When Roots Lose Contact

It has been speculated that during periods of water deficit, roots may shrink and lose contact with the soil, with a consequent reduction in root water uptake. Due to the opaque nature of soil, however, this process has never been observed in situ for living plants. Through x-ray tomography and image analysis, we have demonstrated the formation and dynamics of air gaps around roots. The high spatial resolution required to image the soil–root gaps was achieved by combining tomography of the entire sample (field of view of 16 by 16 cm, pixel side 0.32 mm) with local tomography of the soil region around the roots (field of view of 5 by 5 cm, pixel side 0.09 mm). For a sandy soil, we found that when the soil dries to a water content of 0.025 m 3 m −3 , gaps occur around the taproot and the lateral roots of lupin ( Lupinus albus L.). Gaps were larger for the taproot than the laterals and were caused primarily by root shrinkage rather than by soil shrinkage. When the soil was irrigated again, the roots swelled, partially refilling the gaps; however, large gaps persisted in the more proximal, older part of the taproot. Gaps are expected to reduce water transfers between soil and roots. Opening and closing of gaps may help plants to prevent water loss when the soil dries, and to restore the soil–root continuity when water becomes available. The persistence of gaps in the more proximal parts is one reason why roots preferentially take up water from their more distal parts.

[1]  P. E. Weatherley,et al.  ROOT CONTRACTION IN TRANSPIRING PLANTS , 1982 .

[2]  M. Caldwell,et al.  Hydraulic lift: consequences of water efflux from the roots of plants , 1998, Oecologia.

[3]  H. M. Taylor,et al.  Diurnal variations in root diameter. , 1970, Plant physiology.

[4]  Jean Serra,et al.  Image Analysis and Mathematical Morphology , 1983 .

[5]  Hiroyuki Nishiyama.  Water gradient profiles at bean plant roots determined by neutron beam analysis , 2005 .

[6]  J. Sprent,et al.  Effects of sodium chloride and polyethylene glycol on root-hair infection and nodulation of Vicia faba L. plants by Rhizobium leguminosarum , 1986, Planta.

[7]  E. Segal,et al.  Microsensing of Water Dynamics and Root Distributions in Sandy Soils , 2008 .

[8]  J. Lynch,et al.  Substrate flow in the rhizosphere , 1990, Plant and Soil.

[9]  Tammo S. Steenhuis,et al.  Wetting front instability: 2. Experimental determination of relationships between system parameters and two‐dimensional unstable flow field behavior in initially dry porous media , 1989 .

[10]  P. Nobel,et al.  Hydraulic Conductances of the Soil, the Root-Soil Air Gap, and the Root: Changes for Desert Succulents in Drying Soil , 1992 .

[11]  Pierre Soille,et al.  Morphological Image Analysis: Principles and Applications , 2003 .

[12]  Rainer Schulin,et al.  Quantitative Imaging of Infiltration, Root Growth, and Root Water Uptake via Neutron Radiography , 2008 .

[13]  J. Hopmans,et al.  Three dimensional imaging of plant roots in situ with X-ray Computed Tomography , 1997, Plant and Soil.

[14]  E. Steudle,et al.  Water Transport in Onion (Allium cepa L.) Roots (Changes of Axial and Radial Hydraulic Conductivities during Root Development) , 1993, Plant physiology.

[15]  Mark L. Rivers,et al.  Using X-ray computed tomography in hydrology: systems, resolutions, and limitations , 2002 .

[16]  J. Hopmans,et al.  Thermal Neutron Computed Tomography of Soil Water and Plant Roots , 2007 .

[17]  W. R. Gardner.  DYNAMIC ASPECTS OF WATER AVAILABILITY TO PLANTS , 1960 .

[18]  P J Kramer,et al.  Observation of a water-depletion region surrounding loblolly pine roots by magnetic resonance imaging. , 1990, Proceedings of the National Academy of Sciences of the United States of America.

[19]  Xuecheng Zhou,et al.  Visualization of Plant Root Morphology in situ Based on X-ray CT Imaging Technology , 2004 .

[20]  Kenichi Soga,et al.  Laboratory Study of Immiscible Contaminant Flow in Unsaturated Layered Sands , 2008 .

[21]  P. M. Neumann,et al.  Short episodes of water stress increase barley root resistance to radial shrinkage in a dehydrating environment , 2006 .

[22]  E. J. Morton,et al.  Non-invasive imaging of roots with high resolution X-ray micro-tomography , 2003, Plant and Soil.

[23]  M. Noordwijk,et al.  Root-soil contact of maize, as measured by a thin-section technique , 2004, Plant and Soil.

[24]  Manfred Krafczyk,et al.  Tomographical Imaging and Mathematical Description of Porous Media Used for the Prediction of Fluid Distribution , 2006 .

[25]  H. Marschner,et al.  Water and nutrient translocation by hyphae of Glomus mosseae , 1992 .

[26]  X. Zhou,et al.  Perception of Bradyrhizobium japonicum Nod factor by soybean [Glycine max (L.) Merr.] root hairs under abiotic stress conditions. , 2004, Journal of experimental botany.

[27]  S. Anderson,et al.  Computed tomographic evaluation of osmotica on shrinkage and recovery of lupin (Lupinus angustifolius L.) and radish (Raphanus sativus L.) roots , 2007 .

[28]  P. Nobel,et al.  Root-soil contact for the desert succulent Agave deserti in wet and drying soil. , 1997, The New phytologist.

[29]  H. M. Taylor,et al.  Shrinkage of Soybean Roots1 , 1983 .

[30]  Anders Kaestner,et al.  Visualizing three-dimensional root networks using computed tomography , 2006 .

[31]  Jan Vanderborght,et al.  Changes in Soil Water Content Resulting from Ricinus Root Uptake Monitored by Magnetic Resonance Imaging , 2008 .

[32]  P. Nye.  The Effect of Root Shrinkage on Soil Water Inflow , 1994 .