Aerenchymatous phellem in hypocotyl and roots enables O2 transport in Melilotus siculus

• Aerenchymatous phellem (secondary aerenchyma) has rarely been studied in roots. Its formation and role in internal aeration were evaluated for Melilotus siculus, an annual legume of wet saline land. • Plants were grown for 21 d in aerated or stagnant (deoxygenated) agar solutions. Root porosity and maximum diameters were measured after 0, 7, 14 and 21 d of treatment. Phellem anatomy was studied and oxygen (O(2)) transport properties examined using methylene blue dye and root-sleeving O(2) electrodes. • Interconnecting aerenchymatous phellem developed in hypocotyl, tap root and older laterals (but not in aerial shoots), with radial intercellular connections to steles. Porosity of main roots containing phellem was c. 25%; cross-sectional areas of this phellem were threefold greater for stagnant than for aerated treatments. Root radial O(2) loss was significantly reduced by complete hypocotyl submergence; values approached zero after disruption of hypocotyl phellem below the waterline or, after shoot excision, by covering hypocotyl phellem in nontoxic cream. • Aerenchymatous phellem enables hypocotyl-to-root O(2) transport in M. siculus. Phellem increases radially under stagnant conditions, and will contribute to waterlogging tolerance by enhancing root aeration. It seems likely that with hypocotyl submerged, O(2) will diffuse via surface gas-films and internally from the shoot system.