Shrinkage of Soybean Roots¹

It has been postulated that shrinkage and subsequent rehydration of roots growing in soil cause significant variations in root‐soil contact resistances to ion and water movements. The research reported here used neutron radiography to evaluate shrinkage of soybean ( Glycine max L. Merr. cv. Bethel) roots growing in soil as hydrated plants became severely wilted. The first experiment evaluated the amount of shrinkage possible for soybean roots as their water potential decreased from 0 to −1.34 MPa. The roots were immersed in polyethylene glycol solutions (PEG 6000) of 0, −0.37, or −1.34 MPa. Root diameters were measured as functions of time and distance from the root tip. Under controlled temperature conditions, roots did not change diameter in the 0 MPa solution but shrank to 0.97 of the original diameter in 2 h when immersed in the −0.37 MPa PEG solution and to 0.7 when immersed in the −1.34 MPa solution for 2 h. The second experiment used neutron radiography to evaluate taproot diameter of three soybean plants located in an unclassified sandy loam soil as hydrated plants lost water and wilted severely. Reproducibility with time of diameter readings on each section of root in the neutron radiography technique was about ± 0.02 mm ( ± 2%) on these three plants. When three hydrated plants wilted severely but recovered overnight, their roots shrank less than 0.02 mm (2%). These results indicated that shrinkages of plant roots growing in moist soil probably cause negligible increases in root‐soil contact resistance. These results do not rule out the possibility that roots will shrink appreciably as the soil around them dries substantially.