Drought Dominates Engineered Lipid Sink Effects on Sorghum Physiology and Carbon Allocation

ABSTRACT Determining the environmental limits that govern how engineered metabolic traits control whole‐plant carbon (C) allocation is essential for developing climate‐resilient bioenergy systems. Enhancing sorghum biomass oil (lipid) content is a promising strategy to boost aboveground energy density, yet its system‐wide effects on root C investment and soil C delivery, particularly under drought conditions, are largely uncharacterized. Here, we provide the first comprehensive assessment of these interactions using whole‐plant 13 CO 2 continuous labeling, depth‐specific sampling of roots and rhizosphere soil, and coordinated measurements of photosynthesis, stomatal anatomy, tissue nitrogen, and root nonstructural carbohydrates. The oil‐enhanced sorghum line (TZ424‐5‐3a) differed from wild‐type (TX‐430) in several aboveground traits, with the largest numerical differences observed under well‐watered conditions, including slightly longer stomata, lower stomatal density, and significantly higher leaf nitrogen concentration and biomass. Importantly, these aboveground changes did not translate into detectable shifts in the allocation of new photosynthate to belowground pools. In contrast, drought acted as a dominant regulatory factor, reorganizing C flow across both sorghum genotypes, suppressing photosynthesis and leaf water status, and increasing root nitrogen and root nonstructural carbohydrate concentrations. With depth, drought increased 13 C enrichment in shallow roots but reduced enrichment in deep roots; however, deep rhizosphere soils had increased 13 C enrichment, indicating contrasting depth‐dependent patterns of tracer recovery between roots and soil. Ultimately, soil moisture availability was the overriding determinant of belowground C partitioning and vertical distribution of recovered 13 C, superseding the influence of the engineered lipid sink. These findings provide new mechanistic insight into the environmental constraints on C allocation in an engineered bioenergy crop and identify soil moisture as the major regulator of the short‐term recovery and vertical distribution of recently fixed C.