Water Transport in an Unsaturated Medium to Roots with Differing Local Geometries

Pearl millet [Pennisetum glaucum (L.) R. Br.: syn. P. americanum (L.) Leeke] develops substantially lower predawn leaf water potentials than cowpea [Vigna unguiculata (L.) Walp.] under conditions of soil dehydration, but overall root length densities are greater in millet than in cowpea. The hypothesis was tested that, due to differences in root geometry and root length density, root systems of millet become less efficient in water uptake than those of cowpea as soil dries. Plants were grown in an artificial medium in pots or 1.15-m tubes in a greenhouse and subjected to soil drying. Root geometries, root length densities, and water potentials of cowpea and millet were measured as the rooting medium dried. Distribution of roots was quantified by calculating a clumping ratio (total root length density divided by the root length density in the region surrounding individual root segment axes) at several depths within the profile. Overall root length densities were higher in millet than in cowpea, and root distributions differed. Millet roots were clumped locally along the root axis and globally within the profile, with the highest root length densities at the surface. Cowpea root lengths were one-half as dense as millet along the root axis and distributed fairly uniformly throughout the profile. These data were used in two-dimensional modeling of horizontal water transport from soil to branched segments of root axes of cowpea and millet to test the hypothesis. Simulations in which millet water potential became substantially lower than cowpea, and millet root length was twice as great as cowpea, predicted little difference in cumulative water flow to the roots of millet or cowpea during soil dehydration. This prediction is consistent with observations. Simulated water velocity vector fields indicated that millet's dense and clumped local root geometry became less efficient in water uptake under soil water deficits than cowpea's less dense and more evenly distributed root geometry because water uptake by millet was mainly at the root tips; in contrast, most of the root length of cowpea contributed to water uptake. Simulations conducted with millet root axes having different root length densities indicated that the low efficiency of millet in water uptake was due to the combined effects of local clumping and high root length density. When input parameters for roots and soil were varied, conclusions from the simulations remained the same.

Duke Scholars

Author Zbigniew J. Kabala Civil and Environmental Engineering