Cultivar-specific coordination of root plasticity and root antioxidant defense underlies drought adaptation in...

Alfalfa (Medicago sativa L.) is a high-value perennial forage crop whose productivity is severely limited by drought. Although root plasticity and antioxidant defense each contribute to drought tolerance, whether these two dimensions are coordinated or trade off across cultivars remains unclear.

A glasshouse experiment used three alfalfa cultivars (Zhaodong, ZD; Aohan, AH; Golden-empress, GE) and four soil water levels (well water - 75%-80% of water-holding capacity, WHC, mild drought - 60%-65% WHC, moderate - 45%-50% WHC, severe drought - 30%-35% WHC) in a randomized complete block (RCB) design with eight replications per treatment combination. The eight replications were divided into two groups (four for root morphology and four for physiology/biochemistry) to avoid destructive-sampling interference between measurement categories.

ZD and AH reached peak shoot biomass under mild drought, whereas GE exhibited optimal growth under well-watered conditions. In all three cultivars, root biomass, total root length, root surface area and root volume were highest under severe drought, indicating drought-induced root plasticity that potentially enhances soil exploration. MDA, O₂ ^-, H₂O₂ increased with drought severity, whereas catalase, ascorbate peroxidase, glutathione peroxidase, glutathione reductase and ascorbic acid showed cultivar-specific modulation. H₂O₂ was negatively correlated with ascorbic acid (r = -0.90) and glutathione reductase (r = -0.75).

These findings highlight cultivar-specific strategies for balancing morphological plasticity and biochemical defense. ZD showed the most balanced coordination of root expansion and antioxidant responses, with lower oxidative damage and stable biomass, making it a candidate for breeding programs targeting both drought resilience and yield stability under variable or moderate water availability, whereas GE is better suited to irrigated systems.