Effect of Oxygen Availability and Salinity on Early Life History Stages of Salt Marsh Plants. II. Early Seedling Development Advantage of Spartina alterniflora Over Phragmites australis (Poaceae)

Abstract

In salt marsh soils, germination and the first phases of seedling development often occur under dark, hypoxic or anoxic, and saline conditions. Spartina alterniflora and Phragmites australis seedling development were examined under covaried oxygen and salinity concentrations in growth chamber experiments. First, the effects of oxygen and salinity on seedling development were tested in the dark, using a 5 5 factorial design. Oxygen did not affect P. australis plumule growth at oxygen concentrations from 21 down to 2.5%. Plumules were longer at ≤10 than at ≥25 g NaCl/L. Root growth was maximum in 21% oxygen, at ≤10 g NaCl/L and reduced at all salinities in oxygen concentrations ≤10%. No plumule or root growth occurred under anoxia. Salinity did 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 was inhibited in anoxia. Under low oxygen and high salinity both plumule and root elongation were reduced. Coleoptile and mesocotyl elongation were greatest in 2.5 and 5% oxygen, and shortest in anoxia. The percentage 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 otherwise. 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 stage of a small plumule enclosed in the coleoptile. Within 48 h of illumination in the presence of CO2, roots emerged and the plumule elongated inside the coleoptile at salinities up to 40 g NaCl/L, while the external 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 development. The ability of S. alterniflora seedlings to continue development under external anoxia and high salinity gives that species a competitive advantage over P. australis in high salinity and/or poorly drained marshes.