Leaf Photosynthetic Traits at Different Canopies of Tea Plants

The canopy photosynthetic capacity is one of the dominant factors determining the productivity of tea plants. The photosynthetic traits, physiological and ecological factors as well as their interactions of 10-year old tea plant(Longjing 43 cultivar) at surface, middle, low positions in the canopy receiving 100%, 50%~70%, 0%~15%photosynthetically active radiation(PAR) were investigated in this paper. The received PAR decreased sharply from the surface to low canopy, while CO2 concentration was the highest at low canopy. There were magnificent decreases of leaf mass area ratio(LMA), total nitrogen(NA), total carbon(CA), chlorophyll contents based on leaf area from the surface to low canopy. However, the contents of total N and C were not significantly different among positions in tea plant when calculated based on leaf mass. Leaves at the surface and middle positions, but not low canopy showed typical light response curves fitting well to the Farquhar model. The light-saturated photosynthesis rate(Pn), stomatal conductance(Gs), transpiration rate(Tr) also decreased markedly from surface canopy to low canopy, while concentration of intercellular CO2 (Ci) was the highest at low canopy. Photosynthetic nitrogen use efficiency(PNUE) of surface canopy and middle canopy were significantly higher than that of low canopy. The overall results suggested that the decreased Pn within the leaves of tea plant from surface to low canopy was mainly related to the reduced capacity of photosystem and carboxylation efficiency caused by decreased light intensity instead of low Gs. More research is needed to illustrate the optimum canopy structure shaped by rational agronomical management which could improve the canopy photosynthetic capacity of tea plants.