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Oxygen gas is one of the environmental factors closely related to photosynthesis and respiration. In this article, we investigated the effects of long-term exposure to different O2 concentrations on growth and phytochemical contents in leaf lettuce. In order for provide exposure to low O2 for longer periods, we developed a new growth chamber combined with N2 gas generator. Plants were hydroponically cultured under 3, 10 or 21% O2. The leaf area of the plants grown at 3% O2 was significantly smaller than that for plants grown at 21% O2; however, there was no significant difference in dry weight. Owing to this, the specific leaf area of plants grown at 3% O2 was significantly lower than that for plants grown at 21% O2. While the nitrogen content of plants grown at 3% O2 was significantly lower than that for plants grown at 21% O2, the anthocyanin content of plants grown at 3 and 10% O2 was significantly higher than that for plants grown at 21% O2. According to results, we discussed that reduced leaf expansion observed in plants grown at low O2 was the result of the decreased nitrogen.

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