Increase of propionic acid production in broccoli florets influences the resistance to carbon dioxide injury

Abstract

During controlled atmosphere (CA) storage, high concentrations of carbon dioxide (CO2) can cause injury to fruit and vegetables. This study aimed to uncover the metabolism pathway or metabolite associated with CO2 injury. Our results showed that CO2 injury of broccoli florets with symptom of off-odor occurred at ≥ 3 % CO2 under 4 % O2. Based on the transcriptome analysis results of Yellow ‘Fuji’ apple before and after CO2 injury, the propanoate metabolism pathway and 11 related genes were found to be enriched, and the differential expression levels of homologous genes in broccoli were measured with RT-qPCR. Among the 11 homologous genes, eight showed a response to CO2 injury in broccoli. Gas chromatography-mass spectrometer (GC-MS) analysis showed propionic acid contents in CA chambers at injured concentrations (3 – 5 % CO2) were much higher than uninjured concentrations (1 – 2 % CO2), and the change patterns during uninjured and injured periods were also obviously different. Gas chromatograph (GC) data indicated propionic acid contents in tissues of broccoli florets were kept low and stable at 1 – 2 % CO2, but at 3 % (CO2 injury concentration threshold) for only 24 h, the content rose and continuously increased with higher CO2 concentration, which implied that propionic acid is a good indicator of CO2 injury. Our further research showed exogenous propionic acid fumigation alleviated CO2 injury through the enhancement of cell membrane stability via improving antioxidant capacity. Finally, RT-qPCR analysis indicated that exogenous propionic acid treatment caused the opposite changes in expression levels of seven genes from 2 % CO2 (uninjured concentration) to 3 % CO2 (injured concentration) among above eight CO2 injury-responded genes in broccoli. Our findings demonstrated that the content of propionic acid in broccoli florets influences the resistance to CO2 injury by better maintenance of antioxidant ability. This research provides new insights for understanding the mechanism of high CO2 injury and exploring methods to alleviate the disorder through the changes of endogenous metabolites.