Carbon dioxide assimilation and photosynthetic electron transport of tea leaves under nitrogen deficiency

Tea plant is famed in humid and sub-humid of tropical regions, sub-tropical regions, and is a leaf-harvested crop. Nitrogen is the most important nutrient for increasing quality of tea leaves. Therefore, large amounts of nitrogen fertilizer are increasingly applied by tea farmers. Appropriate application of nitrogen fertilizer aroused people's concern. This research of physiological response to N deficiency stress will be helpful for appropriate application of nitrogen fertilizer for tea farmers and elucidate a mechanistic basis for the reductions in carbon dioxide (CO₂) assimilation.

To elucidate a mechanistic basis for the reductions in carbon dioxide (CO₂) assimilation under nitrogen (N) deficiency tea leaves, changes in chlorophyll (Chl), carbohydrates, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and chlorophyll fluorescence transient were examined together with six N treatment (0, 50, 100, 300, 1200 or 6000 μM N). Root, stem and leaves dry weight (DW) increased as N supply increased from 0 to 300 μM, then remained unchanged. The reductions in CO₂ assimilation of N-deficient leaves paralleled with high intercellular CO₂ concentration. Rubisco activity, protein and Chl content increased linearly or curvilinearly over the range of leaf N content examined except unchanged as leaf N from 2.15 to 2.79 g m^-2. Chlorophyll fluorescence transient from N-deficient leaves displayed a depression at the P-step, accompanied by a new step at about 150 μs (L-step). F_v/F_m, RE_o/ET_o, ET_o/ABS, S_m, ET_o/CS_o, PI_abs, PI_{tot, abs}, were decreased in N-deficient leaves but increased DI_o/CS_o, DI_o/RC and DI_o/ABS. Regressive analysis showed that CO₂ assimilation decreased linearly or curvilinearly with decreasing initial rubisco, PI_abs and Leaf Chl, respectively. Therefore, we concluded the decreased photosynthetic electron transport capacity, leaf chl content and initial rubisco activity are probably the main factors contributing to decreased CO₂ assimilation under N deficiency.

The decreased photosynthetic electron transport capacity, leaf Chl content and initial rubisco activity are probably the main factors contributing to decreased CO₂ assimilation under N deficiency.