Young Sophie N R, Sack Lawren, Sporck-Koehler Margaret J, Lundgren Marjorie R
Lancaster Environment Centre, Lancaster University, Lancaster, UK.
Department of Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA, USA.
J Exp Bot. 2020 Aug 6;71(16):4629-4638. doi: 10.1093/jxb/eraa234.
Since C4 photosynthesis was first discovered >50 years ago, researchers have sought to understand how this complex trait evolved from the ancestral C3 photosynthetic machinery on >60 occasions. Despite its repeated emergence across the plant kingdom, C4 photosynthesis is notably rare in trees, with true C4 trees only existing in Euphorbia. Here we consider aspects of the C4 trait that could limit but not preclude the evolution of a C4 tree, including reduced quantum yield, increased energetic demand, reduced adaptive plasticity, evolutionary constraints, and a new theory that the passive symplastic phloem loading mechanism observed in trees, combined with difficulties in maintaining sugar and water transport over a long pathlength, could make C4 photosynthesis largely incompatible with the tree lifeform. We conclude that the transition to a tree habit within C4 lineages as well as the emergence of C4 photosynthesis within pre-existing trees would both face a series of challenges that together explain the global rarity of C4 photosynthesis in trees. The C4 trees in Euphorbia are therefore exceptional in how they have circumvented every potential barrier to the rare C4 tree lifeform.
自50多年前首次发现C4光合作用以来，研究人员已在60多次研究中试图了解这种复杂特性是如何从祖先的C3光合机制进化而来的。尽管C4光合作用在植物界反复出现，但在树木中却极为罕见，真正的C4树木仅存在于大戟属植物中。在这里，我们考虑了C4特性中可能限制但不排除C4树木进化的方面，包括量子产量降低、能量需求增加、适应性可塑性降低、进化限制，以及一种新理论，即树木中观察到的被动共质体韧皮部装载机制，加上在长路径上维持糖和水运输的困难，可能使C4光合作用在很大程度上与树木的生命形式不相容。我们得出结论，C4谱系向树木习性的转变以及现有树木中C4光合作用的出现都将面临一系列挑战，这些挑战共同解释了C4光合作用在全球树木中罕见的原因。因此，大戟属中的C4树木在如何规避罕见的C4树木生命形式的每一个潜在障碍方面是例外的。
