University of Illinois, Carl R. Woese Institute for Genomic Biology and Departments of Crop Sciences and of Plant Biology, Urbana, IL, USA.
Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, USA.
J Exp Bot. 2019 Jan 1;70(1):357-365. doi: 10.1093/jxb/ery345.
C4 leaves confine Rubisco to bundle sheath cells. Thus, the size of bundle sheath compartments and the total volume of chloroplasts within them limit the space available for Rubisco. Rubisco activity limits photosynthesis at low temperatures. C3 plants counter this limitation by increasing leaf Rubisco content, yet few C4 species do the same. Because C3 plants usually outperform C4 plants in chilling environments, it has been suggested that there is insufficient chloroplast volume available in the bundle sheath of C4 leaves to allow such an increase in Rubisco at low temperatures. We investigated this potential limitation by measuring bundle sheath and mesophyll compartment volumes and chloroplast contents, as well as leaf thickness and inter-veinal distance, in three C4 Andropogoneae grasses: two crops (Zea mays and Saccharum officinarum) and a wild, chilling-tolerant grass (Miscanthus × giganteus). A wild C4 Paniceae grass (Alloteropsis semialata) was also included. Despite significant structural differences between species, there was no evidence of increased bundle sheath chloroplast volume per leaf area available to the chilling-tolerant species, relative to the chilling-sensitive ones. Maximal theoretical photosynthetic capacity of the leaf far exceeded the photosynthetic rates achieved even at low temperatures. C4 bundle sheath cells therefore have the chloroplast volume to house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling.
C4 叶片将 Rubisco 局限在维管束鞘细胞中。因此，维管束鞘腔室的大小和其中的叶绿体总容积限制了 Rubisco 的空间可用性。Rubisco 活性在低温下限制光合作用。C3 植物通过增加叶片 Rubisco 含量来克服这种限制，但很少有 C4 物种这样做。由于 C3 植物在寒冷环境下通常优于 C4 植物，因此有人认为 C4 叶片的维管束鞘中没有足够的叶绿体体积来允许 Rubisco 在低温下增加。我们通过测量三种 C4 禾本科植物（玉米、甘蔗和耐寒野生草类芒属）的维管束鞘和叶肉腔室体积和叶绿体含量，以及叶片厚度和叶脉间距离，来研究这种潜在的限制。还包括一种野生 C4 黍属禾本科植物（短柄草）。尽管物种之间存在显著的结构差异，但与对寒冷敏感的物种相比，耐寒物种的叶片每单位面积可用的维管束鞘叶绿体体积并没有增加的证据。叶片的最大理论光合能力远远超过了即使在低温下也能达到的光合速率。因此，C4 维管束鞘细胞具有容纳足够 Rubisco 的叶绿体体积，以避免在寒冷期间限制 C4 光合作用。
