Effects of 5-Aminolevulinic Acid on Physiological and Biochemical Changes of In Vitro-Cultured Apple Plantlets Under NaCl-Induced Salinity Stress

The present study was carried out to evaluate the effects of 5‑aminolevulinic acid (ALA) on physiological and biochemical changes of three in vitro-cultured genotypes of apples under NaCl-induced salinity stress. Virus-free shoots of three genotypes of apple (‘Bud 9,’ ‘Semnan,’ ‘Makamic’) were cultured on MS medium containing different levels of NaCl (0, 30, 60, and 90 mM) and ALA (0, 2.5, 5, 10, and 20 µM). After prolonged salinity, physiological and phytochemical variations of regenerated plantlets with or without NaCl and ALA treatments were recorded. Salinity reduced the chlorophyll and carotenoid content of regenerated plantlets in all studied genotypes. Still, the accumulation of total phenol and flavonoid, as well as the activities of superoxide dismutase, ascorbate peroxidase, catalase, and peroxidase as well as phenylalanine ammonia-lyase, increased with increasing salinity levels. However, the anthocyanin accumulation did not respond to salinity stress. Exogenous ALA in 5 µM and particularly in 10 µM was effective in enhancing chlorophyll and carotenoid content as well as increasing the activities of superoxide dismutase, ascorbate peroxidase, and phenylalanine ammonia-lyase enzymes and the accumulation of total phenol, flavonoid, and anthocyanin. According to the results obtained, Bud 9 was resistant to salinity stress. A combination of salinity stress (60 mM NaCl) and ALA (10 µM) could be suggested to increase the commercial benefits of apples for anthocyanin production.