Regulation of drought tolerance in apple by amino acids and zinc oxide nanoparticles: integrating physiological, biochemical, and MGIDI-based trait analysis

Recently, the foliar application of bio-stimulants has gained considerable attention as an effective strategy to mitigate the adverse effects of drought stress (DS) in fruit orchards. The objective of this study was to evaluate the combined effects of foliar-applied amino acids (AA) and zinc oxide nanoparticles (NZ) on the physiological, biochemical, nutritional, and yield-related responses of apple trees subjected to different drought stress levels, and to identify the most effective treatment combinations using the Multi-trait Genotype–Ideotype Distance Index (MGIDI).This study aimed to investigate the effects of three drought stress levels (75%, 50%, and 25% of field capacity (FC)) combined with three concentrations of AA (AMINOMAX Nature-L; 1, 2, and 4 ml·L⁻¹) and three concentrations of NZ (ZnO NPs; 10, 20, and 30 mg·L⁻¹) on the physiological traits, fruit quality, and defense mechanisms of apple trees. A well-watered treatment maintained at 100% field capacity without AA or NZ application was included as the control. Drought stress reduced relative water content by approximately 30–45%, fruit yield by 35–60%, chlorophyll content by 25–40%, and fruit weight by 20–50% depending on stress severity. Conversely, combined application of AA and NZ nanoparticles improved these traits by 10–35% compared with untreated drought-stressed plants. According to the Multi-trait Genotype–Ideotype Distance Index (MGIDI), four combined treatments at 50% field capacity FC50AA4NZ30, FC50AA2NZ30, FC50AA4NZ20, and FC50AA4NZ10 exhibited the lowest distances from the ideal ideotype, reflecting superior multi-trait performance and high adaptability under moderate stress conditions. The high communality values for most traits confirmed the adequacy of the factor model in explaining the correlation structure among variables. Most treatment effects and their interactions were statistically significant ( P < 0.05). These findings suggest that the simultaneous application of AA and NZ can effectively enhance drought tolerance and improve fruit performance in apple trees.