Assimilation of Gaseous Ammonia by Sunflower Leaves during Photosynthesis

Abstract

Illuminated (150 �mol m-2 s-1 PAR) sunflower (Helianthus annuus L.) leaves were exposed to 15N-labelled NH3 gas (30 or 300 �l NH3 l-1) or supplied with NH4Cl (2, 20 or 200 mM) through the cut leaf base for up to 1 h. The photosynthetic carbon dioxide uptake and the transpiration were continuously monitored. The leaves were extracted and the concentrations of ammonia, glutamine, glutamate, glycine, serine and alanine were determined as well as the patterns of 15N incorporation. Although the rates of CO2 uptake and transpiration were unchanged in intact leaves, the ammonia level increased from 3 to 42 �mol g fresh wt-1 during feedings of 300 �l NH3 l-1 for 25 min. In contrast, NH4Cl (20 and 200 mM) inhibited photosynthesis immediately and increased the evaporation rate while the total ammonia level was still less than 10 �mol g fresh wt-1. The 15N-labelling pattern suggested that the NH3 assimilation proceeded via glutamine synthetase and glutamate synthase. 15N was transferred into glycine and serine presumably by the activity of glycollate pathway enzymes. The experiments demonstrate that the photosynthetic CO2 uptake and ammonia assimilation by intact sunflower leaves were not inhibited by NH3 concentrations orders of magnitudes above typical concentrations of this compound in the atmosphere. NH3 feedings through the gas phase provide a tool for the examination of nitrogen metabolism in intact leaves.