Empirical studies have shown that plant photosynthetic responses to environmental change can vary over time as a result of acclimation. However, the Earth System Models (ESMs) that are used to project climate rarely simulate photosynthetic acclimation due to a lack of reliable formulas. Theoretical, optimality-based formulas for acclimation offer a way to include this important process into models without adding parameters and increasing uncertainty. These theoretical formulas have been developed for C3, but not for C4 plants. The current uncertainty of long-term responses of C4 photosynthesis is critical because of the environmental and economic importance of C4 plants. C4 photosynthesis accounts for approximately 23% of total gross primary production (GPP), and thus has large effects on climate projections. A model for the long term response of C4 photosynthesis also informs projections of future climate benefits to economically critical C4 crop species, such as corn, sugar cane, millet, and sorghum. Here, we propose derivations for the theoretical acclimation of C4 photosynthesis that are suitable for use in ESMs, and predictions for future C4 crop production and yield, as well as to better understand the evolution and spread of C4 plants, and the competition between C3 and C4 species.
Results/Conclusions
Our theoretical model projects changes in C4 photosynthesis when acclimated to temperature, partial pressures of atmospheric gasses, and light. The theoretical model shows that investment in photosynthetic enzymes changes in response to each condition, with increases in light and temperature tending to increase investment in carbon capture and increases in CO2 tending to decrease investment in carbon capture. This resulted in net photosynthesis that increased with light and temperature and did not respond to changes in atmospheric concentrations. linearly with light, due to increased regeneration of photosynthetic enzymes. Net photosynthesis also increased non-linearly without limit in response to temperature. The lack of response of C4 carbon assimilation to temperature or CO2 suggest that future conditions may have little impact on carbon uptake by C4 species. This also suggests that these conditions may decrease competitive dominance of C4 over C3 species if they result in increased C3 photosynthesis. We predict that temperature will be the driving force for increased C4 photosynthesis in the future, and that current ESMs are underestimating this increase, and thus overestimating warming.