Relationship between wound-healing process of citrus leaf tissues and successful infection through wounds by Xanthomonas campestris pv. citri (hasse) dye.

Abstract

In attached citrus leaves wounded and incubated in a low humidity environment, hypertrophy and subsequent cell divisions were observed at inner 5-6 cell layers apart from the wound and followed by development of meristem tissues composed of thin dividing cells (TDC-layer).Cell walls of parenchyma outside the TDC-layer became lignified. Infection by Xanthomonas campestris pv. citri was completly inhibited after formation of the TDC-layer. In detached leaves wounded and incubated in a moist environment, irregular cell divisions following hypertrophic changes were observed from 1 to 6 cell layers bordering the wound, but any TDC-layers were not observed. Incubation period from wounding to occurrence of cell divisions was immutable irrespective of humidity. The wound tissues incubated in a moist environment were highly susceptible to infection by X. c. pv. citri until the cell walls in the outer tissues were lignified. Therefore, the susceptible period to infection was about 2 times longer than that of attached leaves incubated in a low humidity. Wound healing was most rapid at 30-35C and markedly retarded below 20C. Overwintering leaves in the field were susceptible to infection for about 20 days or more when wounded in January to early March, and for about 10 days when wounded in mid March to April. Strong winds or any other wound-causing factors, even if they accompanied no rainfall, are concluded as important factors for the occurrence of citrus bacterial canker disease.