Root system traits as key determinants of drought tolerance in safflower (Carthamus tinctorius L.) genotypes under stress conditions

Safflower ( Carthamus tinctorius L.) is gaining recognition as a significant oilseed crop suitable for arid environments, attributed to its deep and extensive root system; nevertheless, the genetic foundation of its drought tolerance is still inadequately understood. To address this gap, we evaluated 75 safflower genotypes in two complementary greenhouse experiments designed to capture drought responses at both the seedling and reproductive stages. Experiment 1, conducted under sand-culture conditions, assessed root morphology and seedling vigor following three weeks of water deficit, while Experiment 2, conducted in field soil under net-house conditions, evaluated grain yield and its components following approximately eight weeks of water deficit. In both experiments, plants were subjected to non-stress (100% field capacity) and drought-stress (50% field capacity) regimes within a factorial, completely randomized design with three replications. Genotype × drought interactions were significant for every trait measured ( p < 0.01), underscoring substantial genetic variability in drought response within the panel. Cluster analysis resolved this variability into five distinct genotypic groups with contrasting drought-response strategies. As expected, water deficit reduced most root and yield-related traits—seedling length, root dry weight, diameter, volume, surface area, and water content declined by 30–50%, yield components fell by 27–41%, and grain yield losses of up to 45%, with root volume and surface area reduced by as much as 50%. Notably, specific root length and root vigor index followed the opposite trend, nearly doubling under stress, suggesting an adaptive shift toward finer, more resource-efficient root systems under limited water availability. Two genotypic clusters emerged as particularly drought-resilient, each reflecting a distinct tolerance strategy: Cluster 2 was characterized by high specific root length and seedling vigor, consistent with efficient early-stage water and resource capture, whereas Cluster 4 combined high root surface density with superior grain and biological yield, indicating an advantage in sustaining productivity under prolonged stress. Both clusters recorded the highest Stress Tolerance Index values, corroborating their drought resilience through an independent quantitative measure. collectively, root surface density, specific root length, and root diameter were identified as the most informative traits for discriminating drought tolerance in safflower. Genotypes from Clusters 2 and 4 represent promising genetic resources for breeding programs aimed at developing drought-tolerant safflower cultivars for water-limited agricultural systems.