Plant growth and photosynthesis are strongly influenced by environmental factors. Oxygen gas is an
important factor in photosynthesis, photorespiration, and respiration. In a previous study, we suggested
that long-term hypoxia treatment affected leaf expansion and leaf nitrogen and carbon contents. In the
present study, we measured the effects of long-term exposure to five O2 treatments (0.4, 2, 7, 10, and
21%) on plant growth, photosynthetic characteristics, and nitrogen, carbon, sucrose and phytochemicals
content in red leaf lettuce grown under two light intensities (180 and 350 mol m−2 s−1). The leaf area
of plants grown at 0.4 or 2% O2 was significantly smaller than that of plants grown at 21% O2. The specific
leaf area of plants grown under hypoxia was significantly lower than that of plants grown at 21% O2.
The net CO2 assimilation rate peaked at 7% O2. While total nitrogen content in leaf gradually increased
with O2 concentration, total carbon and sucrose content in leaf gradually decreased with increased O2
concentration. Polyphenol content in plants grown under hypoxia was significantly higher than that
of plants grown at 21% O2 under both light conditions. Anthocyanin content of plants grown at 0.4 or
2% O2 was significantly higher than that of plants grown at 21% O2. From these results, we infer that
photosynthesis was at 7% O2 enhanced by inhibited photorespiration, and the interaction of O2 and light
conditions was affected by the accumulation of polyphenol and anthocyanin.