Differential responses of citrus root systems to biostimulants: the role of γ-aminobutyric acid in root promotion, phytohormones, and rhizosphere microbiology

BACKGROUND: In intensive citrus plantation systems, root growth is often limited by various abiotic factors, leading to poor nutrient uptake and consequently thus low fruit yield and quality. Therefore, selection of effective biostimulant type to promote well-developed root system is fundamental for citrus production. However, the effects of varied biostimulants on citrus crops remain poorly understood. Thus, this study aimed to investigate the effects of fourteen biostimulants on citrus root growth following application. RESULTS: Our results revealed that the citrus root system exhibited differential responses to the various biostimulants. Among all treatments, γ-aminobutyric acid (GABA) proved to be the most effective in promoting root growth, significantly increasing root biomass, enhancing phytohormone levels, and stimulating root morphogenesis. Compared to the CK treatment, GABA application increased the total root length, surface area and volume by 19.3%, 36.3%, and 23.4%, respectively. GABA application also induced shifts in the rhizosphere microbial community structure. Structural equation modeling further confirmed that GABA application had a negative direct effect on microbial community composition, which indirectly promoted root phytohormone synthesis and ultimately enhanced citrus root morphological development. CONCLUSION: This study expands our understanding of how biostimulants promote crop root development and identifies GABA as a promising effective biostimulant for enhancing citrus root growth. Consequently, GABA application represents an effective strategy for promoting root development in intensive citrus plantation systems.