Genotypic Variation in Maize Root Hydrotropism and Its Association with Shoot Growth and Water Use Efficiency Under Partial Root–Zone Drying

Drought severely limits maize yields. Water–saving irrigation methods like partial root–zone drying (PRD) can improve water use efficiency (WUE) but often result in variable yield responses among genotypes. We hypothesized that differences in root hydrotropism might contribute to some of this variability. Seven maize varieties were evaluated for hydrotropic response in a controlled moisture–gradient assay and then grown for five weeks under fixed PRD versus full irrigation in a greenhouse. The different maize varieties exhibited distinct hydrotropic behaviors: roots of V6 and V7 bent toward water much faster and more strongly, while V2 responded slowly with minimal curvature. Under PRD, genotypes also differed in root distribution and shoot performance. However, hydrotropism alone did not guarantee good shoot maintenance. One strongly hydrotropic genotype (V7) still suffered a large biomass reduction under PRD. Overall, genotypes that maintained better shoot water status, along with larger stem diameter and higher shoot water content, achieved the highest WUE under PRD. These results indicate that root hydrotropism varies widely in maize varieties. This variation was associated with shoot traits and WUE under PRD, suggesting that the benefit of hydrotropism for drought adaptation may depend on complementary shoot characteristics. Breeding for drought–resilient maize may therefore require combining strong root hydrotropism with the ability to maintain shoot function under water deficit.