Leaf versus root nitrogen uptake by the surfgrass Phyllospadix torreyi

Abstract

Phyllospadix torreyi 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 Km of 17.4 µM and a Vmax of 125.1 µmol N g dry wt(-1) h(-1). Km and Vmax 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.