EFFECT OF OXYGEN AVAILABILITY AND SALINITY ON EARLY-LIFE HISTORY STAGES OF SALT-MARSH PLANTS .2. EARLY SEEDLING DEVELOPMENT ADVANTAGE OF SPARTINA-ALTERNIFLORA OVER PHRAGMITES-AUSTRALIS (POACEAE)

In salt marsh soils, germination and the first phases of seedling deve lopment often occur under dark, hypoxic or anoxic, and saline conditio ns. Spartina alterniflora and Phragmites australis seedling developmen t were examined under covaried oxygen and salinity concentrations in g rowth chamber experiments. First, the effects of oxygen and salinity o n seedling development were tested in the dark, using a 5 X 5 factoria l design. Oxygen did not affect P. australis plumule growth at oxygen concentrations from 21 down to 2.5%. Plumules were longer at less than or equal to 10 than at greater than or equal to 25 g NaCl/L. Root gro wth was maximum in 21% oxygen, at less than or equal to 10 g NaCl/L an d reduced at all salinities in oxygen concentrations less than or equa l to 10%. No plumule or root growth occurred under anoxia. Salinity di d not affect S. alterniflora mesocotyl emergence, which was fastest in anoxia and hypoxia. Mesocotyls did not emerge from the spikelet in 21 % oxygen. In contrast, plumule growth was fastest in 21% oxygen, but w as inhibited in anoxia. Under low oxygen and high salinity both plumul e and root elongation were reduced. Coleoptile and mesocotyl elongatio n were greatest in 2.5 and 5% oxygen, and shortest in anoxia. The perc entage of mesocotyls elongating was also highest in 2.5% oxygen. Rapid S. alterniflora coleoptile and mesocotyl elongation in anoxia enables its seedlings to escape the impact of the stressful environment where its seeds can germinate, but the seedlings could not survive otherwis e. In separate experiments, S. alterniflora seeds were germinated and the seedlings grown in the dark for 10 d, then exposed to light for 4 d. Prior to illumination the seedlings did not develop beyond the stag e of a small plumule enclosed in the coleoptile. Within 48 h of illumi nation in the presence of CO2, roots emerged and the plumule elongated inside the coleoptile at salinities up to 40 g NaCl/L, while the exte rnal environment remained anoxic. Without CO2, plumule growth and root development did not occur. This suggested oxygen was produced inside the coleoptile by the photosynthesizing plumule, and triggered root de velopment. The ability of S. alterniflora seedlings to continue develo pment under external anoxia and high salinity gives that species a com petitive advantage over P. australis in high salinity and/or poorly dr ained marshes.