A review of progress in research and scaling-up methods of crop water use efficiency

Increasing crop water use efficiency (WUE) is an effective way of alleviating agricultural water scarcity. The scaling up of water use efficiency is the basis for mutual representation, verification and application of achievements at various scales. This paper summarized the main observation technologies of leaf-scale, plant-scale and plantation-scale WUE. At present, the widely used methods include leaf gas exchange measurement, carbon isotopic discrimination, pot weighing method, eddy covariance system, etc. Carbon isotope discrimination provides a new idea for the study of long-term cumulative effects of crop water use conditions which is also available at every scale. We reviewed the impacting factors and the related physiological mechanisms of crop water use at multi-scale WUE. Crop WUE at each scale was regulated by stomatal conductance and crops usually regulated stomatal aperture to response to temperature, humidity, CO₂ and other interactive environmental factors. Stomatal optimization theory essentially sought optimal state of stomata under complex environmental conditions to coordinate the process of photosynthesis and transpiration of crops. Instantaneous WUE at leaf scale cannot directly represent water use status at larger spatial and temporal scales. Thus we also discussed the feasibility of scaling up WUE from leaf to plant to plantation scales and analyzed the main limiting factors at each scale transfer. We pointed out the difficulties in transfer from leaf to plant in terms of WUE. It mainly focused on three points-uncertainty in leaf and light distributions, plant nighttime respiration and transpiration, and plant physiological adjustment mechanisms. Research on leaf to plant to plantation scale transfer was mainly influenced by canopy internal resistance, boundary layer resistance, soil evaporation, night transpiration of crops, crop water use and assimilates partitioning mechanism. Finally, existing research achievements on scale transfer were summarized. At present, WUE scale transfer depended mainly on improvement of models and observation methods. The transfer from leaf to plant focused on separation of water use during day and night and photosynthetic characteristics of each part of the crop. For transfer from leaf to plant to plantation scale, studies explored efficient ways. First, studies understood the structure of evapotranspiration and confirmed the characteristics of water use. Second, studies used the relationship between stomatal and canopy conductance as breakthrough point via models to explore transfer mechanisms. Actually, several models had already been established and applied in this respect.