Auxins and polyamines in relation to differential in vitro root induction on microcuttings of two pear cultivars

The internal levels of indole-3-acetic acid (IAA) and polyamines (PAs) and the metabolism of indole-3-butyric acid (IBA) were studied in relation to the in vitro rooting process of two pear cultivars, the easy-to-root Conference and the difficult-to-root Doyenne d'Hiver. Doyenne d'Hiver required about a 10 times higher concentration of IBA to achieve a rooting percentage similar to that of Conference. One- or two-day exposures to IBA were sufficient to stimulate rooting but with different efficiency for each cultivar. Longer exposure to auxin strongly increased the root number in Conference, whereas root elongation was inhibited in both cultivars. The metabolism of IBA in both cultivars was not significantly different when IBA was used at a high concentration to stimulate maximal rooting in Doyenne d'Hiver. IBA was mainly conjugated into IBA glucose, which was accumulated, and a small amount was converted into free IAA in both cultivars. However, in Doyenne d'Hiver this metabolic pathway appears to be active only at a higher exogenous IBA concentration. At a high IBA concentration more callus was formed by Doyenne d'Hiver, indicating that the cells of Doyenne d'Hiver are not capable of responding to the hormone in the same manner as Conference cells. Anatomic observations indicated that the capacity to induce initial dividing cells was more efficient in Doyenne d'Hiver, but subsequently the number of root primordia formed and root development were much reduced relative to Conference. A possible correlation between these processes and an early increase followed by a decrease of free IAA was seen in Conference. By day 4, a significant increase in IAA conjugates and free putrescine was observed in Doyenne d'Hiver. This higher putrescine content may be related to the lower amount of root development. Together with previous studies these results indicate that differences in the uptake and metabolism of applied auxins may affect rooting ability and the subsequent development of adventitious roots in microcuttings of pear.