Effects of 2-pyrazinecarboxylic acid treatment on juice sac sugar retention and lignification delay in postharvest ‘Sanhongmiyou’ pummelo

Objective The ‘Sanhongmiyou’ pummelo variety is not suitable for long-term storage due to juice sac lignification caused by excessive lignin accumulation during fruit maturation and postharvest storage. This study aimed to investigate the effects of 2-pyrazinecarboxylic acid treatment on delaying lignification and maintaining flavor quality in ‘Sanhongmiyou’ pummelo during storage.

Method ‘Sanhongmiyou’ pummelo was used as the experimental material. Fruits were treated by immersing in 500 μmol·L⁻¹ 2-pyrazinecarboxylic acid solution + 0.1% (φ) tween 80 solution for 2 min at room temperature, with 0.1% tween 80 solution used as the control. Samples were collected at 0, 4, 8, 12, 16, and 20 days postharvest, and changes in juice sac sugar component contents, lignin content, as well as the activity of key enzymes associated with lignin biosynthesis and the expression levels of their primary coding genes were determined.

Result Compared to control, 2-pyrazinecarboxylic acid treatment significantly reduced lignin content in the juice sacs of ‘Sanhongmiyou’ pummelo during storage. At 20 days postharvest, lignin content increased by 87.41% in the control group, while it increased by only 41.85% in the treatment group. Comprehensive analysis of enzyme activity and gene expression revealed that, compared to control, 2-pyrazinecarboxylic acid treatment significantly decreased the activities of key enzymes involved in lignin biosynthesis (POD, F5H, CAD, CCoAOMT, C4H, and 4CL), with average reduction rates of 17.97%, 9.14%, 12.26%, 29.39%, 23.70%, and 27.10%, respectively. Furthermore, the treatment significantly downregulated the expression levels of major coding genes corresponding to these key enzymes (CmPOD3, CmF5H1, CmCAD4, CmCAD5, CmCCoAOMT1, CmCYP73A16, and Cm4CL2). Additionally, compared to control, the treatment remarkably enhanced the contents of glucose, fructose, sucrose, and soluble sugars, with average increase rates of 21.33%, 16.12%, 12.78%, and 5.51%, respectively. Correlation analysis further indicated that lignin content were extremely significantly positively correlated with the activity of its key biosynthetic enzymes, and the increase in lignin content was significantly negatively correlated with the reduction in sugar components.

Conclusion 2-Pyrazinecarboxylic acid notably suppresses the expression of pivotal enzyme genes involved in lignin biosynthesis, effectively inhibiting enzymatic activity, delaying the lignification process within juice sacs, reducing sugar component depletion, and ultimately preserving the flavor quality of ‘Sanhongmiyou’ pummelo. This research provides a scientific foundation and technical guidance for elucidating the underlying action mechanism of 2-pyrazinecarboxylic acid and enhancing postharvest storage strategies for ‘Sanhongmiyou’ pummelo.