Alkaline soils have a great influence on apple production in Northern China. Therefore, comprehensive evaluations of tolerance to such stress are important when selecting the most suitable apple rootstocks. We used hydroponics culturing to test 17 genotypes of apple rootstocks after treatment with 1:1 Na 2 CO 3 and NaHCO 3. When compared with the normally grown controls, stressed plants produced fewer new leaves, and had shorter roots and shoots and lower fresh and dry weights after 15 d of exposure to alkaline conditions. Their root/shoot ratios were also reduced, indicating that the roots had been severely damaged. For all stressed rootstocks, electrolyte leakage (EL) and the concentration of malondialdehyde (MDA) increased while levels of chlorophyll decreased. Changes in root activity (up or down), as well as the activities of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) were rootstock-dependent, possibly reflecting their differences in alkali tolerance. Using an alkali injury index (AI), coefficients of adversity resistance (ARC), cluster analysis, and evaluation of their physiological responses, we classified these 17 genotypes into three groups: (1) high tolerance: Hubeihaitang,
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