A global perspective of ground level, 'ambient' carbon dioxide for assessing the response of plants to atmospheric CO2

For most studies involving the response of plants to future concentrations of atmospheric carbon dioxide (CO[sub 2]), a current concentration of 360-370 µatm is assumed, based on recent data obtained from the Mauna Loa observatory. In the present study, average seasonal diurnal values of ambient CO[sub 2] obtained at ground level from three global locations (Australia, Japan and the USA) indicated that the average CO[sub 2] (at canopy height) can vary from over 500 µatm at night to 350 µatm during the day with average 24-h values ranging from 390 to 465 µatm. At all sites sampled, ambient CO[sub 2] rose to a maximum value during the pre-dawn period (03.00-06.00 hours); at sunrise, CO[sub 2] remained elevated for several hours before declining to a steady-state concentration between 350 and 400 µatm by mid-morning (08.00-10.00 hours). Responses of plant growth to simulations of the observed variation of in situ CO[sub 2] were compared to growth at a constant CO[sub 2] concentration in controlled environment chambers. Three diurnal patterns were used (constant 370 µatm CO[sub 2], constant 370 during the day (07.00-19.00 hours), high CO[sub 2] (500 µatm) at night; or, high CO[sub 2] (500 µatm) at night and during the early morning (07.00-09.00 hours) decreasing to 370 µatm by 10.00 hours). Three plant species — soybean (Glycine max, L (Merr.), velvetleaf (Abutilon theophrasti L.) and tomato (Lycopersicon esculentum L.) — were grown in each of these environments. For soybean, high night-time CO[sub 2] resulted in a significant increase in net assimilation rate (NAR), plant growth, leaf area and biomass relative to a constant ambient value of CO[sub 2] by 29 days after sowing. Significant increases in NAR for all three species, and significant increases in leaf area, growth and total biomass for two of the three C[sub 3] species tested (velvetleaf and soybean) were also observed after 29 days post..