Water efflux from the surface of field‐grown grass roots. Observations by cryo‐scanning electron microscopy

Field‐grown roots of maize, oats, barley and crabgrass collected at different times of day and night have been examined by cryo‐scanning electron microscopy. Droplets and blobs (their high water content confirmed by X‐ray microanalysis) were observed on the young root surfaces and within interstices of the rhizosheaths of guttating plants, particularly in the early morning. Plants collected at midday and in the afternoon had very few or none of these watery deposits. I propose that water released from the root surface during the night allows the expansion of root‐cap mucilage into the surrounding soil. When transpiration resumes, the mucilage is dried and binds tightly to the surrounding soil particles, thus stabilizing the coherent rhizosheath. This nocturnal water effflux may also facilitate nutrient uptake into the root when transpiration resumes. The water efflux to the rhizosheaths may be driven by root pressure and thus differs from the passive water loss from the root thought to occur during hydraulic lift.