was lower in Kashkadarya and Fergana regions compared to Tashkent region. The exception was the third year of testing, where the average indicators were higher in the Fergana region (Figures 5 and 6).    A decrease in the average length of the growing season was also noted. The manifestation of late ripeness in this combination is associated with the influence of the paternal parent S-2016, which has a longer growing season compared to the local maternal form L-248.

Conclusions
It follows from the experiments that, as a rule, in the more northern growing zone (Tashkent region), both the average duration of the growing season and the amplitude of the trait variability increase compared to the indicators in the Fergana and Kashkadarya regions. New early ripening forms of cotton, the most optimal for the Tashkent region [(Bukhara 6 x L-h)