Leaf versus root nitrogen uptake by the surfgrass Phyllospadix torreyi

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 b e very important for nutrient acquisition.Ammonlum and nitrate+nitrite uptake by surfgrass leaves was measured under flowing water and modelled (Michaelis-Menten) as a functlon of external concentrations.Leaf NH,' uptake showed a Km of 17.4 PM and a V, , , , , of 125.1 pm01 N g dry wt-' h-' Km and V, , , values for leaf NO3 +NO2-uptake were 10.1 pM and 54.5 pm01 N g dry wt.' h-', respectively.Ammonium available to the roots had little discernible effect on NH,' uptake by leaves Low NH,' uptake rates by roots ( ~0 . 2 pm01 N g dry wt.' h-'] suggest that surfgrass acquires most of its nitrogen via its leaves.