Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA.
Proc Natl Acad Sci U S A. 2012 Jun 26;109(26):10403-8. doi: 10.1073/pnas.1203769109. Epub 2012 Jun 11.
Declining CO(2) over the Cretaceous has been suggested as an evolutionary driver of the high leaf vein densities (7-28 mm mm(-2)) that are unique to the angiosperms throughout all of Earth history. Photosynthetic modeling indicated the link between high vein density and productivity documented in the modern low-CO(2) regime would be lost as CO(2) concentrations increased but also implied that plants with very low vein densities (less than 3 mm mm(-2)) should experience substantial disadvantages with high CO(2). Thus, the hypothesized relationship between CO(2) and plant evolution can be tested through analysis of the concurrent histories of alternative lineages, because an extrinsic driver like atmospheric CO(2) should affect all plants and not just the flowering plants. No such relationship is seen. Regardless of CO(2) concentrations, low vein densities are equally common among nonangiosperms throughout history and common enough to include forest canopies and not just obligate shade species that will always be of limited productivity. Modeling results can be reconciled with the fossil record if maximum assimilation rates of nonflowering plants are capped well below those of flowering plants, capturing biochemical and physiological differences that would be consistent with extant plants but previously unrecognized in the fossil record. Although previous photosynthetic modeling suggested that productivity would double or triple with each Phanerozoic transition from low to high CO(2), productivity changes are likely to have been limited before a substantial increase accompanying the evolution of flowering plants.
白垩纪 CO2 含量下降被认为是推动被子植物特有的高叶片脉密度（7-28 毫米毫米-2）进化的原因。光合作用模型表明，在现代低 CO2 环境中记录的高脉密度与生产力之间的联系将随着 CO2 浓度的增加而丧失，但也暗示具有非常低脉密度（小于 3 毫米毫米-2）的植物在高 CO2 环境中会面临实质性的劣势。因此，可以通过分析替代谱系的同时历史来检验 CO2 和植物进化之间的假设关系，因为像大气 CO2 这样的外在驱动因素应该影响所有植物，而不仅仅是开花植物。但目前并未发现这种关系。无论 CO2 浓度如何，低脉密度在整个历史中在非被子植物中同样常见，而且足够普遍，包括森林树冠，而不仅仅是始终生产力有限的专性荫生物种。如果将非开花植物的最大同化率限制在远低于开花植物的水平，那么模型结果可以与化石记录相协调，从而捕获与现存植物一致但以前在化石记录中未被识别的生化和生理差异。尽管之前的光合作用模型表明，每次从低 CO2 到高 CO2 的显生宙过渡，生产力都会增加一倍或两倍，但在开花植物进化伴随的大量增加之前，生产力的变化可能受到限制。
