Do roots mind the gap?

Abstract

Aims: Roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the root-soil interface. Do gaps actually limit the root water uptake, or do they form after water flow in soil is already limiting? Methods: Four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray CT at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential. Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements. Results: Transpiration started to decrease at soil matric potential ψ between -5 kPa and -10 kPa. Air-filled gaps appeared along tap roots between ψ = -10 kPa and ψ = -20 kPa. As ψ decreased below -40 kPa, roots further shrank and gaps expanded to 0.1 to 0.35 mm. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size. Conclusions: Gaps formed after the transpiration rate decreased. We conclude that gaps are not the cause but a consequence of reduced water availability for lupins. © 2012 The Author(s).