Background/Question/Methods
Terrestrial ecosystems are currently strong sinks for atmospheric carbon dioxide (CO₂). Sustainability of this sink capacity under predicted elevated CO₂ scenarios depends at the most fundamental level on plant physiological processes that regulate carbon assimilation, allocation, and respiration. Despite the crucial contribution of plant carbon metabolism to the global carbon cycle, we know remarkably little about the effects of elevated CO₂ on the molecular mechanisms and gene networks regulating most metabolic pathways. Furthermore, we lack information on how these mechanisms operate throughout a leaf’s life-span. Understanding how leaf age affects regulation of carbon metabolism under different CO₂ levels is particularly relevant in evergreen species which photosynthesize throughout most of the year and may consequently increase the carbon sequestration potential of an ecosystem. The Duke Forest FACE facility provides a unique opportunity to investigate the effects of elevated CO₂ on an important evergreen tree species – loblolly pine (Pinus taeda, L.) Using cDNA microarrays, we compared the expression levels of 390 carbon metabolism transcripts (genes) between trees growing under ambient CO₂ and trees under elevated (ambient + 200ppm) CO₂. We contrasted the gene expression results from two foliage cohorts: one-year-old (old) and current-year needles (young).
Results/Conclusions
Sixty-five percent of genes were affected by the CO₂ treatment at least once during the life-span of the pine needles. However, the number of genes affected varied with leaf age and only 15%, on average, were differentially expressed between CO₂ conditions across time and age classes. Elevated CO₂ altered the expression of 27% of the genes in young needles but only 4% in old needles. Although the magnitude of the effect was clearly distinct, many of the trends within metabolic groups were similar between age classes. For instance, the group of pathways associated with carbon uptake showed the largest percentage of donwregulated genes in both cohorts. This result has important implications for leaf-level analysis of photosynthetic responses to elevated CO₂. On the other hand, several individual genes were affected by the CO₂ treatment in young needles but not in old ones. Such genes included those coding for a beta-amylase (starch degradation), a cytochrome C oxidase protein (mitochondrial electron transport), ATP synthases (energy production), and hydroxylmethyltransferases (photorespiration). Identification of genes associated with a specific cohort of needles is important for future targeted molecular analysis and for the development of theoretical models that incorporate leaf age as a factor in the response of vegetation to rising atmospheric CO₂.