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

Abstract

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 alkaline 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; fine roots 1.2 mm) occur at all depths (1-60 cm). Some second and third order roots are negatively geotropic and grow up to the mineral soil surface. The epidermis of all roots has a thick, eventually lignified outer wall. Except for the thinnest, all roots have a single-layered, thick-walled exodermis, which is first suberized and later lignified. Root hairs are never present. A hypodermis composed of several layers of lignified cells (up to 8-cells-thick) is next to the exodermis and forms the outer cortex. Radial series of thin walled and slightly lignified cells sporadically occur in the outer cortex of the thinnest roots, but there are no passage cells in the exodermis, which is continuous. The remaining inner cortex is composed of unlignified parenchyma with air canals and a completely lignosuberized endodermis 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 one-third of the cortical parenchyma adjacent to the hypodermis. Fungal coils, 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 apical meristem are sometimes colonized. At sites of appressoria, coils of AM hyphae occur within an epidermal cell and exodermal and hypodermal cells beneath. Intercellular hyphae with intracellular branch arbuscules (Arum-type) are common in the inner cortex. There is evidence of a dieback of the highest 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 samples. Glomus spp. dominated followed by Gigaspora spp. The findings are related to a more general understanding of growth and AM colonization in long-lived roots of tropical woody monocotyledons. Palm roots, in particular, are slow growing and are protected by massive hypodermal layers.