Atmospheric [CO2] is rising, with significant
consequences for plant function in natural and managed ecosystems. Currently we
cannot fully explain the effects of elevated [CO2] on vegetation
under field conditions, where interactions with abiotic and biotic factors are
important. To better understand plant responses to elevated [CO2] we
have combined genomic, biochemical, physiological and ecological investigation
of soybean grown in the field at the SOYbean Free-Air
Concentration Enrichment (SOYFACE) facility at the University of Illinois. Soybean was grown in
four plots at ambient [CO2] (~380 ppm) and four plots at elevated [CO2]
(~550 ppm), from sowing until harvest. This provided a model system, where low
genetic and environmental variability between experimental units increased the
ability to detect subtle treatment effects. The impact of elevated [CO2]
on dark respiration is a controversial subject, with prior studies variously
reporting stimulation, inhibition or no change in CO2 efflux. The principal
molecular response of soybean to elevated [CO2] was increased gene
expression for many components of respiratory metabolism, including glycolysis,
the TCA cycle and mitochondrial electron transport. These molecular responses
were reflected in greater pool sizes of key carbon metabolites and greater
rates of respiratory oxygen uptake and carbon efflux. The integrated genomic,
biochemical and physiological responses provide unique evidence for stimulated
respiration at elevated [CO2]. Greater respiration will partially
offset the stimulation of photosynthesis by elevated [CO2] at
whole-plant and ecosystem scales, while also generating additional energy and
carbon-skeletons. Gene expression for some associated biosynthetic pathways was
altered. Gene expression for cellulose synthesis was greater at elevated [CO2],
but gene expression for lignin synthesis did not change. Greater cellulose to
lignin ratios can alter the rate of leaf litter decomposition. In summary,
microarray analysis revealed previously unknown changes in gene expression
which underlie key physiological and ecological responses of soybean to
elevated [CO2].