On specific influence of the agroecological conditions of humid subtropics of Russia on productive potential of Actinida deliciosa (Kiwifruit)

Actinida deliciosa (kiwi), recognized for its delicious taste and health-promoting properties, is successfully cultivated in the subtropical regions of the planet. The peculiar climate of the humid subtropics of Russia influences the actinidia plants, in particular, the onset of phenological phases. During fruit formation (June-September), actinidia plants need irrigation depending on the amount of precipitation and temperature conditions. Here we show that in the humid subtropics of Russia, the periods of kiwi fruit formation differ from year to year for precipitations, which coincides with the moisture deficit in the soil. For the first time, we have developed a simple physiological indicator to promptly estimate water status of kiwi plants for timely irrigation based on the cell sap concentration in petioles which correlates with soil moisture in the 0-60 cm layer. Purpose of this work is to determine the complex of agroecological factors affecting the moisture supply and productivity of A, deliciosa in the changing weather conditions of the humid subtropics of Russia. The study was performed on late-ripening cv. Hayward planted in 1988 (a 5½4 m scheme, the total site area of 5.5 ha, three-tiered palmette shaping; the Adler experimental station of the Vavilov FRC All-Russian Institute of Plant Genetic Resources, Krasnodar Territory, 2016-2020). The average daily and maximum temperatures were recorded from May to October, the http://www.pogodaiklimat.ru data were used for precipitation. Moisture content of the soil, taken in 10-cm layers along the depth of the root layer (0-60 cm), was measured by the thermostat-weight method. The concentration of cell sap (CCS, %) in the leaves was measured refractometrically from May to September in ten plants (for 3 decades of each month). The yield of each bush ( n = 30 in total, three replicates) was recorded. Annually, the average weight of a berry was determined using 100 berries. The weather conditions of the growing seasons significantly differed and showed abnormal precipitations and extreme temperatures (above 30 °С), affecting the water regime of plants and productivity. In May-June 2016-2020, the water content of leaf tissues was high (CCS 4.96-5.25 %), therefore, during the periods of budding, flowering, and fruit setting, the moisture supply of the plantings was optimal. In August, fruits began to grow actively and the initiation of the next-year generative organs occurred. However, high insolation and the temperature rise above 30 °С increased evaporations, while the atmospheric precipitations were minimal. The soil moisture reserves from May to October were optimal and ranged from 80 to 90 % of the least moisture capacity over the entire depth of the soil profile. However, the CCS increased from mid-September and exceeded 10 % at the end of the month despite the complete soil saturation with moisture. Tension in the environment and the peak of physiological processes caused water deficit in the leaves up to the CCS > 7 % which irrigation still could level. At the end of August or at the beginning of September (depending on weather conditions), the CCS steadily rose above 8 %, regardless of irrigation. During this period, a redistribution of water fractions from bound to free ecologically active form occurs. The earlier the transition of water and assimilation from the leaves of kiwi plants to fruits begins, the higher their weight and yield. Thus, our findings show that in the humid subtropics of Russia, the studied agroecological factors have the greatest effect on the water regime and performance of A. deliciosa cv. Hayward plants during the period of fruit formation (in August-September).