Nitrogen uptake by tropical orchids

Abstract

The uptake of nitrate and ammonium by two terrestrial orchids ( Bromheadia finlaysonia and Cymbidium sinense) and an epiphytic orchid ( Dendrobium “White Fairy”) in solution culture was studied. The rates of uptake of nitrate and ammonium were linear, with a higher uptake rate for ammonium. The rates of nitrate uptake for Cymbidium and Bromheadia were in the range of 0.3–0.4 μmole g fw −1 hr −1 and for Dendrobium the rate was 0.9 μmole g fw −1 hr −1. These rates were considerably lower than those of most major crops. SEM studies showed that the velamen of Bromheadia was two cells thick, whereas that of Dendrobium and Cymbidium was eight to 10 cells thick. It is unlikely that the velamen is the major factor in restricting the influx of nitrate or ammonium. Nitrate reductase (NR) and glutamine synthetase (GS) activity was present in the roots and leaves of the three orchids. NR activity was high in the roots but low in the leaves. The reverse was true for GS. The activity of NR and GS was low, but high enough to account for the rate of nitrate or ammonium uptake. It appears that the movement of ions across the transfer junction at the exodermis plays a major regulatory role in ion uptake by orchid roots.