Paper Reviewed
Canales, F.J., de la Haba, P., Barrientos, E. and Agüera, E. 2016. Effect of CO2 enrichment and increased nitrogen supply on the induction of sunflower (Helianthus annuus L.) primary leaf senescence. Canadian Journal of Plant Science 96: 1002-1013.

Plants grown at elevated atmospheric CO2 concentrations exhibit many changes in their physical and chemical characteristics, as well as alterations in various physiological functions. Leaf senescence is one of the phenomena that may be included in this category of affected processes. Sometimes elevated CO2 shortens the time required to begin senescence (Miglietta et al., 1998), while at other times it may delay senescence (Li et al., 2000) or have no effect upon it at all (Lake and Hughes, 1999).

The latest group of scientists to investigate this topic is Canales et al. (2016). Working with sunflower (Helianthus annuus L.), the team of four Spanish researchers investigated the interactive effects of elevated CO2 and nitrogen supply on this important agricultural crop. More specifically, they grew eight-day-old plants in controlled environment chambers for a period of 34 days under either ambient (400 ppm) or elevated (800 ppm) CO2 and two different nitrate concentrations (10mM, low, or 25 mM, high). Multiple measurements (dry weight, leaf surface area, protein content, photosynthetic pigment levels, CO2 fixation rate, plant oxidative status and changes in the enzymes involved in nitrogen metabolism) were then made on the plants at 8, 14, 24 and 34 days after enrichment "to examine the possible effect of increased nitrogen supply and elevated atmospheric CO2 concentrations on the induction of primary leaf senescence in sunflower, and on the biochemical and physiological processes involved in leaf ontogeny." And what did those measurements reveal?

In the words of the authors, and as shown in the figure below, "dry weight and leaf area increased under elevated CO2 conditions, especially in plants grown with increased nitrate availability." Additionally, it was determined that plants growing under elevated CO2 and increased nitrogen supply conditions had a higher CO2 fixation rate and "increased activity by antioxidative and nitrogen-metabolism-related enzymes than [plants] grown under elevated CO2 with low nitrogen supply," which findings Canales et al. say "indicate that CO2 enrichment and increased nitrate availability slow down the induction of senescence." In contrast, the researchers report that "signs of senescence appeared earlier, and were more marked, in plants grown under elevated CO2 conditions and with limited nitrogen supply, suggesting that the induction of senescence process is directly linked to the leaf C/N ratio." Thus, as the air's CO2 concentration rises in the future, it would appear that leaf senescence of sunflower will either be advanced or delayed depending on plant nitrogen availability.

Figure 1. Changes in dry weight (left panel) and leaf area (right panel) during sunflower primary leaf development for plants growing under different atmospheric CO2 concentrations (400 or 800 ppm) and levels of nitrogen availability (10 or 25 mM NO3^-). Adapted from Canales et al. (2016).

References
Lake, J.C. and Hughes, L. 1999. Nectar production and floral characteristics of Tropaeolum majus L. grown in ambient and elevated carbon dioxide. Annals of Botany 84: 535-541.

Li, J.-H., Dijkstra, P., Hymus, G.J., Wheeler, R.M., Piastuchi, W.C., Hinkle, C.R. and Drake, B.G. 2000. Leaf senescence of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment. Global Change Biology 6: 727-733.

Miglietta, F., Magliulo, V., Bindi, M., Cerio, L., Vaccari, F.P., Loduca, V. and Peressotti, A. 1998. Free Air CO2 Enrichment of potato (Solanum tuberosum L.): development, growth and yield. Global Change Biology 4: 163-172.