Tue, Aug 16, 2022: 5:00 PM-6:30 PM
ESA Exhibit Hall
Background/Question/Methods
The evolution of carbon concentrating mechanisms has led to the diversification of flowering plants into new ecological niches. Models suggest that C4 photosynthesis evolved to counter increased photorespiration resulting from hot, low CO2 conditions of the last 30 million years. However, there are limited empirical data evaluating fitness in closely-related C3, C4, and C3-C4 intermediate genotypes, such that the ecological drivers of C4 evolution remain hypothetical. Tribulus is a promising system to study drivers of C4 evolution, because most species grow as sprawling vines on hot desert surfaces, where leaf temperatures frequently exceed 40 °C. Thus, unlike most other C4 lineages, their heat tolerance allows for direct testing of hypotheses related to photorespiration at high temperatures, while minimizing confounding factors such as heat stress. Our first objective was to screen the Tribulus genus to determine which species are C3, C4, or C3-C4 intermediate functional types. Our second objective was to evaluate physiological performance under photorespiratory conditions thought to promote C4 evolution. We grew Tribulus species in 32/22 °C growth chambers and measured leaf gas exchange to characterize photosynthetic pathways used by each species. We then evaluated photorespiratory costs and carbon assimilation during leaf acclimation to a temperature of 41 °C.
Results/Conclusions
We identify C3 photosynthesis in five species of Tribulus and C4 photosynthesis in three species. In addition, we demonstrate the presence of C3-C4 intermediate traits in Tribulus cristatus, a species from semi-arid landscapes in southern Africa. Leaf gas exchange attributes demonstrate that T. cristatus decreases photorespiratory costs using a physiology termed C2 photosynthesis. The C2 pathway localizes photorespiratory release of CO2 to the bundle sheath tissues, allowing it to be recaptured. As observed in C2 species in other plant lineages, T. cristatus exhibited a light-dependent decrease in intercellular CO2 concentration at low CO2 conditions. Enzyme assays indicate that this occurs in T. cristatus without upregulation of a C4 cycle. Increasing the leaf temperature to 42°C led to a significant increase in photorespiratory costs and a significant decrease in photosynthesis and water-use efficiency in C3 species but not T. cristatus. Our results demonstrate that reassimilation of photorespired CO2 through C2 photosynthesis is particularly advantageous above 40°C, that is, conditions that promote high rates of photorespiration. Thus, the evolution of traits that recapture carbon lost to photorespiration may facilitate the transition from C3 to C4 photosynthesis.
The evolution of carbon concentrating mechanisms has led to the diversification of flowering plants into new ecological niches. Models suggest that C4 photosynthesis evolved to counter increased photorespiration resulting from hot, low CO2 conditions of the last 30 million years. However, there are limited empirical data evaluating fitness in closely-related C3, C4, and C3-C4 intermediate genotypes, such that the ecological drivers of C4 evolution remain hypothetical. Tribulus is a promising system to study drivers of C4 evolution, because most species grow as sprawling vines on hot desert surfaces, where leaf temperatures frequently exceed 40 °C. Thus, unlike most other C4 lineages, their heat tolerance allows for direct testing of hypotheses related to photorespiration at high temperatures, while minimizing confounding factors such as heat stress. Our first objective was to screen the Tribulus genus to determine which species are C3, C4, or C3-C4 intermediate functional types. Our second objective was to evaluate physiological performance under photorespiratory conditions thought to promote C4 evolution. We grew Tribulus species in 32/22 °C growth chambers and measured leaf gas exchange to characterize photosynthetic pathways used by each species. We then evaluated photorespiratory costs and carbon assimilation during leaf acclimation to a temperature of 41 °C.
Results/Conclusions
We identify C3 photosynthesis in five species of Tribulus and C4 photosynthesis in three species. In addition, we demonstrate the presence of C3-C4 intermediate traits in Tribulus cristatus, a species from semi-arid landscapes in southern Africa. Leaf gas exchange attributes demonstrate that T. cristatus decreases photorespiratory costs using a physiology termed C2 photosynthesis. The C2 pathway localizes photorespiratory release of CO2 to the bundle sheath tissues, allowing it to be recaptured. As observed in C2 species in other plant lineages, T. cristatus exhibited a light-dependent decrease in intercellular CO2 concentration at low CO2 conditions. Enzyme assays indicate that this occurs in T. cristatus without upregulation of a C4 cycle. Increasing the leaf temperature to 42°C led to a significant increase in photorespiratory costs and a significant decrease in photosynthesis and water-use efficiency in C3 species but not T. cristatus. Our results demonstrate that reassimilation of photorespired CO2 through C2 photosynthesis is particularly advantageous above 40°C, that is, conditions that promote high rates of photorespiration. Thus, the evolution of traits that recapture carbon lost to photorespiration may facilitate the transition from C3 to C4 photosynthesis.