Medium pH is generally adjusted to 5.8 to 6.0 for plant tissue culture. Our research indicated that pH generally falls between 5.5 and 7.5 in an ordinarily made medium which can be directly used for apple tissue culture without adjusting pH. Repeated adjustment of pH by adding NaOH and HCl leads to the increase in Na+ and Cl− concentration and decrease in Mg2+ and Ca2+ concentration in the medium due to precipitation. To determine the pros, cons, and necessity of pH levels while making medium in plant tissue culture, subculture proliferation, adventitious root induction, and organ regeneration, the apple cultivars Fuji, Golden Delicious, Jonagold, and Gala were used and hardness of the medium and the ion content of Na+, Cl−, Mg2+, and Ca2+ in the medium under different pH were measured. In the lower pH range of 5.0–5.5, plantlets could be subcultured and grew normally; however, the medium did not solidify or solidified poorly resulting in problems associated with handling. No significant difference was found among the treatments when pH ranged 6.0–8.0 in terms of proliferation, adventitious root induction, and adventitious bud regeneration from leaves, except a slight decrease in shoot number proliferation in ‘Jonagold’ and in adventitious bud regeneration from leaves in ‘Fuji’ and ‘Golden Delicious’ at pH above 7.5. The hardness of the medium increased with the increased pH. The superfluous Cl− and Na+ generated during the process of overadjusting pH to 7.0 by adding NaOH and then readjusting to 6.0 by adding HCl significantly affected the proliferation, rooting, and organ regeneration of apple plantlets. A relative broad range of medium pH (5.5–7.5) is suitable for apple tissue culture. We suggest that it is not necessary to always adjust medium pH to 5.8–6.0 in apple tissue culture; especially the repeated adjustment should be avoided.
This work was supported by Nature Science Foundation of Hebei, China (C2012204091, C2014204115) and Hebei Province Hundred Talents Program, China.
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