The supply of nutrient ions by diffusion to plant roots in soil |
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Authors: | M C Drew P H Nye |
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Institution: | (1) Soil Science Laboratory, Department of Agricultural Science, University of Oxford, UK |
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Abstract: | Summary Measurements were made of the diffusion of P32-labelled phosphate to single roots of onion, leek and rye-grass growing in an Upper Greensand sandy loam (UGS) and a Coral
Rag Clay (CRC) to which different amounts of phosphate had been added. Concentration-dependent diffusion coefficients for
phosphate ions in the soils were calculated from phosphate desorption isotherms in calcium chloride. The experimental uptake
by roots of known dimensions was compared with supply expected by diffusion to a cylindrical model root of the same dimensions.
Allowance was made for absorption by the root hairs on rye-grass roots.
Phosphate absorption by a cm length of intact root was found to continue for at least 16 days for onion, 10 days for leek
and 5 days for rye-grass. Over a wide range of conditions (phosphate concentrations, soils, plant species), experimental uptake
was close to the maximum calculated to be possible for the diffusion model except on one soil at a high level of phosphate.
Although the concentration of phosphate in the soil solution at the root boundary appeared to be reduced to a small fraction
of the initial concentration, because of the extreme non-linear form of the desorption isotherm less than 1/2 of the P32 exchangeable pool of P was considered to contribute to diffusion.
Phosphate uptake by rye grass could only be accounted for if the root hairs were active. Although only a small fraction of
the uptake is derived from inside the root hair cylinder, this increases the efficiency of the central root 2.3 fold by providing
a zone close to the central root through which phosphate moves very readily. |
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