Abstract：
C2 photosynthesis is a photosynthetic pathway in which photorespiratory CO2 release and refixation are enhanced in leaf bundle sheath (BS) tissues. The evolution of C2 photosynthesis has been hypothesized to be a major step in the origin of C4 photosynthesis, highlighting the importance of studying C2 evolution. In this study, physiological, anatomical, ultrastructural, and immunohistochemical properties of leaf photosynthetic tissues were investigated in six non-C4 Tribulus species and four C4 Tribulus species. At 42°C, T. cristatus exhibited a photosynthetic CO2 compensation point in the absence of respiration (C*) of 21 µmol mol-1, below the C3 mean C* of 73 µmol mol-1. Tribulus astrocarpus had a C* value at 42°C of 55 µmol mol-1, intermediate between the C3 species and the C2 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 C3 mean of 11%. Tribulus astrocarpus thus exhibits a weaker C2 (termed sub-C2) phenotype. Increased allocation of mitochondria to the BS and decreased length-to-width ratios of BS cells, were present in non-C4 species, indicating a potential role in C2 and C4 evolution.
摘要：
C2光合作用是光合途径，其中在叶束鞘（BS）组织中增强了光呼吸CO2的释放和固定。C2光合作用的演变被认为是C4光合作用起源的重要一步，强调研究C2进化的重要性。在这项研究中,生理,解剖学,超微结构，在6种非C4Tribulus物种和4种C4Tribulus物种中研究了叶片光合组织的免疫组织化学特性。在42°C时，T.cristatus在没有呼吸（C*）的情况下表现出21µmolmol-1的光合CO2补偿点，低于73µmolmol-1的C3平均C*。Tribulusastrocarpus在42°C时的C*值为55µmolmol-1，介于C3物种和C2T.cristatus之间。甘氨酸脱羧酶（GDC）在BS组织中的分配与较低的C*相关。Tribuluscrystatus和T.astrocarpus将其GDC的86％和30％分配给BS组织，分别,远高于11%的C3平均值。因此，Tribulusastrocarpus表现出较弱的C2（称为sub-C2）表型。线粒体向BS的分配增加，BS细胞的长宽比降低，存在于非C4物种中，表明在C2和C4进化中的潜在作用。
