Comparative analysis of the drought tolerance of promising domestic and foreign apple varieties on clonal rootstock 54-118

The Oryol Oblast (Russia) is characterized by years with intensive droughts and dry winds in June and July, i.e., during vegetation and fruiting of fruit and berry crops. These periods make up about 20-55 %. We set out to evaluate the drought tolerance of apple-tree varieties based on their leaf water content indicators under the conditions of the Oryol Oblast. The research objects were the following apple varieties: ‘Ligol’ (Polish selection), ‘Honey Crisp’ (American selection), ‘Rozhdestvenskoe’, ‘Pamyat Semakina’, ‘Minister Kiselev’, ‘Orlovskoe Polesie’, ‘Vyatich’,‘Orlovsky Partizan’, and ‘Zdorovye’ (VNIISPK selection) on a medium-grown rootstock 54-118. The experiment was laid out in the fall of 2016; the planting scheme was 5 x 2.5 m. The drought tolerance of the apple varieties was studied by the method of artificial dehydration in 2-fold repetition (5 leaves in each repetition). Leaves were dried in an Espec PSL- 2KPH environmental test chamber in metal vessels at a temperature of 105 °C to constant weight. Water loss by apple leaves was determined at the time of completion of the dehydration process (following 4 h). To determine the ability of leaves to restore their water content, leaves were weighed and spread out for wilting. After 4 h, the leaves were weighed and left to saturate with water for 12 h. Over two years of research during the vegetation period under the specified conditions, an average level of leaf tissue water content was observed in all the studied varieties. The highest index of water deficit under the field conditions in August 2019-2020 was observed in ‘Orlovsky Partizan’ (12.3 and 10 %). On average, during the research period after drought simulation, apple-tree varieties were characterized by an average level of water deficit. Its lowest level (on average for the vegetation period, i.e., June, July, and August) was noted in ‘Rozhdestvenskoe’ (29.5 %) and ‘Vyatich’ (27.4 %) varieties. A high ability to restore the water content of leaf tissues after drought and subsequent saturation of their water under laboratory conditions was noted in ‘Rozhdestvenskoe’ (128.5 %) and ‘Orlovsky Partizan’ (127.7 %) varieties. Our studies showed that apple varieties on medium-grown rootstock 54-118 under the conditions of the Oryol Oblast have an average index of drought tolerance.