PS 36-187 - Reassimilation of CO₂ in the leaves of Tribulus species (Zygophyllaceae): implications for the evolution of C₄ photosynthesis

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.