The study investigated the peculiarities of yield formation in 72 potato genotypes belonging to different maturity groups under contrasting hydrothermal conditions during 2022–2025 in Zhytomyr Polissia region. Yield assessments were conducted on the 65th (T1) and 80th (T2) days after planting and at final harvest (T3). Significant interannual variability in yield accumulation rates was observed. In favorable years (2022 and 2025), the rate of yield formation during the T1-T2 period reached 0.40–0.51 t/ha/day, whereas in 2023 it decreased by 43–58 % (P < 0.001). The most critical phase was T2-T3 (days 80–115), when in the drought year of 2023 yield accumulation rates declined by 69–86 % compared with 2022 (P < 0.001). Final yield in favorable years reached 23-28 t/ha, whereas in 2023 it decreased by 2.5–2.7 times (P < 0.001). In 2024, under extreme drought conditions, yield remained 20–28 % lower than in 2022 (P < 0.001), and differences between maturity groups were not significant (P > 0.05). The coefficient of variation increased to 39 % under stress conditions. Mid-maturing genotypes realized the highest yield potential in favorable years (up to 28.2 t/ha in 2025, P < 0.01) but showed greater variability under drought conditions. Early-maturing genotypes had lower maximum productivity but demonstrated relatively more stable responses. Plasticity analysis according to the Eberhart and Russell method revealed changes in β-coefficients during the growing season. At stage T1, all groups exhibited a stable response type (β < 1). By final harvest, mid-maturing genotypes shifted to an intensive response type (β = 1.04), whereas early (β = 0.81) and medium-early (β = 0.91) genotypes retained a stable or moderately plastic adaptation type. Strong correlations were found between productivity and hydrothermal indicators. Final yield showed the strongest association with yield accumulation rate during the T2-T3 period (r = 0.973; P < 0.001), soil moisture index GWET (r = 0.912; P < 0.001), and the number of days with Tmax > 30 °C (r = −0.879; P < 0.001). The regression model (R² = 0.972; P < 0.0001) showed that each 0.1 increase in GWET raises yield by 1.8 t/ha, while each additional day with high temperature decreases it by 0.48 t/ha. It was demonstrated that the decisive period of yield formation is the T2-T3 phase, while the key limiting factors are soil moisture deficit and the number of days with Tmax > 30 °C during July–August. The obtained results may be used for predicting the productivity of potato genotypes under conditions of interannual climatic variability.

Key words: potato, breeding material, ecological plasticity, yield dynamics, hydrothermal stress, moisture deficit, Tmax > 30 °C, GWET, β-coefficient, predictive modeling, adaptability, Zhytomyr Polissia.