Uptake and metabolism of indolebutyric acid during root formation on Malus microcuttings

In this study, the content of active internal indolebutyric acid (I BA) and of IBA-derived indoleacetic acid (IAA) was related to adventitious root formation on the stem base of microcuttings of apple. Maximum root formation was obtained after an incubation of 3 days on medium supplemented with 32 pM IBA or 5 days on medium containing 3-2 or 10 [im of the hormone. Two to three weeks after the start of the auxin treatment, the maximum root number was attained. The absorbed IBA-3H accumulated predominantly in the bottom 1 mm of the stem, i.e. the location where the roots emerge. The metabolism of [3H]-IBA was examined in this part of the apple shoots in relation to adventitious root formation in vitro. The internal content of free IBA (IBA'nt) corresponded to about 60% of the IBA concentration in the medium from 6 h to 3 days of incubation. The internal content of IBA-derived IAA (IAA'nt) was about 15% of the IBA concentration in the medium. At incubation periods of 2 and 3 days the shorter period required a higher IBAmt and IAA'nt content to produce the same amount of roots. This observation indicates that in those cases, the mode of action of IBA in root formation resembles a dose effect of the active auxin components in the stem base. Over the whole range of experimental conditions (6-120 h, OT-32 p\i), however, the product of incubation time and exogenous concentration or internal content of auxins was not related to the number of roots. The percentage of label recovered in the IBA‘m and IAA'm fractions was the same at suboptimal and optimal IBA concentrations for root formation, and did not change between 0-7 and 3 days after auxin addition. Therefore, root formation was not related to a specific type of auxin metabolism.