Scaling relations between leaf and plant water use efficiencies in rainfed Cotton

Water use efficiency (WUE) relates two important processes of the plant atmosphere continuum namely carbon assimilation (via photosynthesis) and water utilization (via transpiration). Our desire to trade-off WUE between accurate measurement at leaf level (WUEL) and effective implementation at plant level (WUEP) demands accurate scaling relations. Conventional mid-day, fully expanded, single-leaf measurements of WUEL are found to be poorly correlated with WUEP, thus questioning the applicability of scaling relations. This research is aimed at obtaining optimal time-window and canopy position to characterize and upscale WUEL for effective field level implementation. Leaf gas exchange parameters were monitored in a rainfed Cotton field at five canopy positions for one crop cycle, and further correlated with WUEP considering individual measurements as well as their spatial averages. Optimal time-window showing highest correlation with WUEP has occurred during 15:00 to 16:00 h irrespective of leaf position and growth stage. Deviation with mid-day measurements of WUEL are largely attributed to stomatal regulation of water vapour via unregulated water stress conditions. Scaling relations between WUEL and WUEP are linear with correlation strengths ranging from 0.52 (west bottom) to 0.80 (plant top). At leaf level, WUE is controlled by variations in photosynthetic photon flux density (ρ = 0.80) and vapour pressure deficit (ρ = 0.78), whereas at plant level, WUE is controlled by relative humidity (ρ = 0.77) and net solar radiation (ρ = 0.85). Our findings can help in developing alternate water management strategies to improve WUE in rainfed Cotton fields of tropical humid climate.