Experiments using split-root chambers on water uptake from soil macropores by sunflowers

The purpose of this study was to use split-root chambers to determine whether sunflowers (Helianthus annuus), which possess high transpirational demand, can preferentially extract macropore water when they are given the option of using either micropore or macropore water. Sunflower plants were grown in split-root chambers with half of their roots in a macroporous soil and half in a microporous soil of identical mineralogy. The two chambers were irrigated with water of different stable hydrogen isotope compositions (δD) (tap water and melted snow water). By measuring the δD of the sunflower xylem sap, it was possible to determine from which soil type the sunflowers were extracting water. It was found that sunflowers did not preferentially extract macropore water. Since sunflower plants possess very high transpiration rates on a whole-plant basis, and because transpiration remains high even under soil drying, we believe sunflower roots must have a consistently very low water potential compared with the soil water potentials of either the macroporous or microporous soils. If root water potentials are sufficiently low, then root water uptake cannot discriminate between macropores and micropores.