Abstract: Plant water use efficiency (WUE) is a key eco-physiological index reflecting the coupling relationship between carbon and water cycles of vegetation ecosystems. In the meanwhile, the variability of WUE can reflect the responses of terrestrial plants to global change. With the development and application of stable isotope technique and eddy covariance technology, the concepts and measurement methods of WUE are improved, and the research scales about WUE are extended. To further understand the variation characteristics and regulation mechanisms of WUE would help to reveal the plant eco-physiological adaptation mechanism in different habitats, and promote the understanding of the coupling relationship between the carbon and water cycles of terrestrial ecosystems. At present, the research on ecosystem WUE is still in its initial stage, and how to correlate the WUE on different scale levels will be an important research field in the future. In this paper, the affecting factors of leaf WUE, the interspecific differences in plant WUE, the relationships between WUE and leaf characters, and the research progress on WUE at ecosystem scale are reviewed, and the future research aspects are proposed.

Key words: Redundancy, Redundancy degree, Optimal growth redundancy degree, Agro-ecosystems management