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Water Extraction by Single Plant Roots¹

ABSTRACT

Application of computer assisted tomography to x-ray attenuation measurements has been used to study the drawdowns in soil-water content associated with single plant roots. Drawdowns have been measured at different depths along radish (Raphanus sativus cv long scarlet) roots growing in soil at two different initial water contents while the plants were subjected to either a high or low transpirational demand. This novel approach has provided the most detailed observations of this type yet obtained, particularly by a nondestructive technique. Soil resistance to water flow is clearly of significance even at the high soil-water potentials used since it has been observed to markedly influence water uptake. Comparisons between these experimentally determined drawdowns and those predicted by an analytical approach indicate that this model does not adequately describe the extraction process. In particular, the assumption that a plant root acts as a uniform absorbing cylinder along its length is clearly erroneous. While a numerical model gives a closer fit to the experimental data, it is clear that improvements in the physical concepts on which this is based are necessary to accurately describe the position and shape of the drawdowns.

¹ Contribution from Soil Science and Plant Nutrition, School of Agriculture, Univ. of Western Australia, Nedlands, Western Australia, 6009.

² Research Officer and Associate Professor of Soil Science, respectively.

Received for publication May 23, 1985.

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