Adding N to Water-Stressed Sorghum bicolor Shifts Root Functional Traits and Uptake Strategies, but Without Short-Term Improvements in Water Use Efficiency or Productivity.

Given the co-limitation of water and nitrogen (N) acquisition under drought, it is essential to mechanistically explore how plants respond to these limitations individually and in combination. To understand the general effect that N availability plays in water use efficiency across Sorghum bicolor ssp. bicolor, which has diverse mechanisms conferring tolerance to water limitations, we grew six genotypes ranging in drought tolerance under greenhouse conditions under three treatments: full water/high N, limited water/high N, and limited water/low N availability. We measured a suite of above- and below-ground plant traits to understand growth allocation and physiological responses. Under high N but limited water, two genotypes increased allocation to fine root production and reduced allocation to coarse root production, which could limit deep water foraging under field conditions. In contrast, limited N and water availability together stimulated coarse root production in those genotypes. High N with limited water did not increase net photosynthesis or total plant biomass production compared to limited N and water availability in any genotype. These results suggest that adding N to S. bicolor ssp. bicolor is unlikely to be an effective strategy for rescuing growth or yield under drought conditions. Moreover, limiting coarse root growth may have detrimental effects under field conditions. It is crucial for future studies to follow up on the effects of N and water availability on the grain yield of S. bicolor ssp. bicolor under water-limited field conditions to better understand the potentially negative effects of high nitrogen on water foraging under drought.