Lh. Ziska et al., A global perspective, of ground level, 'ambient' carbon dioxide for assessing the response of plants to atmospheric CO2, GL CHANGE B, 7(7), 2001, pp. 789-796
For most studies involving the response of plants to future concentrations
of atmospheric carbon dioxide (CO2), a current concentration of 360-370 mu
atm is assumed, based on recent data obtained from the Mauna Loa observator
y. In the present study, average seasonal diurnal values of ambient CO2 obt
ained at ground level from three global locations (Australia, Japan and the
USA) indicated that the average CO2 (at canopy height) can vary from over
500 mu atm at night to 350 mu atm during the day with average 24-h values r
anging from 390 to 465 mu atm. At all sites sampled, ambient CO2 rose to a
maximum value during the pre-dawn period (03.00-06.00 hours); at sunrise, C
O2 remained elevated for several hours before declining to a steady-state c
oncentration between 350 and 400 mu atm by mid-morning (08.00-10.00 hours).
Responses of plant growth to simulations of the observed variation of in s
itu CO2 were compared to growth at a constant CO2 concentration in controll
ed environment chambers. Three diurnal patterns were used (constant 370 mu
atm CO2, constant 370 during the day (07.00-19.00 hours), high CO2 (500 mu
atm) at night; or, high CO2 (500 mu atm) at night and during the early morn
ing (07.00-09.00 hours) decreasing to 370 mu atm by 10.00 hours). Three pla
nt species soybean (Glycine max, L (Merr.), velvetleaf (Abutilon theophrast
i L.) and tomato (Lycopersicon esculentum L.) - were grown in each of these
environments. For soybean, high night-time CO2 resulted in a significant i
ncrease in net assimilation rate (NAR), plant growth, leaf area and biomass
relative to a constant ambient value of CO2 by 29 days after sowing. Signi
ficant increases in NAR for all three species, and significant increases in
leaf area, growth and total biomass for two of the three C3 species tested
(velvetleaf and soybean) were also observed after 29 days post sowing for
the high night/early morning diurnal pattern of CO2. Data from these experi
ments suggest that the ambient CO2 concentration experienced by some plants
is higher than the Mauna Loa average, and that growth of some agricultural
species at in situ CO2 levels can differ significantly from the constant C
O2 value used as a control in many CO2 experiments. This suggests that a re
assessment of control conditions used to quantify the response of plants to
future, elevated CO2 may be required.