Physiological variables influenced by “Candidatus Liberibacter asiaticus” infection in two citrus species

Abstract

“Candidatus Liberibacter asiaticus” (CLas) is the bacterium associated with the citrus disease known as Huanglongbing (HLB). This study evaluated the influence of CLas infection on a number of key plant physiological variables concerning photosynthesis, cell integrity, reactive oxygen species scavengers’ activity, and osmoregulation of two different species of citrus (the pomelo Citrus maxima (Burm.) Merr. and the mandarin C. reticulata cv. Tankan), relative to their measured CLas infection load. Results indicated all measured physiological variables excepting soluble sugar were affected by increased CLas infection titers, wherein the variety C. maxima proved overall more resistant than C. reticulata. CLas infection was linked in both plants to decrease in chlorophyll concentration, cell membrane permeability and malondialdehyde, as well as increased free proline and starch contents. Chlorophyll fluorescence measurements taken 9 months after grafting the mandarin C. reticulata with CLas scions revealed a significant decrease in the photosynthesis variables Fv/Fm, Y(II) and QL, whilst NPQ increased significantly; C. maxima plants, on the other hand, did not show significant differences until the 12th month from infection exposure. The variables SOD, CAT, POD, and soluble protein initially increased and later decreased. In addition, progression of CLas replication in both citrus species was accompanied by rapid changes in three reactive oxygen species scavenging enzymes in C. maxima, while the pattern was different in C. reticulata. We hypothesize the observed interspecific differences in physiological change are related to their relative resistance against CLas infection. These results provide a scaffold for better describing the pathogenesis, selecting the most resistant breeds, or even validating pertaining omics research: ultimately these detailed observations can facilitate the diagnosis of CLas infection.