CO2 Enrichment at Night Affects the Growth and Yield of Common Beans

Many experiments to determine the crop yield increase expected with rising atmospheric carbon dioxide concentration added carbon dioxide only during the daytime, without tests of whether elevation of carbon dioxide at night affected plant growth. In this experiment, two cultivars of common bean (Phaseolus vulgaris L.) were grown in the field for 4 yr in open-top chambers in which CO2 was added during the day or the night, both day and night, or not at all. CO2 was added to increase the concentration by 180 mu mol mol(-1) above that of the ambient outside air, which averaged 392 mu mol mol(-1) during the daytime and 503 mu mol mol(-1) at night. In the two cultivars, the average increase in seed yield above the ambient CO2 treatment was 42% with daytime CO2 enrichment and 77% with continuous enrichment. In both cultivars, continuous CO2 enrichment increased the area of individual leaflets by about 40% compared with daytime enrichment. Leaflet size was the same with daytime CO2 enrichment and with no enrichment. In one of the cultivars, nighttime CO2 elevation alone increased leaflet size and seed yield. The results indicated that CO2 enrichment at night had important consequences for the growth and yield of these bean cultivars, and that daytime-only CO2 enrichment greatly underestimated the yield increase likely to occur with rising atmospheric CO2 concentrations.