Response of root morphology and anatomy of two alfalfa cultivars with contrasting root system architecture to phosphorus deficiency under drought stress

Introduction The mechanisms underlying adaptations of root system architecture (RSA) and root anatomy to phosphorus (P) deficiency and drought stress remain unclear in alfalfa ( Medicago sativa L.). This study investigated the root response strategies of two alfalfa cultivars (WL354HQ: WL354; Longdong: LD) with contrasting RSA under P deficiency and drought, and the relationship between root traits and shoot P uptake. Methods Root morphology and anatomy were assessed under varying P (0, 5, 20 mg P kg -1 soil) and water (80 ± 5% and 40 ± 5% field capacity) conditions using visual rhizoboxes, with two years field validation. Results Under low P and drought stress within the rhizoboxes, WL354 increased fine roots in soil layers below 60 cm, maintained xylem vessel area (XVA) in lateral roots below 90 cm, enlarged cortical cell size (CCS) in taproots and 2 nd lateral roots across several soil layers, and reduced cortical cell files (CCF) in both taproots and lateral roots throughout all soil layers. Conversely, LD merely maintained CCS in taproots and 2 nd lateral roots across several soil layers, and reduced CCF in taproots across all soil layers and in 1 st lateral roots above 90 cm. Simple linear correlation analysis revealed that shoot P uptake was positively correlated with total root surface area in the rhizobox experiment and with xylem vessel area of the taproot in the field experiment. Discussion Under P deficiency and drought, WL354 implemented a resource acquisition strategy by increasing resource capture and transport in deep soil, and uptake efficiency. In contrast, LD exhibited a relatively conservative strategy focused on uptake efficiency. An increase in total root surface area and xylem vessel area of the taproot may help promote shoot P uptake. These findings offer insight into root trait-based strategies for improving alfalfa productivity in low-P and drought regions.