Allometric relationships between nitrogen uptake and transpiration to untangle interactions between nitrogen supply and drought in maize and sorghum

Worldwide supply of water and nitrogen (N) are critical to crop yield with multiple interactions that need to be untangled. We used an allometric framework to quantify the dynamic relationships between crop growth, transpiration and N uptake in maize (Zea mays L.) and sorghum (Sorghum bicolor L.) using published data from experiments in France, New Zealand and United States. Our analysis highlighted a two-fold effect of drought on crop growth: a direct effect whereby crop growth is reduced proportionally to the reduction in transpiration, and an indirect effect mediated by a drought-induced crop N deficit, quantified as N nutrition index (NNI), the ratio between actual and critical plant N concentration (the minimum plant N concentration for achieving maximum crop mass). Under drought, sorghum maintained NNI and transpiration efficiency (biomass per unit of transpiration) whereas both NNI and transpiration efficiency declined in drought-stressed maize in comparison to well-watered crops. Nitrogen uptake per unit of transpiration (N/T) is a valuable trait for analyzing the combined effect of water and N deficit on water use efficiency (biomass per unit of evapotranspiration). However, both N/T and crop N concentration decline allometrically with biomass, hence comparisons of genotypes, environments and management practices would be biased unless they are scaled by crop biomass.