The study was conducted at the Southwest Florida Research and Education Center (SWFREC), University of Florida (UF), between 2012 and 2013. It focused on evaluating oxidative stress metabolism in two-year-old 'Valencia' sweet orange (Citrus sinensis) plants grafted onto Swingle rootstock (Citrus paradisi × Poncirus trifoliata). Both healthy and HLB-affected plants were cultivated under controlled greenhouse conditions. Leaf samples, ranging from young to fully expanded stages, were analyzed to observe biochemical responses to HLB infection. Early-stage HLB-affected leaves appeared asymptomatic but later developed blotchy patterns, characteristic of the disease. Hydrogen peroxide (H₂O₂) levels increased in both healthy and HLB-affected leaves, with significantly higher concentrations observed in the latter. Healthy leaves showed H₂O₂ levels ranging from 0.5 to 3.8 µmole per gram of fresh weight (FW), while affected leaves exhibited levels from 0.56 to 6.5 µmole per gram FW, especially in fully expanded leaves. Enzymatic activities related to oxidative stress were also evaluated. Catalase (CAT) and ascorbate peroxidase (APX) activities increased during the leaf expansion phase but declined in fully expanded leaves, with a sharper decrease observed in HLB-affected samples. The reduced CAT and APX activity in affected leaves contributed to the accumulation of H₂O₂, exacerbating oxidative stress. Guaiacol peroxidase (GPOD) activity was low during early leaf expansion but increased in fully expanded leaves. HLB-affected leaves showed significantly higher GPOD activity, possibly contributing to elevated H₂O₂ levels. Glutathione reductase (GR) activity, vital for maintaining redox balance by regenerating reduced glutathione (GSH), was higher in healthy leaves but declined in HLB-affected samples. This decline suggested impaired recycling of GSH, disrupting redox homeostasis and weakening antioxidant defenses. In contrast, glutathione-S-transferase (GST) activity was elevated in HLB-affected leaves, likely as an adaptive response to detoxify reactive oxygen species (ROS). However, the combination of increased GST and reduced GR activity led to a depletion of reduced glutathione, further intensifying oxidative stress. Overall, the study highlights the disruption of oxidative stress metabolism in HLB-affected sweet orange leaves. The compromised antioxidant defense system, characterized by reduced CAT, APX, and GR activities, contributes to increased cellular damage. These findings provide insights into plant defense mechanisms and suggest potential intervention strategies for managing HLB-induced stress.
