Effect of light intensity on ammonia assimilation in maize leaves

The effect of light on the metabolism of ammonia was studied by subjecting detached maize leaves to 150 or 1350 μmol m(-2) s(-1) PAR during incubation with the leaf base in 2 mM (15)NH4Cl. After up to 60 min, leaves were extracted. Ammonia, glutamine, glycine, serine, alanine, and aspartate were separated by isothermal distillation and ion exchange chromatography. (15)N enrichments were analyzed by emission spectroscopy. The uptake of ammonium chloride did not influence CO2 assimilation (8.3 and 17.4 μmol m(-1) s(-1) at 150 and 1350 μmol m(-2) s(-1) PAR, respectively). Leaves kept at high light intensity contained more serine and less alanine than leaves from low light treatments. Within 1 h of incubation the enrichment of ammonia extracted from leaves rose to approximately 20% (15)N. In the high light regime the amino acids contained up to 15% (15)N, whereas in low light (15)N enrichments were small (up to 6%). The kinetics of (15)N incorporation indicated that NH3 was firstly assimilated into glutamine and then into glutamate. After 15 min (15)N was also found in glycine, serine and alanine. At high light intensity nearly half of the (15)N was incorporated in glycine. On the other hand, at low light intensity alanine was the predominant (15)N sink. It is concluded that light influences ammonia assimilation at the glutamine synthetase reaction.