Comparison of morphological, physiological, and related-gene expression responses to saline-alkali stress in eight apple rootstock genotypes

Abstract

• Saline-alkali inhibited the growth of all tested apple rootstock by restricting root development and reducing plants biomass accumulation. • The morphological, and physiological response performance largely differed among genotypes. • Saline-alkali induced up-regulation of signal transduction related genes expressions to enhancing the stress signal output. • Eight genotypes of apple rootstocks were classified into 3 types: highly tolerant, tolerant and susceptible. Grafting cultivars onto rootstocks capable of high saline-alkalinization tolerance is a promising way to improve apple production under specific regions with saline-alkali stress. The morphological, physiological, and related-gene expression responses of eight apple rootstock genotypes, including ‘Zumi’, ‘Marubakaido’, ‘Qingzhen 1’, ‘SH.6’, ‘M.26’, ‘B.9’, ‘M.9-T337’ and ‘Mark 9’, to saline-alkali stress were investigated under a 1:1 molar ratio of 75 mM sodium chloride (NaCl) and sodium bicarbonate (NaHCO 3 ) conditions. Morphological observations suggested that saline-alkali inhibited root development, limited biomass accumulation, and caused varying degrees of injury symptoms on leaves. Moreover, root activity and photosynthesis capacity of most genotypes declined. Still, the sodium (Na + ) /potassium (K + ) ratios, relative electrolyte conductivity (REC), malondialdehyde (MDA), proline, soluble protein, and soluble sugar contents were increased differently in all genotypes. Furthermore, the antioxidative enzyme activities (superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD), and catalase (CAT)) and the expression levels of stress signal transduction related genes ( MdRD22, MdRD29A, MdRD29B, MdAREB1A, MdAREB1B , and MdDREB2A ) were increased or upregulated in some saline-alkali tolerance genotypes while decreased or downregulated in other sensitive genotypes. To sum up, the studied apple rootstocks were classified as highly tolerant (‘Zumi’ and ‘Qingzhen 1’); tolerant (‘M.9-T337’, ‘Marubakaido’ and ‘SH.6’); and susceptible (‘Mark 9’, ‘M.26’, and ‘B.9’) genotypes based on their saline-alkali stress indexes (SAI), average saline-alkali tolerance trait indexes (SATTI) and clustering analysis. These findings suggested that the rootstocks in current study are potential candidates for apple rootstocks against various levels of saline-alkali stress.