Abstract:

Changes of hydrogen peroxide (H₂O₂) as well as peroxidase (POD) in the tissues of diseased, healthy, mechanical wounded and phytoplasma graft-transmitted tissue-cultured plantlets of different paulownia clones were investigated using histochemical methods. H₂O₂ localization by DAB staining method showed that the H₂O₂ did not significantly accumulated in both diseased and healthy plantlets of intact plants. After wounding treatment, both diseased and healthy tissues induced H₂O₂ accumulation, and the H₂O₂ in healthy tissues was more than the diseased ones. POD activity in the vascular bundles of severe diseased plants was highest, followed by that of plants with light symptoms, and then asymptomatic and uninfected plantlets. Mesophyllous tissues was lower POD activity than the veins. At the early stage (within 3 days) of graft-inoculation by means of inserting infected scion onto healthy stock, changes in the POD activity and H₂O₂ content had similar pattern as the mechanical wounding reaction, and these effects became weaker 6 days later after graft inoculation. The sustaining H₂O₂ content and high POD activity were detected at the graft union site as well as in the stock in 20 days post graft inoculation using infected scion which appeared witches-broom symptom. Moreover, the systematic induction of H₂O₂ and POD was also detected in the distal parts of main stem or veins of some leaves. H₂O₂ localization by means of KI/starch assay revealed that more H₂O₂ was found in the cell surface of cortex and pith as well as on the wall of conduct tube of xylem in the healthy section and less H₂O₂ was found in the corresponding tissues of diseased plants. When the diseased stem segments were treated with different concentration of H₂O₂ for 1 hour and then in vitro cultured on MS medium, it was demonstrated that the H₂O₂ in 25 to 100 mmol·L^-1 concentrations could obviously relieve symptom on the cultured explants. Ascorbate acid and lowly aerating culture conditions also suppressed symptom development. Certain paulownia clones showed some difference in H₂O₂ accumulation and POD activity in response to phytoplasma infection, which might probably relate to the resistances to phytoplasmas.

Key words: in vitro cultured paulownia clone, witches&rsquo, -broom phytoplasma, hydrogen peroxide, peroxidase, histolocalization