Tribulus (Zygophyllaceae) as a case study for the evolution of C and C photosynthesis.

C photosynthesis is a photosynthetic pathway in which photorespiratory CO release and refixation are enhanced in leaf bundle sheath (BS) tissues. The evolution of C photosynthesis has been hypothesized to be a major step in the origin of C photosynthesis, highlighting the importance of studying C evolution. In this study, physiological, anatomical, ultrastructural, and immunohistochemical properties of leaf photosynthetic tissues were investigated in six non-C Tribulus species and four C Tribulus species. At 42°C, T. cristatus exhibited a photosynthetic CO compensation point in the absence of respiration (C) of 21 µmol mol, below the C mean C of 73 µmol mol. Tribulus astrocarpus had a C value at 42°C of 55 µmol mol, intermediate between the C species and the C T. cristatus. Glycine decarboxylase (GDC) allocation to BS tissues was associated with lower C. Tribulus cristatus and T. astrocarpus allocated 86% and 30% of their GDC to the BS tissues, respectively, well above the C mean of 11%. Tribulus astrocarpus thus exhibits a weaker C (termed sub-C) phenotype. Increased allocation of mitochondria to the BS and decreased length-to-width ratios of BS cells, were present in non-C species, indicating a potential role in C and C evolution.