Effects of gradual versus step increases in carbon dioxide on Plantago photosynthesis and growth in a microcosm study

This study investigated the effects of a gradual versus step increases in carbon dioxide (CO2) on plant photosynthesis and growth at two nitrogen (N) levels. Plantago lanceolata were grown for 80 days and then treated with the ambient CO2 (as the control), gradual CO2 increase and step CO2 increase as well as low and high N additions for 70 days. While [CO2] were kept at constant 350 and 700 mol mol for the ambient and step CO2 treatments, respectively, [CO2] in the gradual CO2 treatment was raised by 5 mol mol day, beginning at 350 mol mol and reaching 700 mol mol by the end of experiment. The step CO2 treatment immediately resulted in an approximate 50% increase in leaf photosynthetic carbon fixation at both the low and high N additions, leading to a 20–24% decrease in leaf N concentration. The CO2-induced nitrogen stress, in return, resulted in partial photosynthetic downregulation since the third week at the low N level and the fourth week at the high N level after treatments. In comparison, the gradual CO2 treatment induced a gradual increase in photosynthetic carbon fixation, leading to less reduction in leaf N concentration. In comparison to the ambient CO2, both the gradual and step CO2 increases resulted in decreases in specific leaf area, leaf N concentration but an increase in plant biomass. Responses of plant shoot:root ratio to CO2 treatments varied with N supply. It decreased with low N supply and increased with high N supply under the gradual and step CO2 treatments relative to that under the ambient CO2. Degrees of those changes in physiological and growth parameters were usually larger under the step than the gradual CO2 treatments, largely due to different photosynthetic C influxes under the two CO2 treatments. © 2002 Elsevier Science B.V. All rights reserved.