X-ray Microanalysis of the Seminal Root of Sorghum bicolor with Particular Reference to Silicon

Sorghum bicolor (L.) Moench. cv. P508.GB plants were grown inwater culture for 1 week, when the seminal roots were harvestedand sampled at five positions starting from the base: 0.0, 0.25,0.50 and 0.75 of the axis length, and a sub-apical position,11 mm behind the tip. Mineral distribution in bulk frozen rootsegments was investigated using SEM and X-ray microanalysis.The elements detected were potassium, chlorine, sulphur, sodium,phosphorus, calcium and silicon. The first four occurred inall root zones. Phosphorus was ubiquitous, but appeared to accumulatein the pericycle protoplasm. Calcium and silicon exhibited themost variation along the seminal axis. Calcium was present inall tissues at the base, but decreased acropetally, being detectedin only the outer cortical and epidermal walls of the subapicalzone. Silicon was present at low levels in protoplasts and wallsof most root tissues, and accumulated in the endodermal protoplastand walls. Deposition in walls is initiated coincident withthe earliest stages of secondary wall thickening. Silicon contentof the inner tangential wall of the endodermis exhibits a decreasingacropetal gradient along the axis length. It is absent frommost cell walls of the sub-apical zone. Silicon pathways inthe root, and silica aggregate formation in relation to thesurrounding ionic environment, are discussed. Sorghum bicolor (L). Moench, seminal root, cryostage, SEM, X-ray microanalysis, ion localization, silicon, endodermis