Root apical meristems of Allium porrum L. as affected by arbuscular mycorrhizae and phosphorus

Arbuscular mycorrhizal (AM) fungi significantly improve plant growth in soi ls with low phosphorus availability and cause many changes in root morpholo gy, similar to those produced by increased P nutrition, mainly depending on root apex size and activity. The aim of this work was to discriminate betw een the morphogenetic role of AM fungi and P in leek (Allium porrum L.) by feeding mycorrhizal and nonmycorrhizal plants with two nutrient solutions c ontaining 3.2 or 96 muM P and examining specific parameters related to adve ntitious root apices (apex size, mitotic cycle, anc RNA synthesis). The res ults showed that AM fungi blocked meristem activity as indicated by the hig her percentages of inactive apices and metaphases in the apical meristem of mycorrhizal plants, whereas the high P supply lengthened the mitotic cycle without blocking the apices, resulting in steady, slow root growth. The po ssible involvement of abscisic acid in the regulation of root apex activity is discussed.