PHOSPHATE UPTAKE AND ANATOMY OF UNTHICKENED AND SECONDARILY THICKENED ADVENTITIOUS ROOTS OF FIELD‐GROWN WHITE CLOVER ( TRIFOLIUM REPENS L.)

S ummary This study reports on the uptake of phosphate by, and the anatomy of, unthickened roots of primary structure and secondarily thickened roots of intact white clover ( Trifolium repens L.) plants which were washed from turves from grazed pasture. Phosphate uptake was measured over 24 h by supplying 6 mm root segments with [ 32 P] labelled nutrient solution. The total quantity of labelled phosphate translocated from the treated segment was 10 times greater in unthickened root (405.1 pmol) than in secondarily thickened root (40.7 pmol). A higher proportion of accumulated phosphate was translocated from unthickened than from thickened roots. Features of unthickened primary root structure including presence of thin‐walled, intact epidermal and cortical cells, a lack of suberization except for Casparian bands in the endodermis, heavy infections of endomycorrhiza and presence of root hairs, were consistent with an ability to absorb and transport ions rapidly. Several features of secondarily thickened roots indicated these to be less adapted to absorb and transport ions rapidly including invasion of the outermost layers of cells by bacteria, disintegration of cells, and suberin lamellae encasing cells of the phellem. Suberization of the periderm was thought to reduce rate of phosphate movement into the root apoplast. Roots which were secondarily thickened were not infected by endomycorrhiza and had no root hairs.