Stress induced ethylene evolution and its possible relationship to auxin-transport, cytokinin levels, and flower bud induction in shoots of apple seedlings and bearing apple trees

Mechanically induced stress (MIS) was imposed on apple shoots by bending horizontal shoots upward and vertical shoots downward and its effect on internal ethylene evolution, polar auxin transport and cytokinin levels studied. Induction of stress caused a significant rise in internal ethylene production in the tissues under stress, decreased polar auxin transport and cytokinin levels and increased the percentage of floral buds in the shoots. There were only quantitative differences between bending up or down of the shoots. In general, unstressed horizontal shoots had higher ethylene production rates and exhibited lower auxin transport and cytokinin levels compared to vertical shoots which may explain the reason for higher flower bud formation. Parallel trials with seedlings using the ethylene action inhibitor silver-thiosulphate (STS) confirmed that ethylene concentrations in the tissues may be responsible for the reduction of endogenous basipetal auxin transport and Z/ZR levels in the shoots. It is suggested that the main stimulae for mechanically induced stress responsiveness in shoots of apple plants is ethylene and ethylene regulated changes in auxin transport and cytokinin levels. Flower bud formation may be regulated by some of these phytohormonal changes.