Unexpected decrease in yield and antioxidants in vegetable at very high CO2 levels

Global warming is partly due to the increase in atmospheric CO2 concentration. In the last two decades, many investigations have studied the effect of high CO2 levels on crops. However, nearly all studies have examined the effects of CO2 enrichment up to about 1200 mu mol mol(-1). In general, more CO2 is beneficial to plant growth because plants feed on CO2. Here we tested the effect of CO2 levels from 400 to 5000 mu mol mol(-1) on the yield and antioxidant capacity of Chinese cabbage, Brassica chinensis, and lettuce, Lactuca sativa. Total antioxidant capacity was measured using the ferric ion reducing antioxidant potential assay and the diphenyl-picryl-hydrazylhydrate free radical detection. Our results show that, as expected, CO2 concentrations of 1000-3000 mu mol mol(-1) enhance the yields and contents of polyphenols, flavonoids and vitamin C in both vegetables. However, surprisingly, further increase up to 5000 mu mol mol(-1) CO2 decreased yields and contents of polyphenols, flavonoids and vitamin C. Our findings thus show that very high atmospheric CO2 levels are impeding the growth of tested plants. They also suggest that agricultural food and production could be depleted at high atmospheric CO2 levels.