Comprehensive Biochemical Responses of Apple Tree Genotypes to Repeated Cycles of Drought and Rehydration

Repeated drought–rehydration episodes expose perennial fruit trees to alternating stress and recovery phases, while treatment-level averages can obscure genotype-specific response strategies. We evaluated seven biochemical markers in fourteen apple (Malus domestica Borkh.) genotypes during two consecutive drought–rehydration cycles: proline, malondialdehyde (MDA), total phenolics, total flavonoids, and the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX). Both drought cycles increased all measured markers, with the strongest overall response at the end of the second drought cycle (T6). Relative to the control, mean values at T6 increased by 272.4% for proline, 136.4% for MDA, 45.3% for total phenolics, 76.5% for total flavonoids, 109.1% for SOD, 89.0% for CAT, and 117.5% for POX. Genotype-specific log₂(Stress/Control) profiles at the first (T2) and second (T6) drought maxima revealed contrasting combinations of osmotic adjustment, oxidative damage, antioxidant-enzyme activity, and secondary-metabolite accumulation. B11 combined comparatively low MDA induction with strong antioxidant-enzyme responses, whereas HL1282 and HL1579 showed lower proline and MDA induction together with stronger phenolic, flavonoid, and selected enzyme responses. In contrast, ‘Idared’ showed strong proline and MDA induction. The first two principal components explained 75.1% of the total variance. Overall, the results identify distinct genotype-specific biochemical response strategies and show that coordinated marker profiles provide substantially more information than individual biomarkers or treatment averages alone.