Indoleacetic acid induces xylem formation instead of rooting in walnut (Juglans regia L.) microcuttings

Although the hormone indole-3-acetic acid (IAA) is known to induce adventitious root formation in many plants, it does not have this effect in walnut (Falasca et al., 2000). Therefore, we analysed the histological events induced by IAA. As early as day 5 in culture, IAA enhanced the formation of secondary xylem from vascular cambial cells, whereas it did not induce phloem production. Xylem production increased with the duration of culture, and "meristematization" of the ray cells and ray distortion occurred, The cambial cells also produced callus with xylogenesis, Late in culture, in the de novo formed secondary xylem, nests of non-lignified cells were observed. Polyphenol deposition in some vessels and tylosis were also observed, suggesting the presence of persistently high levels of IAA in the xylem region. The early xylem production and its increase over time in the stem and the conspicuous xylogenesis in the callus suggest that the high auxin level irreversibly channels target (cambial) cells towards xylem formation, possibly preventing the alternative programme of rhizogenesis.