High temperature acclimation of C₄ photosynthesis is linked to changes in photosynthetic biochemistry

With average global temperatures predicted to increase over the next century, it is important to understand the extent and mechanisms of C ₄ photosynthetic acclimation to modest increases in growth temperature. To this end, we compared the photosynthetic responses of two C ₄ grasses (Panicum coloratum and Cenchrus ciliaris) and one C ₄ dicot (Flaveria bidentis) to growth at moderate (25/20°C, day/night) or high (35/30°C, day/night) temperatures. In all three C ₄ species, CO₂ assimilation rates (A) underwent significant thermal acclimation, such that when compared at growth temperatures, A increased less than what would be expected given the strong response of A to short-term changes in leaf temperature. Thermal photosynthetic acclimation was further manifested by an increase in the temperature optima of A, and a decrease in leaf nitrogen content and leaf mass per area in the high- relative to the moderate-temperature-grown plants. Reduced photosynthetic capacity at the higher growth temperature was underpinned by selective changes in photosynthetic components. Plants grown at the higher temperature had lower amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase and cytochrome f and activity of carbonic anhydrase. The activities of photosystem II (PSII) and phosphenolpyruvate carboxylase were not affected by growth temperature. Chlorophyll fluorescence measurements of F. bidentis showed a corresponding decrease in the quantum yield of PSII (Φ_PSII) and an increase in non-photochemical quenching (Φ_NPQ). It is concluded that through these biochemical changes, C₄ plants maintain the balance between the various photosynthetic components at each growth temperature, despite the differing temperature dependence of each process. As such, at higher temperatures photosynthetic nitrogen use efficiency increases more than A. Our results suggest C₄ plants will show only modest changes in photosynthetic rates in response to changes in growth temperature, such as those expected within or between seasons, or the warming anticipated as a result of global climate change.