Assimilation of Gaseous Ammonia by Sunflower Leaves during Photosynthesis

Abstract

Illuminated (150 µmol m^-2 s^-1 PAR) sunflower (Helianthus annuus L.) leaves were exposed to ¹⁵N-labelled NH₃ gas (30 or 300 µl 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 µmol g fresh wt^-1 during feedings of 300 µl 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 µmol 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.