Winter Klaus, Holtum Joseph A M, Smith J Andrew C
Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancón, Republic of Panama.
Centre for Tropical Biodiversity and Climate Change, James Cook University, Townsville, QLD, 4811, Australia.
New Phytol. 2015 Oct;208(1):73-8. doi: 10.1111/nph.13446. Epub 2015 May 14.
The key components of crassulacean acid metabolism (CAM) - nocturnal fixation of atmospheric CO2 and its processing via Rubisco in the subsequent light period - are now reasonably well understood in terms of the biochemical reactions defining this water-saving mode of carbon assimilation. Phenotypically, however, the degree to which plants engage in the CAM cycle relative to regular C3 photosynthesis is highly variable. Depending upon species, ontogeny and environment, the contribution of nocturnal CO2 fixation to 24-h carbon gain can range continuously from close to 0% to 100%. Nevertheless, not all possible combinations of light and dark CO2 fixation appear equally common. Large-scale surveys of carbon-isotope ratios typically show a strongly bimodal frequency distribution, with relatively few intermediate values. Recent research has revealed that many species capable of low-level CAM activity are nested within the peak of C3 -type isotope signatures. While questions remain concerning the adaptive significance of dark CO2 fixation in such species, plants with low-level CAM should prove valuable models for investigating the discrete changes in genetic architecture and gene expression that have enabled the evolutionary transition from C3 to CAM.
景天酸代谢（CAM）的关键组成部分——夜间固定大气中的二氧化碳，并在随后的光照期通过核酮糖-1,5-二磷酸羧化酶/加氧酶（Rubisco）对其进行处理——就定义这种节水型碳同化模式的生化反应而言，目前已得到较为充分的理解。然而，从表型上看，植物相对于常规C3光合作用参与CAM循环的程度差异很大。取决于物种、个体发育和环境，夜间二氧化碳固定对24小时碳积累的贡献可在接近0%到100%之间连续变化。尽管如此，并非所有可能的光暗二氧化碳固定组合都同样常见。对碳同位素比率的大规模调查通常显示出强烈的双峰频率分布，中间值相对较少。最近的研究表明，许多具有低水平CAM活性的物种嵌套在C3型同位素特征的峰值范围内。虽然关于此类物种中暗二氧化碳固定的适应性意义仍存在疑问，但具有低水平CAM的植物应成为研究促成从C3到CAM进化转变的遗传结构和基因表达离散变化的有价值模型。
