Jennifer D. Cure

Crop responses to carbon dioxide doubling: a literature survey

Abstract Atmospheric carbon dioxide (CO 2 ) concentration will probably double by the middle of the next century. Since this is widely expected to increase crop yields, the Department of Energy has established a research program to gather data on the effects of CO 2 on plants and to develop models that can be used to predict how plants will behave in a future high-CO 2 world. This paper identifies strengths and weaknesses in the knowledge base for modelling plant responses to CO 2 . It is based on an extensive tabulation of published information on responses of ten leading crop species to elevated CO 2 . The response variables selected for examination were: (a) net carbon exchange rate, (b) net assimilation rate, (c) biomass accumulation, (d) root:shoot ratio, (e) harvest index, (f) conductance, (g) transpiration rate and (h) yield. The results were expressed as a predicted percentage change of the variable in response to a doubled CO 2 concentration. In most instances, a linear model was used to fit the response data. Overall, the net CO 2 exchange rate of crops increased 52% on first exposure to a doubled CO 2 concentration, but was only 29% higher after the plants had acclimatized to the new concentration. For net assimilation rate, the increases were smaller, but fell with time in a similar way. The C 4 crops responded very much less than C 3 crops. The responses of biomass accumulation and yield were similar to that for carbon fixation rate. Yield increased on average 41% for a doubling of CO 2 concentration. The variation in harvest index was small and erratic except for soybean, where it decreased with a doubling of CO 2 concentration. Conductance and transpiration were both inversely related to CO 2 concentration. Transpiration decreased 23% on average for a doubling of CO 2 . Crop responses to CO 2 during water stress were variable probably because high CO 2 both increased leaf area (which increases water use) and reduced stomatal conductance (which decreases water use). However, low nutrient concentrations limited the responses of most crops to CO 2 . The absolute increase in photosynthetic rate in response to high CO 2 concentration was always greater in high light than in low light, but this was not necessarily true of the relative increase. In all except one study, responses to CO 2 were larger at high temperature than at low. Most of these studies were done in high light intensity. In low light intensity, the effect of temperature on the CO 2 response was smaller.

Direct effects of increasing carbon dioxide on vegetation

CO/sub 2/ is an essential environmental resource. It is required as a raw material of the orderly development of all green plants. As the availability of CO/sub 2/ increases, perhaps reaching two or three times the concentration prevailing in preindustrial times, plants and all other organisms dependent on them for food will be affected. Humans are releasing a gaseous fertilizer into the global atmosphere in quantities sufficient to affect all life. This volume considers the direct effects of global CO/sub 2/ fertilization on plants and thus on all other life. Separate abstracts have been prepared for individual papers. (ACR)

Soybean growth and yield response to elevated carbon dioxide

Abstract Soybeans ( Glycine max L. Merr. ‘Bragg’) were grown in seeded rows in open-top field chambers and exposed continuously to a range of elevated CO 2 concentrations through-out the 1982 and 1983 growing seasons. During 1983, a water stress treatment was also imposed. Comparison of vegetative growth with a similarly conducted pot experiment showed an increased ration of leaf area to total top dry weight in the seeded row plants, but generally similar qualitative effects of elevated CO 2 . Careful recording of mainstem leaf emergence rates and reproduction stages showed no consistent effect of CO 2 under well watered conditions, but in 1983 there was a distinct modification by high CO 2 of the water stress-induced hastening of the time to physiological maturity. In 1982, and for the well watered plants in 1983, standing biomass at maturity was increased significantly by elevated CO 2 , but harvest index decreased and yield was (statistically) unaffected by the treatment. The yield responses calculated for a doubling of the current CO 2 concentrations for these well watered treatments were 1.07 and 0.93, respectively. In the water stress treatment in 1983, however, harvest index did not decrease in the presence of elevated CO 2 , and a highly significant yield response occurred (1.41 at 700 μll −1 ).

Alterations in Soybean Leaf Development and Photosynthesis in a Co2- Enriched Atmosphere

This study was conducted to characterize changes in the canopy photosynthetic leaf area of developing soybean (Glycine max [L.] Merr. cv Lee) exposed to a CO2-enriched atmosphere. Young, vegetative plants were exposed to 350 or 700 μL L-1 CO2 for 15 d. Plant dry mass and total leaf area were greater in the CO2-enriched environment. Emergence and expansion rates of main stem leaves increased at high CO2, but the areas of individual leaves at full expansion were affected very little (5%-10% greater than controls). More rapid leaf expansion rates occurred in the light and dark. Under CO2-enriched conditions, the net CO2 exchange rates of all leaves on the main stem were higher before and after full expansion. Stomatal conductance was lower in high CO2 only after leaves approached full expansion. Leaf development on the lateral branches also was increased at high CO2, accounting for 40% of the total increase in leaf area by the end of the experiment. We conclude that more rapid rates of leaf development under CO2 enrichment likely resulted from increased photosynthesis rates and that both direct and indirect effects were involved.