Chlorophyll Retention Reduces Storability and Pathogen Defense in a Novel Citrus Brown Flavedo Mutant

Residual chlorophylls in harvested crops induce “green seed problem”, which dramatically reduces seed shelf life and storage. However, the effect of residual chlorophylls on fruit storability was largely unexplored. Here, a novel citrus brown flavedo mutant ‘Zong Cheng’ (MT) was explored to understand the effect of residual chlorophylls on fruit postharvest disease and storage. MT fruit remained brown during long-term storage, and residual chlorophyll was observed in the flavedo of MT during the whole storage period. Compared with ‘Lane Late Navel Orange’ (WT), the firmness of MT decreased, while decay incidence, weight loss, respiratory intensity, and the release of ethanol in MT increased during low-temperature storage. The disease infection and decay rate of MT are obviously higher than WT during on-tree or natural storage. Inoculation experiments revealed reduced resistance against Penicillium italicum of MT, further confirming the worse pathogen defense of MT. Perturbations in many stress-responsive phytohormones, primary metabolites, and volatiles were associated with changes in storability and pathogen defense of MT fruit. Noticeably, compared with WT, the content of hydrogen peroxide (H2O2) and the activities of related enzymes were increased in the flavedo of MT during storage. Further studies indicated that chlorophyll retention was closely associated with fewer starch granules and decreased starch content in the flavedo of MT, which might result in decreased firmness and the worse storability of MT. Hence, residual chlorophyll in MT causes excessive ROS accumulation and firmness decrease, and thus reduces storability and pathogen defense in MT fruit. These findings contribute to our understanding of how residual chlorophylls affect fruit storability and disease infection, and help future citrus improvement with an enhanced intrinsic defense mechanism.