Evaluating Analytical Procedures for Quantifying Ammonium in Leaf Tissue

The effects of different methods of sample preparation, extraction, and storage on the recovery of the combined pool of ammonia plus ammonium (NH 3 + NH 4 + ) from `Washington' navel orange leaves previously incubated in solutions of increasing NH 4 Cl concentrations were assessed. Procedures and instruments for quantifying NH 3 + NH 4 + were tested for their sensitivity, reproducibility, and freedom from interference by amino acids. Reliable recoveries of NH 3 + NH 4 + free from amino acid interference, were obtained with oven-dried (60C) leaves ground to pass through a 40-mesh screen, extracted by homogenization in 10% TCA or by shaking in 2% acetic acid, and then filtered and analyzed on the basis of differences in electrical conductance between the sample and the reference cell. Methods measuring NH 3 + NH 4 + in KCl extracts by reaction with salicylate-nitroprusside in the presence of hypochlorite were compromised by significant color formation due to amino acids. Using fresh or freeze-dried leaf samples resulted in lower recoveries than use of oven-dried samples. Storage at -20C of fresh or oven-dried leaf samples in 10% TCA before or after homogenization and filtration did not alter NH 3 + NH 4 + levels, whereas storage of these samples at 4C increased NH 3 + NH 4 + levels.