Scaling relations between leaf and plant water use efficiencies in rainfed Cotton – Role of environmental and biophysical parameters

Water use efficiency (WUE) relates two important processes of the plant atmosphere continuum namely net carbon assimilation (via photosynthesis) and water utilization (via evapotranspiration). Our desire to trade-off WUE between accurate measurement at leaf level (WUE_L) and effective implementation at plant level (WUE_P) demands accurate scaling relations. Conventional mid-day, fully expanded, single-leaf measurements of WUE_L are found to be poorly correlated with WUE_P, thus questioning the applicability of scaling relations. This research is aimed at obtaining optimal time-window and leaf canopy position to characterize and upscale WUE_L 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 WUE_P considering individual measurements as well as their spatial averages. Optimal time-window showing highest correlation with WUE_P has occurred during 15:00 to 16:00 hours irrespective of canopy leaf position and growth stage. Deviation with mid-day measurements of WUE_L low during boll bursting stage (7.38 ± 4.69 %) and high during germination and seedling emergence stage (17.27 ± 5.37 %). These changes are largely attributed to stomatal regulation of water vapour via unregulated water stress conditions. Scaling relations between WUE_L and WUE_P 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.