Effect of Nitrogen Form and Root-zone pH on Growth and Nitrogen Uptake of Tea ( Camellia sinensis ) Plants

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. Tea plants were hydroponically cultured with NH 4+ , NO 3− and NH 4+ +NO 3− , at pH 4·0, 5·0 and 6·0, which were maintained by pH stat systems. Plants supplied with NO 3− showed yellowish leaves resembling nitrogen deficiency and grew much slower than those receiving NH 4+ or NH 4+ +NO 3− irrespective of root-zone pH. Absorption of NH 4+ was 2- to 3·4-fold faster than NO 3− 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 NH 4+ or NH 4+ +NO 3− . 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 NO 3− . Low root-zone pH reduced root growth only in NO 3− -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. Tea plants are well-adapted to NH 4+ -rich environments by exhibiting a high capacity for NH 4+ assimilation in their roots, reflected in strongly increased key enzyme activities and improved carbohydrate status. The poor plant growth with NO 3− 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.