Effect of root zone pH and form and concentration of nitrogen on accumulation of quality‐related components in green tea

Green tea quality is greatly influenced by concentrations of free amino acids, polyphenols (mainly catechins) and caffeine. The present study investigated the principal relationship between selected nutritional factors (form and concentration of N supply, root zone pH) and accumulation of these quality‐related components of tea (Camellia sinensis (L.)) plants. Tea plants were hydroponically cultured with NH₄⁺ , NO₃⁻ and NH₄⁺ + NO₃⁻ at pH 4.0, 5.0 and 6.0 in one experiment and supplied with varying N concentrations (0.75, 2.0 and 4.5 mmol L−1, NH₄⁺ /NO−3 = 3:1) in another experiment. Concentrations of free amino acids were considerably higher in NH₄⁺ ‐ than in NO₃⁻ ‐fed plants. This was attributed to the much greater absorption of NH₄⁺ compared with NO₃⁻ . Furthermore, the relative allocation of absorbed N to free amino acids, particularly theanine and glutamine, was substantially increased by NH₄⁺ nutrition, suggesting that NH₄⁺ was more readily assimilated than NO₃⁻ into theanine. The concentration of caffeine was increased in NH₄⁺ ‐ and (NH₄⁺ + NO₃⁻ )‐supplied plants, whereas concentrations of catechins were reduced in (NH₄⁺ + NO₃⁻ )‐fed plants. Root zone pH did not influence concentrations of most free amino acids in young shoots, with the exception of theanine, which increased at low pH (4.0) irrespective of N form; this likely stemmed from an accumulation effect, as growth decreased more strongly than N absorption. Raising the N supply increased plant N allocation to free amino acids. The increase was most striking for arginine, while theanine was only marginally affected. This may have adverse consequences for green tea quality, as less favourable taste characteristics have been attributed to arginine.