Glycine Betaine Mitigates Flooding and Drought Damage in Maize by Regulating Respiration, ROS Homeostasis, and Metabolism.

Flooding (FS) and drought (DS) reduce maize yield and water use efficiency by inducing anaerobic respiration and excessive reactive oxygen species (ROS). We tested foliar glycine betaine (GB) in three maize genotypes, GD668 (water-tolerant), ZF505 (water-sensitive), and GD0810 (drought-tolerant), subjected to FS or DS at the V3 stage using a factorial design of genotype × water stress × GB, with measurements at Day 4, Day 8, and after rewatering or drainage. Both stresses inhibited root growth, decreased leaf relative water content (RWC) and water potential (Wp), destabilized instantaneous water use efficiency (IWUE), and increased hydrogen peroxide (H2O2), superoxide (O2 -), and membrane permeability (Mp), with ZF505 most affected. GB improved root traits, increased RWC and Wp, stabilized IWUE, and reduced H2O2, O2 -, and Mp. GB also enhanced lactate dehydrogenase (LDH) activity, suppressed excessive pyruvate decarboxylase (PDC) activity, and limited ethanol (ETOH) and pyruvic acid (PA) accumulation. Transcriptomic and metabolomic analyses of GD668 and ZF505 indicated that GB up-regulated starch and sucrose metabolism and glycolysis genes (e.g., ZmSPS, ZmHK), modulated ZmPDC, and regulated lipid and betaine-related genes (ZmMGD, ZmDGD, ZmSQD2, ZmCHPT1, ZmBADH), supporting central carbon flux and membrane stability. A two-year field study (2022-2023) confirmed genotype-dependent mitigation, with the strongest effect in ZF505. Overall, the results indicate that GB stabilizes IWUE under opposite water stresses by moderating fermentative intensity and supporting membrane lipid remodeling, with stronger benefits in the sensitive genotype.