Inorganic nitrogen fertilizers induce changes in ammonium assimilation and gas exchange in Camellia sinensis L.

Abstract

Camellia sinensis L. is one of the most economically important crops in the world today. The leaves of this plant are harvested for tea production on a massive scale through a process that requires a large amount of inorganic nitrogen (N). However, knowledge of how inorganic nitrogen regulates the growth physiology of C. sinensis L. remains limited. In order to investigate its effects on gas exchange, N assimilation, and the efficiency of N use in C. sinensis L., we carried out experiments with 4 cultivars (WNZ, YS, ZN139, and LJ43) under 4 different treatments: 1) a control treatment, 2) a treatment fed only with NO_3^--N, 3) a treatment fed only with NH_4^+-N, and 4) a treatment fed with a mixture of NO_3^--N and NH_4^+-N in a 1:1 ratio. We found that the application of N, especially in a mixture, encouraged higher foliar N content per unit area (N_f), which in turn promoted glutamine synthetase (GS) activity and glutamate synthase (GOGAT) activity. Our mixture-fed treatment increased N_f, GS, and GOGAT activity by 26.18%, 110.2%, and 92.21%, respectively, for the WNZ cultivar. The mixed-fed treatment also had the highest net photosynthetic (Pn), stomatal conductance (Gs), and transpiration (Tr) rates, while the NH_4^+-fed treatment had the highest intercellular CO_2 content (Ci). The mixture-fed and NO_3^--fed applications caused significant increases in photosynthetic N-use efficiency (PNUE), but the NH_4^+-fed application decreased PNUE and water-use efficiency (WUE). Furthermore, we observed that differences among cultivars for some of these parameters were significant due to different N solutions. We found that the YS and LJ43 cultivars showed no significant response to different N applications (excluding control) in key enzyme activity and Pn, respectively, but this was not the case for most of the other parameters. Our study indicates that the mixture-fed application promotes increased foliar N content and ammonium assimilation, which in turn increases photosynthesis rates, suggesting that the mixed-feeding of plants provides optimal conditions to promote the growth of C. sinensis L. for tea cultivation.