Metal-directed root foraging in hyperaccumulators is masked in soil systems

Metal hyperaccumulators are promising plants for phytoextraction, but the root traits that help them grow toward metal-rich soil remain unclear. Metal-directed root foraging has been proposed as a unique trait of hyperaccumulators, yet inconsistent observations have obscured its functional significance. Here, we combined a meta-analysis of 128 published observations with split-root experiments in Sedum alfredii to evaluate the recurrence, environmental sensitivity, and physiological relevance of metal-directed root foraging. A meta-analysis showed a broad but context-dependent tendency for hyperaccumulators to allocate more roots to metal-enriched patches, and this response was positively associated with shoot metal accumulation and biomass. However, this response was strongly shaped by soil conditions, particularly soil texture and soil source. Split-root experiments further revealed a clear ecotype-specific response in hydroponics. The hyperaccumulating ecotype (HE) allocated more than 60 % of its root biomass to the Cd-enriched compartment, whereas the non-hyperaccumulating ecotype (NHE) showed reduced root allocation to this compartment. This ecotype-specific contrast was not fully retained in the tested soil system: Cd-directed foraging by HE was no longer detectable, whereas Cd avoidance by NHE persisted. These findings identify metal-directed root foraging as a recurrent but soil-dependent trait of hyperaccumulators. By showing that soil conditions can constrain the detectability of this trait, our study reconciles inconsistent observations and points to soil management as a potential strategy to strengthen root foraging and enhance phytoextraction in contaminated soils.