STRESS-INDUCED ETHYLENE EVOLUTION AND ITS POSSIBLE RELATIONSHIP TO AUXIN-TRANSPORT, CYTOKININ LEVELS, AND FLOWER BUD INDUCTION IN SHOOTS OFAPPLE SEEDLINGS AND BEARING APPLE-TREES

Mechanically induced stress (MIS) was imposed on apple shoots by bendi ng horizontal shoots upward and vertical shoots downward and its effec t on internal ethylene evolution, polar auxin transport and cytokinin levels studied. Induction of stress caused a significant rise in inter nal ethylene production in the tissues under stress, decreased polar a uxin transport and cytokinin levels and increased the percentage of fl oral buds in the shoots. There were only quantitative differences betw een bending up or down of the shoots. In general, unstressed horizonta l shoots had higher ethylene production rates and exhibited lower auxi n transport and cytokinin levels compared to vertical shoots which may explain the reason for higher flower bud formation. Parallel trials w ith seedlings using the ethylene action inhibitor silver-thiosulphate (STS) confirmed that ethylene concentrations in the tissues may be res ponsible 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 cyto kinin levels. Flower bud formation may be regulated by some of these p hytohormonal changes.