Effect of ammonium application on the oxalate content of tea plants (Camellia sinensis L.)

Abstract

Tea plants (Camellia sinensis L.) were hydroponically cultured for 9 d in a solution containing ammonium as a sole nitrogen source (0 (control), 50, 100, or 200 mg N L-1 ), and harvested after 0 (control), 1,3,6, and 9 d of treatments to investigate the effect of ammonium on the content and metabolism of oxalate in tea plants. Total and water-soluble oxalate contents in young leaves and roots treated with 50, 100, and 200 mg N L-1 decreased in proportion to the amount of ammonium applied, with levels significantly lower than those without ammonium application. In old leaves, both total and water-soluble oxalate contents showed constant values during the treatment period. The content of total ascorbic acid (AsA) in young leaves increased with the amount of ammonium in the medium. The activity of oxalate oxidase (OxO) in young leaves also increased with the concentration of ammonium in the medium and with the duration of the treatment. However, in roots, total AsA content and OxO activity were not affected. These results suggest that in young tea leaves, the application of ammonium suppressed the synthesis of oxalate via AsA oxidation and stimulated the decomposition of oxalate by the increase of the OxO activity, resulting in the accumulation of AsA and a decrease in oxalate content. On the other hand, in old leaves and young roots, the responses to ammonium were different from those in young leaves, suggesting that the oxalate metabolism may be controlled by different mechanisms in different parts of the tea plants.