The effect of plant mineral nutrition on yield and quality of green tea (Camellia sinensis L.) under field conditions
Field experiments were conducted in 2002 and 2003 to study the responses of tea yield and quality to fertilization. The optimum N application rate of 810 kg N ha-1 a-1 was calculated from 95% of the maximum yield, which was closer to the N level at the lowest TP/AA. The critical soil Nmin in 0-60 cm soil depth was 200-260 kg N ha-1 (spring) and 320-430 kg N ha-1 (autumn), respectively. In spring, the critical NO3- contents were 30-33, 6-7 and 10-11 mg L-1 in the young stem, new fully expanded leaves (NFEL) and in the top old leaves, respectively. The critical AAN content was 714-732 mg L-1 in NFEL. In autumn, the critical indicator contents were 5-6 mg L-1 NO3- and 495-510 mg L-1 AAN in mature leaves, respectively. The optimum N application rate within a season (Nopts) were 240-340 kg N ha-1 (spring) and 130-240 kg N ha-1 (autumn). In summer, shading had strong effects on the Nopty (810 kg N ha-1 a-1 in full sun and 700 kg N ha-1 a-1 under shading) and the critical NO3- levels in young stem (20-33 mg L-1 in full sun and 32-51 mg L-1 under shading). Shading had no effects on the critical soil Nmin and Nopts. The reduced activities of nitrate reductase (NR) under shading might inhibit the synthesis of precursor amino acids of cinnamic acid and the reduced activities of phenylalanine ammonia-lyase (PAL) under shading could inhibit the synthesis of the precursors of catechin. The increased polyphenol oxidase (PPO) under shading enhanced the degradation of polyphenols. The application of K, Mg and S fertilizers could slightly increase the yield and improve the quality in 2003. The activities of NR and PAL were slightly increased by sulphur, but the activity of PPO was inhibited.