The role of ethylene and abscisic acid is both positive and negative. Ethylene is a simple gaseous PGR. As a negative effect, it is synthesized in large amount by tissues undergoing senescence and ripening. It also promotes senescence and abscission of plant organs especially of leaves and flowers.
As a positive effect, ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers. It promotes internode/petiole elongation in deep water rice plants. It helps leaves/ inner parts of the shoot to remain above water