Plant and Soil 226: 29–35, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 29 IAA and ZR content in leek (Allium porrum L.), as influenced by P nutrition and arbuscular mycorrhizae, in relation to plant development A. Torelli 1 , A. Trotta 2∗ , L. Acerbi 2 , G. Arcidiacono 1 , G. Berta 3 and C. Branca 1 1 Dipartimento di Biologia Evolutiva e Funzionale, viale delle Scienze, I-43100 Parma, Italy, 2 Dipartimento di Biologia Vegetale dell’Universit` a di Torino, viale Mattioli 25, I-10125, Torino, Italy and 3 Dipartimento di Scienze e Tecnologie Avanzate dell’Universit` a del Piemonte Orientale ‘Amedeo Avogadro’, corso Borsalino 54, I-15100 Alessandria, Italy. ∗ Corresponding author Key words: Arbuscular mycorrhizae, hormone balance, leek, P nutrition, root morphogenesis Abstract Leek plants (Allium porrum L.), infected or not with the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, were grown in a sand-hydroponic system, fed with a nutrient solution containing 3.2 or 96 μM P and analyzed for root IAA and ZR content, to assess the role played by the fungus and P nutrition on host hormonal balance. IAA was analyzed by HPLC-fluorimetry, ZR by HPLC-UV coupled with a bioassay based on the expression of a phytohormone-regulated GUS reporter gene. Shoot and root weights and shoot FW-DW ratio enhancements, as well as root-to-shoot DW ratio decrement in mycorrhizal plants, were related to P nutrition. Shoot P concentration was increased by mycorrhizae at both P levels, but was comparable in AM plants grown at 3.2 μM P and non mycorrhizal (NM) plants at 96 μM P. Mycorrhizae and P increased IAA at substantially similar values, while P increased ZR much more than mycorrhizae did. These results are discussed in relation to root architecture modifications induced by the AM fungus. Abbreviations: AM – arbuscular mycorrhizal, DW – dry weight, FW – fresh weight, IAA – indole-acetic acid, IPA – indole-propionic acid, MU – 4-methyl-umbelliferyl-glucuronide, NAA – naphthalene-acetic acid, NM – non mycorrhizal, P – phosphorus, ZR – zeatin riboside Introduction Plant development is influenced by several internal and environmental factors (Marschner, 1995). The latter include mycorrhizae, symbiotic associations between soil fungi and the roots of most land plants. This has long been known for ectomycorrhizae due to the obvious modifications they induce in the short roots of their hosts (Harley and Smith, 1983). Recent work has also shown that endomycorrhizae, mainly AM, influence plant development (Berta et al., 1993a). The best known effect of AM is the enhanced growth of AM plants in relatively infertile soils, due to more efficient absorption of low mobile minerals, mainly P (Smith and Gianinazzi-Pearson, 1988). Moreover their modifications of host root development are related to ∗ Tel: +39 11 670 7446/7/9. Fax No: + 39 11 670 7459. E-mail: atrotta@bioveg.unito.it root size and architecture (Berta et al., 1993a; 1995, and references). Our previous results on leek plants infected with the AM fungus Glomus sp. strain E3 showed remarkable differences between NM and AM plants in root architecture when grown at low P nutri- tion. The latter had more numerous, shorter and more branched adventitious roots (Berta et al., 1990). Sub- sequent work on root apical meristem activity related these results to a higher percentage of inactive and necrotic apices, lower mitotic activity and lengthen- ing of the mitotic cycle (Berta et al., 1991). Higher P nutrition resulted in NM plants with a root system substantially similar to that of mycorrhizal AM plants grown at low P nutrition (Trotta et al., 1991b). These effects may be due to improved P nutrition by the fungus and/or its more direct influence on root devel- opment. In both cases, changes in the level of plant hormones may be expected, since they mediate in-