Response of photosynthesis and chlorophyll fluorescence to acute ozone stress in tomato (Solanum lycopersicum Mill.)

The crop sensitivity to ozone (O-3) is affected by the timing of the O-3 exposure, by the O-3 concentration, and by the crop age. To determine the physiological response to the acute ozone stress, tomato plants were exposed to O-3 at two growth stages. In Experiment I (Exp. I), O-3 (500 mu g m(-3)) was applied to 30-d-old plants (PL30). In Experiment II (Exp. II), three O-3 concentrations (200, 350, and 500 mu g m(-3)) were applied to 51-d-old plants (PL51). The time of the treatment was 4 h (7:30-11:30 h). Photosynthesis and chlorophyll fluorescence measurements were done 4 times (before the exposure; 20 min, 20 h, and 2-3 weeks after the end of the treatment) using a LI-COR 6400 photosynthesis meter. The stomatal pore area and stomatal conductance were reduced as the O-3 concentration increased. Ozone induced the decrease in the photosynthetic parameters of tomato regardless of the plant age. Both the photosystem (PS) II operating efficiency and the maximum quantum efficiency of PSII photochemistry declined under the ozone stress suggesting that the PSII activity was inhibited by O-3. The impaired PSII contributed to the reduced photosynthetic rate. The greater decline of photosynthetic parameters was found in the PL30 compared with the PL51. It proved the age-dependent ozone sensitivity of tomato, where the younger plants were more vulnerable. Ozone caused the degradation of photosynthetic apparatus, which affected the photosynthesis of tomato plants depending on the growth stage and the O-3 concentration.