Ethylene production in citrus leaves infected with Xanthomonas citri and its relation to defoliation

Abstract

Xanthomonas citri produced ethylene in the liquid cultures at an early stage of growth, i.e. 3 to 6 h after inoculation. Ethylene production ceased 9 h after inoculation. This pattern was similar in four different media: peptone-glucose broth, citrus leaf extract and synthetic media supplemented with methionine or cysteine. In citrus leaves inoculated with X. citri by a leaf-infiltration method, however, ethylene production began 3 to 5 days after inoculation and increased sharply. The rate of continued ethylene production was directly related to disease symptom development. Maximum ethylene production occurred 10 to 14 days after inoculation when the disease had become severe. The rate of defoliation was directly related to the rate of ethylene production. The leaves inoculated with higher inoculum doses produced ethylene earlier than those with lower inoculum doses. When the lesion area was small, ethylene production was delayed. Ethylene production and defoliation were affected by the site of lesion development. The peak ethylene production and defoliation took place earlier in the leaves inoculated close to the base of the leaf blades than those inoculated close to the apex. In the presence of 10 ppm of ethylene, immature citrus leaves on shoots all fell off by 9 days after treatment and mature leaves by 16 days.