Root structure and arbuscular mycorrhizal colonization of the palm Serenoarepens under field conditions

Serenoa repens (Bartr.) Small is a palm native to the southeastern USA. It is a common understory plant in pine communities on both acid sands and alk aline limestone. Roots have only primary growth and range in thickness from 8.0 mm (first order roots from the stem) to 0.8-2.9 mm (ultimate roots of third to fifth order). The thickest roots occur at soil depths > 20 cm; fin e roots (< 1.2 mm) occur at all depths (1-60 cm). Some second and third ord er roots are negatively geotropic and grow up to the mineral soil surface. The epidermis of all roots has a thick, eventually lignified outer wall. Ex cept for the thinnest, all roots have a single-layered, thick-walled exoder mis, which is first suberized and later lignified. Root hairs are never pre sent. A hypodermis composed of several layers of lignified cells (up to 8-c ells-thick) is next to the exodermis and forms the outer cortex. Radial ser ies of thin walled and slightly lignified cells sporadically occur in the o uter cortex of the thinnest roots, but there are no passage cells in the ex odermis, which is continuous. The remaining inner cortex is composed of unl ignified parenchyma with air canals and a completely lignosuberized endoder mis in old roots. Passage cells were seen the the endodermis of the some of the thinnest roots. Arbuscular mycorrhizal (AM) fungi occur in the outer o ne-third of the cortical parenchyma adjacent to the hypodermis. Fungal coil s, arbuscules and vesicles are found most frequently in the thinnest roots, but also occur sporadically in all root orders. Cells a few mm from the ap ical meristem are sometimes colonized. At sites of appressoria, coils of AM hyphae occur within an epidermal cell and exodermal and hypodermal cells b eneath. Intercellular hyphae with intracellular branch arbuscules (Arum-typ e) are common in the inner cortex. There is evidence of a dieback of the hi ghest order roots during the winter dry season. Profiles of soil and roots have the highest density of AM spores in the surface 10 cm layer. Total AM spore density ranged from 130 to 1100 spores per 50 g soil in different sam ples. Glomus spp. dominated followed by Gigaspora spp. The findings are rel ated to a more general understanding of growth and AM colonization in long- lived roots of tropical woody monocotyledons. Palm roots, in particular, ar e slow growing and are protected by massive hypodermal layers.