Role of root hairs in phosphorus depletion from a macrostructured soil

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One rape (Brassica napus cv. Wesroona) plant and four cotton (Gossypium hirsutum cv. Sicot 3) plants were grown in plastic cells containing soil labelled with 407 kBq of33P g−1 soil. After 5–8 days of growth, the33P depletion zones of all plants were autoradiographed and33P uptake by plants was measured. The autoradiographs were scanned with a microdensitometer and the optical densities at several places within the33P depletion zones of roots were obtained. The volume of soil explored by root hairs was estimated from measurements of root diameters and lengths of roots and root hairs. About half of the total33P depleted by cotion roots came from outside the root hair cylinder whereas most of33P taken up by rape was from within the root hair cylinder. Plants grown in a macrostructured soil may have roots growing in voids, within aggregates or on the surfaces of aggregates. The results of this study demonstrate that root hairs have a strong influence on the accessibility of phosphorus to roots in such a soil, and thus on the phosphorus nutrition of plants.