Physiological and iTRAQ-based proteomic analyses for yellowing of postharvest broccoli heads under elevated O2 controlled atmosphere

Controlled atmosphere storage has been proved to be an effective measure to regulate chlorophyll degradation. The obvious inhibitory effect of CO₂ on chlorophyll degradation was widely studied, but information available on the promotion of chlorophyll degradation by elevated O₂ was limited. The present work aimed to investigate the regulation of different O₂ concentrations on yellowing of broccoli heads during storage by physiology and proteomics. As expected, 40% O₂ + 5% CO₂ treatment promoted the chlorophyll degradation and carotenoid synthesis, and thus to accelerate the yellowing of broccoli heads. In addition, elevated O₂ decreased the ascorbic acid, glucosinolate, and flavonoid contents and increased the MDA content. A total of 189 proteins related to the yellowing of broccoli heads were identified as differentially expressed proteins (fold change ≥1.20 or ≤0.83). These proteins were mainly associated with pigment metabolism, thylakoid structure, photosynthetic system, chloroplast ribosome, antioxidant substance metabolism, and antioxidant enzyme metabolism. The proteomics results showed 40% O₂ + 5% CO₂ treatment disrupted antioxidant metabolism and aggravated oxidative stress compared with 5% O₂ + 5% CO₂ and 20% O₂ + 5% CO₂ treatments, thus leading to the destruction of photosystems I and II, thylakoid membrane, and chloroplast ribosome in broccoli heads. In addition, 40% O₂ + 5% CO₂ treatment accelerated the yellowing of postharvest broccoli heads by down-regulating chlorophyll biosynthetic pathway and the expression of apolipoproteins (light-harvesting complex I, II, and oxygen-evolving enhancer protein) and ribulose-1,5-bisphosphate carboxylase (Rubisco), and up-regulating chlorophyll degradation and carotenoid biosynthetic pathways. This study might improve our understanding of yellowing acceleration mechanisms of broccoli heads by elevated O₂, thereby promoting the development of new broccoli heads preservation technology.