Five decades ago, the first report of a shift from C to CAM (crassulacean acid metabolism) photosynthesis following the imposition of stress was published in this journal. The annual, (Aizoaceae), was shown to be a C plant when grown under non-saline conditions, and a CAM plant when exposed to high soil salinity. This observation of environmentally triggered CAM eventually led to the introduction of the term facultative CAM, which categorises CAM that is induced or upregulated in response to water-deficit stress and is lost or downregulated when the stress is removed. Reversibility of C–to–CAM shifts distinguishes stress-driven facultative-CAM responses from purely ontogenetic increases of CAM activity. We briefly review how the understanding of facultative CAM has developed, evaluate the current state of knowledge, and highlight questions of continuing interest. We demonstrate that the long-lived leaves of a perennial facultative-CAM arborescent species, , can repeatedly switch between C and CAM in response to multiple wet–dry–wet cycles. Undoubtedly, this is a dedicated response to environment, independent of ontogeny. We highlight the potential for engineering facultative CAM into C crops to provide a flexible capacity for drought tolerance.

中文翻译：

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在快车道和慢车道之间切换光合作用：兼性 CAM 和缺水胁迫

五十年前，该杂志发表了第一篇关于施加压力后光合作用从 C 转向 CAM（景天酸代谢）的报告。一年生植物（相思科）在非盐分条件下生长时是 C 植物，而在高盐度土壤中生长时是 CAM 植物。对环境触发的 CAM 的观察最终导致了“兼性 CAM”这一术语的引入，该术语将因缺水压力而诱导或上调的 CAM 分类，并在压力消除时丢失或下调。C 到 CAM 转变的可逆性将应激驱动的兼性 CAM 反应与纯粹个体发生的 CAM 活性增加区分开来。我们简要回顾了对兼性 CAM 的理解是如何发展的，评估了当前的知识状况，并强调了人们持续感兴趣的问题。我们证明，多年生兼性 CAM 树状物种的长寿叶子，可以在 C 和 CAM 之间反复切换，以响应多个湿-干-湿循环。毫无疑问，这是对环境的专门反应，与个体发育无关。我们强调了将兼性 CAM 工程化到 C 作物中以提供灵活的耐旱能力的潜力。