Harnessing root phenotyping and root growth plasticity of landraces of maize ( L.) for enhancing waterlogging tolerance in fragile and challenging agro-ecologies of Eastern Himalaya.

Maize is a major crop of the Eastern Himalayan Region (EHR) which faces significant environmental challenges including waterlogging (WL) under changing climate. Through microcosm and field experiments, this study aimed to evaluate the phenotypic plasticity and adaptive mechanisms of maize landraces under WL conditions at the seedling and flowering stages. Based on the response coefficient and waterlogging tolerance coefficient 14 landraces at the seedling stage were found to be WL tolerant whereas RCM-12-19, RCM-32-19, and RCM-16-19, emerged as WL tolerant at both stages. At seedling stage, Root Length ratio (RLR) has increased under WL stress to the tune of 98.4 % while Root Mass Ratio (RMR) has ranged from 0.09 to 0.47 for control and from 0.10 to 0.55 under WL. Root:shoot ratio varied from 0.10 to 0.88 and 0.11-1.23 under control and WL, respectively and a total of 19 genotypes reflected tolerance trait under WL. Phenotyping of key root traits (brace root angle (BA1), branching, and crown root number) revealed their substantial contribution to stress resilience, as confirmed by principal component analysis (PCA) and regression models. PCA indicates, for root tissue density (RTD) and root fineness (RF) at seedling stage, genotypes like RCM-34-19, RCM-10-19, RCM-23-19, RCM-42-19, and RCM-31-19 are closely associated with the second principal component (PC2). Whereas, at flowering stage, RCM-39-19, RCM-52-19, RCM-34-19, RCM-32-19, RCM-2-19, RCM-43-19, RCM-5-19, RCM-45-19, RCM-47-19, and RCM-50-19 exhibited strong positive loadings on (PC2) for the trait BA1. The results indicate that brace- and crown roots exhibit genotype-dependent architectural plasticity, which reduces the metabolic cost of soil exploration by increasing BA1, branching of brace roots (BB), and the number of brace roots (BO) while decreasing above-ground whorls (BW); thereby improving nutrient uptake from topsoil under WL stress. RCM-12-19, RCM-32-19, RCM-16-19, and RCM-23-19 demonstrated rapid root growth and branching after WL stress at the flowering stage, supporting their potential for breeding WL-tolerant maize. These findings align with the "steep, cheap, and deep" (SCD) root ideotype, where reduced crown root number and deeper root architecture improve nutrient uptake and yield. Genotype RCM-11-19 apart from scoring high through visual scoring was also found to have the highest dry biomass (76.7 g plant-1) and grain yield (12.2 g plant-1) under WL conditions. This research identifies critical root traits and promising genotypes for developing WL-tolerant maize, contributing to sustainable crop production in rain-fed EHR environments.