A selective tissue culture medium for growth of compact (dwarf) mutants of apple

Three strains of 'McIntosh' apple (Malus domestica Borkh.) with growth habits ranging from the standard parent type to extremely compact (dwarf) were grown in vitro as meristem-tip cultures on Murashige and Skoog medium containing a range of concentrations of benzyladenine (BA). All strains exhibited a similar optima (3 to 6 μM BA) for maximum shoot proliferation and culture weight increase. However, tolerance to supra-optimal concentration of this cytokinin was related to growth habit. For example, at 10 μM BA shoot production rates as a percent of the maximum rates were 90%, 20% and zero for the extreme compact, moderate compact and standard strains, respectively. Comparisons among field trees and meristem-tip cultures of all three strains revealed similarities in growth and development.