Selecting dwarf apple (Malus x domestica Borkh.) trees in vitro: multiple cytokinin tolerance expressed among three strains of 'McIntosh' that differin their growth habit under field conditions

Microcuttings of three strains of 'McIntosh' apple that varied in their gro wth habit under field conditions were tested for their tolerance to three c ytokinins (BA, kinetin, and 2iP) and one cytokinin-like compound (TDZ) in v itro. The three strains were 'McIntosh' (standard), 'Macspur' (intermediate ) and 'Wijcik' (dwarf). For each cytokinin 'Wijcik' was more tolerant of hi gh cytokinin levels than either 'Macspur', which showed an intermediate tol erance, or 'McIntosh' which did not tolerate (died) high levels of cytokini ns. These results show that 'Wijcik' exhibits multiple cytokinin tolerance in vitro. The relationship between growth habit and BA tolerance has been r eported earlier. The present study demonstrates that cytokinins can be used to differentiate among growth types in apple.