Changes in Polyphenol Levels in Satsuma (Citrus unshiu) Leaves in Re- sponse to Asian Citrus Psyllid Infestation and Water Stress

Huanglongbing (HLB), also referred to as citrus greening disease has caused significant losses to the citrus industry in the United States and elsewhere. In our previous studies, we observed the fluctuation of some primary and secondary metabolites in response to biotic (psyllids feeding that transfers bacteria; i.e., Candidatus Liberibacter spp. that cause greening disease) and abiotic (water deficit) stress factors in citrus. In the current report, we evaluated the changes in polyphenolic compounds in Satsuma leaves in response to Asian citrus psyllid feeding stress and the water stress. In general, polyphenolic levels increased in Satsuma leaves in response to insect and water stress. Specifically, polyphenols such as chlorogenic acid, rutin, diosmin, luteolin7-O-glucoside, and narangin levels increased significantly in response to both biotic and abiotic stress. On the other hand, while caffeic acid levels significantly increased in water stressed plants, their levels drastically declined to the level of being undetectable in leaves stressed by psyllid feeding. Differences between the two types of stresses were also observed in the levels of apigenin 7-O-glucoside where it decreased significantly in water stressed leaves but not in leaves stressed by psyllid feeding; i.e. changes in the levels of apigenin 7-O-glucoside and caffeic acid were opposite in response to water or feeding stresses. Hesperidin levels were not affected by the water stress or by psyllid feeding. The findings may help to better understanding plant psyllid interactions and may be helpful in developing effective management practices to control the spread of citrus greening disease.