Effects of Trace Elements Manganese, Cobalt, and Molybdenum on Plant Root Traits

Trace elements are essential for plant growth, yet their dose-dependent effects on root system development still remain poorly characterized across diverse plant functional types. We investigated the effects of manganese (Mn), cobalt (Co), and molybdenum (Mo) across 10 concentration gradients (0–500% of standard Hoagland nutrient solution) on root length, tip number, biomass, and activity in four species: Triticum aestivum L., Zea mays L., Ipomoea aquatica Forssk. in Forssk. & Niebuhr, and Canna glauca L. After 30 days of hydroponic culture, our results showed that Mn and Mo elicited significant, concentration-dependent responses across all species. Root traits improved progressively with increasing concentrations from deficiency to optimal levels, but declined when concentrations exceeded the optimal range. The optimal ranges were 25–100% for Mn and 50–150% for Mo, while concentrations above 150% generally proved toxic. By contrast, Co effects were comparatively limited, with significant inhibition only at concentrations exceeding 100%. Species-specific responses were also evident: T. aestivum and Z. mays were more sensitive to Mn and Mo excess, whereas I. aquatica and C. glauca displayed broader tolerance, with C. glauca showing notably high Mo utilization efficiency. Notably, all four root traits exhibited strong positive correlations (r > 0.90, p < 0.001), indicating coordinated morphological and physiological plasticity. Collectively, our findings provide quantitative evidence for species-specific trace element management and contribute to a more nuanced understanding of trace element regulation of root system architecture.