MEPS 134:195-206 (1996)  -  doi:10.3354/meps134195

The kinetics of ammonium and phosphate uptake by leaves and roots of the tropical seagrass Thalassia hemprichii were investigated in laboratory 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), was compared. The leaves from all plant samples showed a clear capacity for both ammonium and phosphate uptake. This uptake could be described by Michaelis-Menten kinetics. v_max ranged between 32 and 37 umol g^-1 leaf dry weight h^-1 for ammonium and between 2.2 and 3.2 umol g^-1 leaf dry weight h^-1 for phosphate. K_m ranged between 21 and 60 uM for ammonium and between 7.7 and 15 uM for phosphate. There 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 uptake rates at natural pore water concentrations. Uptake rates were 22 and 1.0 umol g^-1 root dry weight h^-1 for ammonium and phosphate, respectively. Calculations suggest that at all 3 locations uptake of ammonium and phosphate by roots was probably limited by the diffusion of nutrients in the sediment rather than by their uptake capacity. Evidence was found that the availability of nutrients in the root zone relative to the leaf zone affects the uptake affinity of the leaves. The role of roots versus leaves in supplying plant nutrients is discussed. We concluded that even in the tropics, where water column nutrient concentrations are often very low, leaves clearly have a significant ability for ammonium or phosphate uptake and that in some situations nutrient uptake by the leaves may even be essential in meeting plant nutrient demands.

Ammonium . Phosphate . Nutrient uptake kinetics . Leaves . Roots . Seagrass . Thalassia hemprichii . Indonesia