Response of genetic dwarf apple plants to GA3

Abstract

High temperature has been implicated as the major factor responsible for dwarfing of selected apple (Malus domestica Borkh.) trees of a hybrid population of cv. Goldspur Delicious x cv. Redspur Delicious. Dwarf plants grew only 2.2 cm in 63 days under a ramped temperature regime (night 15°C, day ramped up to 38°C, held for 2 h and ramped down to 15°C—14 h daylength), whereas semi‐dwarf plants grew 26.3 cm. At a constant 27°C (14 h daylength), both dwarf and semi‐dwarf plants grew 26.3 cm. At a constant 27°C (14 h daylength), both dwarf and semi‐dwarf plants grew nearly 50 cm. The gibberellin biosynthesis inhibitor, paclobutrazol, retarded growth of semi‐dwarf plants in both ramped and constant environments and dwarf plants in the constant 27°C environment. It did not further reduce the size of dwarf plants growing under the ramped regime. Gibberellin (GA3) treatment reversed the inhibition of growth caused by paclobutrazol for all plants except it did not restore growth of dwarf plants in the ramped environment. These data suggest that neither pacobutrazoltreated nor untreated dwarf plants growing in the ramped environment (or in the orchard during hot summer months) are able to respond to GA3. In constrast, GA3 was utilized by the paclobutrazol‐inhibited dwarf plants growing at constant 27°C, enabling shoot elongation to take place. It appears that high temperature may have caused alterations in GA target tissues in dwarf plants so that they no longer had the capacity to respond to GA.