REVERSE WATER-FLOW IN SORGHUM ROOTS

Efflux of water from plant roots has implications for nutrient uptake in dry soil, the effectiveness of collection of water from deep in the soil, water parasitism among plants, and the ability of roots to resume water uptake after exposure to dry soil. We measured the minimum soil water potential required for reverse flow in sorghum (Sorghum bicolor (L.) Moench], the diurnal time-course of the efflux, and differences between sorghum genotypes in reverse flow. A split-root system was used in which near-surface roots were subjected to drying and deeper roots were in free water. Efflux of water to the dry soil could first be detected at a soil water potential of about -0.55 MPa, or across a soil water potential gradient of 0.55 MPa, a smaller value than previously reported to induce reverse flow. Outflow was 5 to 6% of daily transpiration during the periods of highest water use. Differences between the genotypes in the amount of water emitted and recaptured and the mean water content of the soil were mimicked by a computer simulation of water uptake and release by plant roots. Plant parameters contributing to simulation of the observed behavior were the relationship of leaf water potential to stomatal closure and root length density. Water efflux from roots into the pot soil began at 1430 h, with the peak rate occurring near the end of the daytime period (1900 h) and cessation of outflow by 2400 h. Initiation of outflow during daytime is both physically reasonable and simulated in computer models. The time-course of outflow, however, was not correctly simulated. Measurements should be made at 1 hour time resolution to capture all reverse flow.