Tuesday, 11 July 2006

Rhizosheath in Cynodon dactylon Growing in a Volcanic Sandy Soil.

Fernando De León-González1, Claudia Hidalgo-Moreno2, and Eduardo Celada-Tornel1. (1) Universidad Autónoma Mteropolitana-Xochimilco, Calzada del Hueso 1100 Col Villa Quietud, Mexico, 16300, Mexico, (2) Colegio de Postgraduados, km 35.5 carretera Mexico-Texcoco, Montecillo, 56230, Mexico

Rhizosheaths has been reported as soil-root structures frequently found in grass species growing under sandy soil conditions. The phenomenon has not been reported for volcanic soils in Mexico. The objective of the present work was to elucidate the morphological traits of rhizosheaths observed in Cynodon dactylon (bermudagrass), an invasive grass growing in the valley of Mexico. Root-soil samples were obtained at Tulyehualco (Xochimilco, Mexico, D.F.) in March 2005. The soil was an Entisol (Typic Ustifluvent) with high sand content (79%). Annual rainfall (550 mm) is concentrated in summer months. The water content at the sampling date was 10 g 100 g-1. Root air-dried samples were observed under stereoscopic and optical microscope coupled to a video camera and an Image Pro® system. All micrometric measurements were made with the adequate calibration corresponding to selected magnification. In order to distinguish between fungal hyphaes and root-hairs, a set of rhizosheaths was immersed in neutral-red stain. This technique helped to observe vascular structures in root-hairs. In other specimens soil was mechanically and carefully retired from the rhizosheaths in order to observe and measure root-hairs dimensions (immersion in blue tripan). Root-hair density and details of sand and silt root-particle contact were obtained analysing Scanning Electron Microscope (SEM) images.Results. Images from stereoscopic microscope evidenced the rhizosheaths in C. dactylon. The mean diameter of soil particle layer covering roots was 0.3 mm (30% of the rhizosheath+root diameter). We observed a dense enmeshment of soil particles trapped by the rhizosheath, indicating a mechanism of soil stabilization for the sandy soil. Micrometric measurements were made from images of specimens without soil particles, stained with tripan blue, and observed under optical microscope. The root-hair length ranged between 100-200 µ, and 10-15 µ width. Images from fresh samples of rhizosheaths immersed in neutral red stain confirmed the vascular structures of root-hairs observed when tripan blue was used. Length and width dimensions, and vascular structures evidenced by the two stains confirmed that the root-soil structure corresponded to a rhizosheath where the root-hairs play a key role in attaching sand and silt particles to main roots. Images from SEM microscope helped to estimate the root-hair density (400 mm-2 , approx.) and to evidence the contact between root-hairs and sand particles. Thinner filaments (in comparison to root-hairs) appeared in SEM images; they could correspond to hyphae filaments. Conclusions. a) Roots of Cynodon dactylon growing in a volcanic sandy soil formed rhyzosheaths which can be an important mechanism to stabilize the sandy soil and also it can be a protection of roots against pests, low temperatures and low water content. According to our knowledge is the first report of rhizosheath structures in volcanic soils of Central México. b) Observations with optical (under blue tripan and neutral red stains) and SEM microscope confirm the contribution of root-hairs attaching sand particles to rhizosheath structure.

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