Impact of Drought Stress on Root System Architecture and Anatomical Adaptations in Forages: A Comprehensive Review

Drought stress represents the most significant threat to global forage productivity, necessitating a robust understanding of adaptive belowground mechanisms. This paper highlights the critical advancements in root system architecture (RSA) and root vascular architecture that underpin resilience in forages. Under water-limited conditions, forages exhibit substantial morphological plasticity, characterized by a 20-40% increase in primary root elongation and a strategic rise in root-to-shoot ratios to prioritize deep-layer moisture acquisition. Anatomically, they optimize hydraulic safety through a 10-40% reduction in xylem vessel diameter, which enhances embolism resistance by 20-35% despite a minor decline in hydraulic conductivity. Metabolic efficiency is further bolstered by the development of root cortical aerenchyma, which reduces cortical area by 30-50% and lowers maintenance respiration costs by up to 60%. Parallel to these metabolic shifts, the reinforcement of endodermal barriers specifically 1.5-2 times thickening of the Casparian strip and a 1.5-2.2 times elongation of root hairs maximizes water-use efficiency and soil-root contact. The review further examines the physiological drivers of these changes, indicating that moderate ABA accumulation can contribute to the maintenance or stimulation of primary root elongation under stress, with reported increases of 20–40% depending on genotype and environmental conditions. By integrating these multi-scale traits with high-throughput phenotyping and symbiotic interactions, this paper provides a framework for breeding climate-resilient forage ideotypes essential for sustaining livestock systems and global soil carbon stocks.