GROWTH-CHARACTERISTICS OF ZOSTERA-MARINA SEEDLINGS UNDER ANAEROBIC CONDITIONS

Seeds of Zostera marina L. are capable of germinating in anaerobic (nitrogen-gassed) seawater (10 parts per thousand) and developing into seedlings with unusual morphology. The axial hypocotyl of seedlings grown in light and dark was characteristically elongated (mean in dark = 32 mm), but the plumule and adventitious roots remained rudimentary and embryonic in appearance. Aerobic seedlings by contrast exhibited little hypocotyl elongation (mean in dark = 5 mm), and plumule and roots were well developed. The stalk-like blade of the cotyledon elongated in both aerobic and anaerobic cultures, but growth of the cotyledonary sheath surrounding the plumule was most marked in the presence of oxygen. When seedlings were grown anaerobically in the dark for 16 days and then transferred to aerobic-light conditions, elongation of the axial hypocotyl ceased instantly, but plumule and cotyledonary sheath growth was evident within 32 h, and by 96 h after transfer there was pronounced greening of the plumule leaves and swelling of the root primordia. Hypocotyl elongation of seedlings from seeds planted in sediment varied directly with planting depth. Elongation of the hypocotyl pushed the cotyledonary node with rudimentary plumule and root primordia up through the sediment to a position below the sediment surface where hypocotyl elongation stopped and plumule and root growth commenced. The cotyledonary blade which grew concomitantly with the axial hypocotyl continued to elongate after protruding from the sediment into the overlying water. Results from experiments in which the distal part of a seedling cotyledon was exposed to aerated water while the rest of the seedling was maintained in N2-gassed water suggested that the cotyledon may function as a conduit for oxygen diffusion to the plumular region of the seedling.