Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/15/1992
Publication Date: N/A
Citation: Interpretive Summary: Under conditions of progressive soil dehydration, pearl millet develops lower predawn leaf water potentials than cowpea, even when the plants were grown together in the same pot. The primary objective of this study were to: (I) quantify differences in geometry and root density of millet and cowpea roots, and (ii) model two-dimensional horizontal water flow to roots under drying conditions to determine whether the differences in root geometry and root length density could be responsible for the lower predawn water potential observed in millet than in cowpea.
Technical Abstract: Pearl millet develops substantially lower predawn leaf water potentials than cowpea under condition 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 system of millet become less efficient in water uptake than those of cowpea as soil dries. Plants were grown in an artificial medium in pots of 1.15 m tubes in a greenhouse and subjected to 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 at several depths within the profile. 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, 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.