Determination of fruit photosynthesis under different CO2 and low PPFD levels
Fruits 94 DAFB were used for determining in-vitro response to different CO2 concentrations, and fruits 114 DAFB were for different low PPFD, respectively, because the mandarin fruits show active photosynthesis during these stages as reported previously [5]. Photosynthesis and respiration rates were measured by oxygen electrode apparatus (Rank Brothers, Cambridge, U.K.)supplemented with halogen lamps (Sumita Optical Glass, Saitama,Japan) as reported earlier [5]. Briefly, 10 rind discs (3 mm diameter)were prepared from equator zone of three fruits with the albedo tissue removed, soaked in 0.05 M phosphate buffer (pH 7.0) for 30 s, and placed in an electrode tank with 3 ml of fresh phosphate buffer at 25◦C. The CO2 concentration, 50–1000 ppm, was adjusted by adding different amount of NaHCO3 solution into the tank and PPFD of 135 ?mol m−2s−1was supplied. The pH in the tank was7.0. Meanwhile, PPFD, from 2.7 to 270 ?mol m−2s−1, was regulated by placing the lamp at various distances from the electrode tank containing 440 ppm CO2. The O2 increase (net photosynthesis) inthe buffer was recorded under light conditions for 15 min, then O2 decrease (respiration) was measured for 10 min under dark by covering the tank with three layers of aluminum foil. Before use, the buffer had been bubbled with N2 gas for 1 h and stored in a desiccator containing 2 M NaOH to remove CO2. Leaf photosynthes is and respiration rates were also measured as a control by using 10 discs (3 mm diameter) from five leaves at the same sampling date.Experiments were repeated at least 3 times and mean value ± SE was presented.