Bundle sheath chloroplast volume can house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling (J Exp Bot)

It has long been thought that Cspecies generally perform less well than Cspecies in cold environments as a consequence of a physical space restriction.  Cspecies tend to accumulate more rubisco under chilling stress to avoid limiting photosynthesis, but there is less capacity for this in Cspecies as rubisco is confined solely to bundle sheath cells (BSCs).  Pignon et al investigated this phenomenon by comparing the chloroplast volume and leaf anatomy of a chilling-tolerant Cspecies to three chilling-intolerant Cspecies.  To achieve accurate determination of these parameters, the authors employed 3D confocal imaging of fixed leaf samples with stained cell walls.  Surprisingly, chilling-tolerant plants had the smallest BSC chloroplast volume of the four species examined.  In addition, calculations of the theoretical maximum rate of rubisco-limited photosynthesis were higher than actual net photosynthetic uptake for chilled plants in all the species tested.  These results suggest that BSC volume does not limit photosynthesis in Cplants growing in cold environments, contradicting the existing hypothesis.  (Summary by Mike Page) J Exp Bot 10.1093/jxb/ery345