Electron Transport in Leaves: A Physiological Perspective

Abstract

The concentration of CO2 ([CO2]) in the atmosphere is projected to exceed 550 ppm by 2050. C3 plants respond directly to growth at elevated [CO2] by stimulation of photosynthesis and reduced stomatal conductance. The stimulation of photosynthesis is the result of increased velocity of carboxylation of CO2 by Rubisco and inhibition of the competing oxygenation reaction. Long-term exposure of C3 plants to elevated [CO2] can also lead to photosynthetic acclimation in which allocation of resources to components of the photosynthetic machinery, including Rubisco, is altered to optimize metabolic efficiency. The decrease in stomatal conductance that occurs in all plants at elevated [CO2] can reduce canopy water use and indirectly enhance carbon gain by ameliorating drought stress. However, canopy micrometeorology constrains reductions in water use at the whole-plant level compared to the leaf level. C4 photosynthesis is not directly stimulated by free-air concentration enrichment (FACE) of CO2 in the field. However, reduced water use can indirectly enhance carbon gain by ameliorating stress in times and places of drought. There are commonalities and important distinctions between plant responses to growth at elevated [CO2] under FACE versus controlled environment chambers. In FACE experiments: (1) the enhancement of photosynthesis and productivity by elevated [CO2] is sustained over time; (2) the decrease in carboxylation capacity and leaf N characteristic of photosynthetic acclimation to elevated [CO2] is consistent with an optimization of metabolic efficiency rather than a general down-regulation of metabolism, and (3) the enhancement effect of elevated [CO2] is greatest for photosynthesis, intermediate for biomass accumulation, and lowest for crop yield. Plant responses to elevated [CO2] have the potential to influence the global carbon cycle and climate in the future, but the complexity of scaling from the leaf to whole plant, canopy, ecosystem and biosphere make it unclear to what extent this will be realized. Elevated [CO2] will probably offset some of the future losses in crop yield caused by increased temperature and drought stress, but not to the extent previously thought. Expanding FACE experimentation to consider multiple elements of global change across a wider geographic range and more ecosystem types should be a priority if we are to minimize the problems, and maximize the benefits, of climate change impacts on ecosystem good and services.