Differential Responses of Thai Fragrant Rice to Silicon Application Enhance Yield and Aroma Under Highland and Lowland Ecosystems

Silicon (Si), a beneficial element accumulated by rice (Oryza sativa L.), enhances productivity and tolerance to biotic and abiotic stresses. Fragrance, primarily driven by 2-acetyl-1-pyrroline (2AP), is a key trait in premium rice markets. This study evaluated the effects of Si on grain yield, yield components, 2AP content, and Si accumulation in three Thai fragrant rice genotypes—BNM4, BNMCMU, and KDML105—under highland and lowland conditions. Plants received four Si application rates: 0 (control), 168, 336, and 504 kg Si ha−1. Si significantly increased yield under lowland conditions, while responses in the highland were genotype-dependent, with only BNMCMU showing significant improvement at the highest Si rate. Silicon accumulation in shoot tissues was consistently higher in the highland than in the lowland across all genotypes. Nevertheless, Si application significantly increased shoot Si content under lowland conditions. A positive correlation between grain yield and shoot Si accumulation was observed under both environments, highlighting the role of Si in yield enhancement. The influence of Si on 2AP concentration was limited, with stronger effects from genotype and environment especially in the highland, where KDML105 consistently exhibited the highest 2AP levels. In the lowland, however, Si application significantly enhanced 2AP content in BNMCMU and KDML105. These findings underscore the significance of genotype × environment interaction and support precision Si application to enhance both yield and aroma in fragrant rice.