Low temperature sensing in tulip (Tulipa gesneriana L.) is mediated through an increased response to auxin

Tulip (Tulipa gesneriana L.) is a bulbous plant species that requires a per iod of low temperature for proper growth and flowering. The mechanism of se nsing the low temperature period is unknown. The study presented in this pa per shows that the essential developmental change in tulip bulbs during col d treatment is an increase in sensitivity to the phytohormone auxin, This i s demonstrated using a model system consisting of isolated internodes grown on tissue culture medium containing different combinations of the phytohor mones auxin and gibberellin. Using mathematical modelling, equations taken from the field of enzyme kinetics were fitted through the data. By doing so it became apparent that longer periods of low temperature resulted in an i ncreased maximum response at a lower auxin concentration. Besides the cold treatment, gibberellin also enhances the response to auxin in the internode s in this in vitro system. A working model describing the relationship betw een the cold requirement, gibberellin action and auxin sensitivity is put f orward. Possible analogies with other cold-requiring processes such as vern alization and stratification, and the interaction of auxin and gibberellin in the stalk elongation process in other plant species are discussed.