Tribulus (Zygophyllaceae) as a case study for the evolution of C 2 and C 4 photosynthesis.

C ₂ photosynthesis is a photosynthetic pathway in which photorespiratory CO ₂ release and refixation are enhanced in leaf bundle sheath (BS) tissues. The evolution of C ₂ photosynthesis has been hypothesized to be a major step in the origin of C ₄ photosynthesis, highlighting the importance of studying C ₂ evolution. In this study, physiological, anatomical, ultrastructural, and immunohistochemical properties of leaf photosynthetic tissues were investigated in six non-C ₄ Tribulus species and four C ₄ Tribulus species. At 42°C, T. cristatus exhibited a photosynthetic CO ₂ compensation point in the absence of respiration (C _* ) of 21 µmol mol ^-1 , below the C ₃ mean C _* of 73 µmol mol ^-1 . Tribulus astrocarpus had a C _* value at 42°C of 55 µmol mol ^-1 , intermediate between the C ₃ species and the C ₂ 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 ₃ mean of 11%. Tribulus astrocarpus thus exhibits a weaker C ₂ (termed sub-C ₂ ) phenotype. Increased allocation of mitochondria to the BS and decreased length-to-width ratios of BS cells, were present in non-C ₄ species, indicating a potential role in C ₂ and C ₄ evolution.
(© 2024 The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)