NUTRIENT-UPTAKE BY LEAVES AND ROOTS OF THE SEAGRASS THALASSIA-HEMPRICHII IN THE SPERMONDE ARCHIPELAGO, INDONESIA

The kinetics of ammonium and phosphate uptake by leaves and roots of t he tropical seagrass Thalassia hemprichii were investigated in laborat ory experiments. Uptake in leaves of plants from 3 different locations , covering the range from coastal to oceanic conditions in the region of investigation (Spermonde Archipelago, South Sulawesi, Indonesia), w as compared. The leaves from all plant samples showed a clear capacity for both ammonium and phosphate uptake. This uptake could be describe d by Michaelis-Menten kinetics. v(max) ranged between 32 and 37 mu mol g(-1) leaf dry weight h(-1) for ammonium and between 2.2 and 3.2 mu m ol g(-1) leaf dry weight h(-1) for phosphate. K-m ranged between 21 an d 60 mu M for ammonium and between 7.7 and 15 mu M for phosphate. Ther e was no significant site difference in uptake characteristics (v(max) and K-m) of ammonium and phosphate. Uptake of ammonium and phosphate by roots was investigated with plants from the intermediate location, Barang Lompo, using an approach which allowed only calculation of upta ke rates at natural pore water concentrations. Uptake rates were 22 an d 1.0 mu mol g(-1) root dry weight h(-1) for ammonium and phosphate, r espectively Calculations suggest that at all 3 locations uptake of amm onium and phosphate by roots was probably limited by the diffusion of nutrients in the sediment rather than by their uptake capacity. Eviden ce was found that the availability of nutrients in the root zone relat ive to the leaf zone affects the uptake affinity of the leaves. The ro le of roots versus leaves in supplying plant nutrients is discussed. W e concluded that even in the tropics, where water column nutrient conc entrations are often very low, leaves clearly have a significant abili ty for ammonium or phosphate uptake and that in some situations nutrie nt uptake by the leaves may even be essential in meeting plant nutrien t demands.