The hydraulic safety zone at the base of barley roots.

A hydraulic constriction of the vessels occurs at the base of the primary roots of barley (Hordeum vulgare L.). The constriction and consequent hydraulic protection result from an extreme shortening of the vessel elements, leading to the accumulation of perforation plates with simple, broad-rimmed perforations which are smaller than those in normal-length vessel elements. It is compensated for by a local increase in the number of tracheary elements and an increase in their diameter. A similar trend of development was observed both at the base of other seminal roots and at the base of stem-borne adventitious roots. The rate at which compensation for the hydraulic constriction occurs could be of crucial importance for the axial resistance of water transport.