ABSTRACT: Phyllospadixtorreyi S. Watson ('surfgrass') is one of very few seagrass species that grow on rocks. Thus, unlike most other seagrasses, nutrient uptake across leaves rather than across roots might be very important for nutrient acquisition. Ammonium and nitrate+nitrite uptake by surfgrass leaves was measured under flowing water and modelled (Michaelis-Menten) as a function of external concentrations. Leaf NH₄⁺ uptake showed a K_m of 17.4 µM and a V_max of 125.1 µmol N g dry wt^-1 h^-1. K_m and V_max values for leaf NO₃^-+NO₂^- uptake were 10.1 µM and 54.5 µmol N g dry wt^-1 h^-1, respectively. Ammonium available to the roots had little discernible effect on NH₄⁺ uptake by leaves. Low NH₄⁺ uptake rates by roots (<0.2 µmol N g dry wt^-1 h^-1) suggest that surfgrass acquires most of its nitrogen via its leaves.

KEY WORDS: Ammonium · Nitrate · Leaf uptake · Root uptake · Phyllospadix · Seagrasses

Published in MEPS Vol. 149 (1997) on April 10
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997