Paper Reviewed
Ruiz-Vera, U.M., De Souza, A.P., Long, S.P. and Ort, D.R. 2017. The role of sink strength and nitrogen availability in the down-regulation of photosynthetic capacity in field-grown Nicotiana tabacum L. at elevated CO2 concentration. Frontiers in Plant Science 8: Article 998.

Agricultural species grown in elevated CO2 environments often, but not always (see, for example, Acclimation: Agricultural Species), exhibit some degree of photosynthetic acclimation or down-regulation, which is typically characterized by reduced rates of photosynthesis resulting from decreased activity and/or amount of the primary plant carboxylating enzyme rubisco. However, in nearly every reported case of CO2-induced photosynthetic acclimation, the reduced rates of photosynthesis displayed by CO2-enriched plants are still typically greater than those exhibited by plants growing at ambient CO2 concentrations. Thus, the biomass of plants growing under elevated CO2 conditions is almost always enhanced relative to that of plants growing under ambient CO2 conditions.

The latest study to examine the topic of down-regulation comes from Ruiz-Vera et al. (2017), who write that "with the continuous increase of atmospheric CO2, it is critical to understand the role of sink limitation in the down-regulation of photosynthetic capacity under agricultural field conditions and the capacity of N availability to mitigate it if agriculture is to meet future demand (Long et al., 2004; Tilman and Clark, 2015)." And in this regard they wonder if down regulation can be avoided by genetically increasing plant sink size and providing sufficient N so as to capitalize on "the full potential photosynthetic benefit of rising CO2 [in] crops."

To investigate this possibility, Ruiz-Vera et al. designed an experiment to assess the potential of nitrogen fertilization to mitigate photosynthetic down-regulation in tobacco (Nicotiana tabacum L.). The experiment was performed at a Free-Air CO2 Enrichment (FACE) facility in Champaign, IL (USA) in the summer of 2015. Two tobacco cultivars of different sink strength were selected for study: Petit Havana (low sink capacity, producing small leaves) and Mammoth (high sink capacity, producing large leaves). After 4 weeks of initial growth in a greenhouse, plants of each cultivar were transplanted outdoors at the FACE facility where they were subjected in a full factorial design to two CO2 levels (400 or 600 ppm) and two nitrogen applications (normal, 150 Kg N/ha or high, 300 Kg N/ha). Then, over the course of the next 48 days the scientists performed a series of measurements (gas exchange, height, specific leaf area, leaf carbon and nitrogen content, leaf carbohydrates, and plant dry weight) in attempt to answer their research question. And what did their data reveal?

In the words of the authors, "high sink strength resulting from rapid growth throughout the experiment appears to have prevented down-regulation in tobacco cv. Mammoth whereas the small stature of cv. Petite Havana appears to have resulted in progressive down-regulation." Nevertheless, despite its occurrence, photosynthetic uptake -- averaged over the growing season -- in Petit Havana was significantly higher (+11%) under elevated CO2 regardless of nitrogen treatment. What is more, Ruiz-Vera et al. report that increased nitrogen "partially mitigated the down-regulation of photosynthesis in cv. Petit Havana."

These findings and others, according to the authors, "confirm that under open-air conditions of CO2 elevation in an agricultural field, down-regulation can be strongly offset in germplasm with a high sink capacity." Therefore, as they conclude, "downregulation of photosynthetic capacity is not inevitable under field conditions where there is no limitation of rooting volume or interference with micro-climate if there is sufficient sink potential and nitrogen supply." And that is wonderful news for those concerned about feeding the ever-growing population of the planet. In the future, there is no reason why society cannot capitalize on the full potential photosynthetic benefit of rising atmospheric CO2 in crops by selecting cultivars with high sink capacity and/or adding supplemental nitrogen during the growing season.

References
Long, S.P., Ainsworth, E.A., Rogers, A. and Ort, D.R. 2004. Rising atmospheric carbon dioxide: plants FACE the future. Annual Review of Plant Biology 55: 591-628.

Tilman, D. and Clark, M. 2015. Food, agriculture & the environment: can we feed the world and save the earth? Daedalus 144: 8-23.