Effect of nitrogen form and root-zone pH on growth and nitrogen uptake of tea (Camellia sinensis) plants

摘要

Background and Aims Tea (Camellia sinensis) is considered to be acid tolerant and prefers ammonium nutrition, but the interaction between root zone acidity and N form is not properly understood. The present study was performed to characterize their interaction with respect to growth and mineral nutrition. Methods Tea plants were hydroponically cultured with NH4+, NO3- and NH4+ + NO3-, at pH 4.0, 5.0 and 6.0, which were maintained by pH stat systems. Key Results Plants supplied with NO3- showed yellowish leaves resembling nitrogen deficiency and grew much slower than those receiving NW or NH4+ + NO3- irrespective of root-zone pH. Absorption of NHT was 2- to 3.4-fold faster than N03 when supplied separately, and 6- to 16-fold faster when supplied simultaneously. Nitrate-grown plants had significantly reduced glutamine synthetase activity, and lower concentrations of total N, free amino acids and glucose in the roots, but higher concentrations of cations and carboxylates (mainly oxalate) than those grown with NH4+ or NH4+ + NO3-. Biomass production was largest at pH 5.0 regardless of N form, and was drastically reduced by a combination of high root-zone pH and NO3-. Low root-zone pH reduced root growth only in NO3--fed plants. Absorption of N followed a similar pattern as root-zone pH changed, showing highest uptake rates at pH 5.0. The concentrations of total N, free amino acids, sugars and the activity of GS were generally not influenced by pH, whereas the concentrations of cations and carboxylates were generally increased with increasing root-zone pH. 9 Conclusions Tea plants are well-adapted to NH4+-rich environments by exhibiting a high capacity for NH4+ assimilation in their roots, reflected in strongly increased key enzyme activities and improved carbohydrate status. The poor plant growth with NO3- was largely associated with inefficient absorption of this N source. Decreased growth caused by inappropriate external pH corresponded well with the declining absorption of nitrogen.