The resilience of barley to drought in a changing climate is determined by its lateral root diameter

Climate change is intensifying droughts and threatening food security. Roots are the plants’ organ for water uptake and are crucial for their adaptation, with their structure being a decisive factor. In barley (Hordeum vulgare), lateral roots form~60% of the total root length and are important for water uptake. Hydraulic conductance scales strongly with root diameter: thicker laterals conduct more water per unit length but demand higher carbon for construction and maintenance. During soil drying, this creates a potential carbon-water trade-off. We test whether such a trade-off exists and whether it shapes drought resilience across environments by comparing the diameter that optimizes the trade-off with that maximizing shoot dry mass (SDM).

We used a functional–structural plant model (OpenSimRoot) to simulate barley growth across five climatically and pedologically contrasting global sites, representing different drought regimes. Simulations covered 50 growing seasons (2000–2049) using projected climate data and site-specific soils. Five lateral root diameter classes were evaluated, and outputs included shoot dry mass, root carbon allocation, and root hydraulic conductance. Drought performance was assessed by jointly considering productivity and efficiency-based metrics related to carbon investment and water transport capacity.

Across all environments, barley performance showed a clear dependence on lateral root diameter, with intermediate diameters generally balancing water uptake capacity and carbon costs. SDW and trade-off analyses converged on to the same diameter, reflecting a general trend. However, site-specific analyses revealed substantial divergence, reflecting differences in climate variability, soil properties, and drought characteristics. In several environments, finer lateral roots did not consistently confer advantages in either hydraulic efficiency or biomass production, challenging the notion of a universally optimal “cheap-root” strategy under drought.

A robust carbon–water trade-off underlies lateral root diameter; the diameter that performs best depends on the environment (climate and soil) and the objective (e.g., maximizing SDW versus efficiency/resilience). When data are pooled across all sites, SDM- and trade-off–based optima coincide, but site-level results differ; therefore, breeding for drought resilience should target site- and objective-specific trait values rather than a single fixed optimum.