The nitrogen and phosphorus nutrition of developing plants of two seagrasses, Posidonia australis and Posidonia sinuosa

The depletion of dry matter, N and P from seeds of Posidonia australis Hook. f. and Posidonia sinuosa Cambridge et Kuo during germination and seedling establishment is described. Seeds of both species showed essentially the same patterns of depletion, which resembled those of terrestrial plants. Seed reserves of N and P were retrieved with an apparent efficiency of ca. 95%, and linear relationships existed between the loss of these nutrients and dry matter from seeds during the first nine months of seedling growth. The distribution of dry matter, N and P amongst plant parts of both species was very similar. Leaves were major sinks for N and P during the first two years of plant development. Leaves contained the highest concentrations of N and rhizomes the highest levels of P. All seedling parts accumulated N and P against large concentration gradients. The environment contributed 100–150 mg N and 17–25 mg P m −2 to seedlings during their two years of growth. In 5-year-old plants, leaf bases contained important reserves of P, and 30–40% of the plant's N and P was associated with dead and morinund tissue. Leaf bases lost 84% of their N and 95% of their P during senescence.