The ratio of plant carbon gain to water use, known as water use efficiency (WUE), has long been recognized as a key constraint on crop production and an important target for crop improvement. WUE is a physiologically and genetically complex trait that can be defined at a range of scales. Many component traits directly influence WUE, including photosynthesis, stomatal and mesophyll conductances, and canopy structure. Interactions of carbon and water relations with diverse aspects of the environment and crop development also modulate WUE. As a consequence, enhancing WUE by breeding or biotechnology has proven challenging but not impossible. This review aims to synthesize new knowledge of WUE arising from advances in phenotyping, modeling, physiology, genetics, and molecular biology in the context of classical theoretical principles. In addition, we discuss how rising atmospheric CO2 concentration has created and will continue to create opportunities for enhancing WUE by modifying the trade-off between photosynthesis and transpiration.

中文翻译：

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用水效率作为提高 C3 和 C4 作物恢复力和生产力的约束和目标

植物碳增加与用水的比率，称为用水效率（WUE），长期以来一直被认为是作物生产的关键制约因素和作物改良的重要目标。WUE 是一种生理学和遗传学上的复杂性状，可以在一系列尺度上进行定义。许多组成性状直接影响 WUE，包括光合作用、气孔和叶肉导度以及冠层结构。碳和水的关系与环境和作物发展的各个方面的相互作用也会调节 WUE。因此，通过育种或生物技术提高 WUE 已被证明具有挑战性，但并非不可能。本综述旨在在经典理论原理的背景下综合表型、建模、生理学、遗传学和分子生物学的进展所产生的 WUE 新知识。此外，