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 in water culture for 1 week, when the seminal roots were harvested and 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 root segments was investigated using SEM and X-ray microanalysis. The elements detected were potassium, chlorine, sulphur, sodium, phosphorus, calcium and silicon. The first four occurred in all root zones. Phosphorus was ubiquitous, but appeared to accumulate in the pericycle protoplasm. Calcium and silicon exhibited the most variation along the seminal axis. Calcium was present in all tissues at the base, but decreased acropetally, being detected in only the outer cortical and epidermal walls of the subapical zone. Silicon was present at low levels in protoplasts and walls of most root tissues, and accumulated in the endodermal protoplast and walls. Deposition in walls is initiated coincident with the earliest stages of secondary wall thickening. Silicon content of the inner tangential wall of the endodermis exhibits a decreasing acropetal gradient along the axis length. It is absent from most cell walls of the sub-apical zone. Silicon pathways in the root, and silica aggregate formation in relation to the surrounding ionic environment, are discussed.