Effect of alternative anions (Cl–vs. SO$ _4^{2-} $) on concentrations of free amino acids in young tea plants

Abstract

AbstractThe quality of green tea is highly dependent on the concentration of free amino acids, whose profile is dominated by the unique amino acid theanine (N5‐ethyl‐glutamine). A high quality is associated with a high amino acid–to–catechin ratio, but previous results indicated that excessive chloride (Cl–) supply is detrimental for amino acid accumulation. Several experiments were conducted to investigate the effect of chloride on growth and concentrations of free amino acids in young tea plants. Soil‐grown tea plants supplied with different levels of potassium (K) as K2SO4 or KCl exhibited increased concentrations of free amino acid in young shoots only when supplied with K2SO4, and the negative effect of KCl supply was mainly due to a reduced concentration of theanine. Concentrations of other nutrients in plant tissues were not influenced. The uptake of Cl– and its interaction with nitrogen (N) uptake were further investigated in a second experiment, in which soil‐cultivated tea plants were supplied with varying amounts of Cl–. Chloride application reduced yield of young shoots, and severity of leaf damage was related to the concentration of Cl– in leaves. Nitrogen uptake was reduced by Cl– addition. To verify whether the decrease of free amino acids was simply a result of inhibited NO$ _3^- $ assimilation, a third experiment was conducted, in which tea plants were NH$ _4^+ $‐fed in the absence or presence (equivalent to the NH$ _4^+ $ concentration) of Cl–. Again, concentrations of theanine and total free amino acids in young shoots were reduced by Cl– supply, but changes of the free–amino acid pool did not contribute to the maintenance of charge balance. However, concentration of theanine in roots, where it is synthesized, was not influenced by Cl–. Total N concentrations of roots and mature leaves, uptake rate of NH$ _4^+ $, and activity of glutamine synthetase in fibrous roots and young leaves were all unaffected by Cl– as well. It is suggested that translocation of theanine from root to shoot and its catabolism in young shoots might be influenced by Cl–.