The Ability of the Key Precursors, Proline, GABA and ZnCl2 to Increase 2-Acetyl-1-Pyrroline Content in Mature Rice (Oryza sativa) Grains of Two Thai Fragrant Rice Varieties Under Field Conditions

Abstract

The key aroma compound in aromatic rice, 2-acetyl-1-pyrroline (2AP), is influenced by various environmental factors and crop management practices. Although the exogenous application of proline, γ-aminobutyric acid (GABA) and zinc chloride (ZnCl₂) has been shown to enhance 2AP levels, no study has conclusively identified which compound is most effective. This study evaluated the effects of foliar application of proline (1000 mg/L), GABA (250 mg/L), ZnCl₂ (1000 mg/L) and a control treatment at the heading stage over five consecutive growing seasons. Two rice varieties, Pathumthani 1 (PTT1) and Hom Malaiman (HMM), were used to compare their responses in 2AP accumulation. The results showed that proline and GABA significantly increased 2AP content, while ZnCl₂ induced only a slight increase. Among the tested compounds, GABA proved to be the most cost-effective. Gene expression analysis revealed that proline and ZnCl₂ enhanced the expression of OsPRODH, supporting the biosynthetic pathway of 2AP via Δ¹-pyrroline-5-carboxylic acid (P5C). In contrast, GABA application upregulated Osbadh2, suggesting a reversible pathway from GABA to γ-aminobutyraldehyde (GABald), which subsequently contributed to the formation of Δ¹-pyrroline and 2AP. None of the treatments significantly affected grain yield, amylose content, chalky grain percentage, or gelatinization temperature, except ZnCl₂, which caused temporary leaf burn. In conclusion, foliar application of these compounds effectively enhanced 2AP levels in rice grains under field condition, with GABA identified as the most promising option due to its high efficacy and low application cost.