Fruit quality, sugar-acid fractions, volatile aroma components, phenolic compounds of early fruit stage ‘Harlikar’ trees onto different dwarfing rootstocks

• Evaluates 5 rootstocks' effects on cold-adapted 'Harlikar' apple quality, filling key research gap. • Discovers rootstock-specific compounds in 'Harlikar' apple, clarifying their quality formation mechanisms. • Z3 and 62–396 rootstocks yield superior fruit quality in comprehensive analysis. To identify dwarfing rootstocks that improve early-season fruit quality of ‘Harlikar’ apple under the cold climate of Northeast China, we monitored key quality attributes, primary metabolites, aroma volatiles and phenolics in 3-year-old trees grafted on five rootstocks. 62–396 produced the largest and most elongated fruit, whereas 60–160 conferred the greatest firmness, soluble solids content and titratable acidity. M9-T337 accumulated the highest fructose, while 62–396 led in malic acid. 60–160 fruit also contained the richest esters and total phenolics. A composite membership-function index ranked overall quality as Z3 > 62–396 > 60–160 > M9-T337 > JM7. Consequently, Z3 and 62–396 are recommended as the most suitable rootstocks for ‘Harlikar’ apple in cold regions.