Hypersensitive Response of Orange-Colored Robinson Tangerines to Colletotrichum gloeosporioides After Ethylene Treatment

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Abstract
Mature Robinson tangerines with about 25% external orange color break did not develop anthracnose caused by C. gloeosporioides when treated with ethylene to obtain a complete orange color. Histochemical tests of the peel removed after ethylene treatment indicated that phenolics and lignin were present in the flavedo in association with infection hyphae produced by appressoria on the peel surface. Ultrastructural changes occurred in cells up to 5 cell diameters from the point of penetration. Cytoplasm of these cells contained mitochondria with dilated cristae and chromoplasts that often had indistinct envelopes and homogeneous stroma. The cytoplasm also contained numerous vesicles apparently derived from dilated endoplasmic reticulum and dictyosomes. Vesicles formed by the dictyosomes usually were contained within multivesicular bodies. These bodies frequently were associated with the plasmalemma before electron-dense-deposits accumulated at the cell wall. Accumulation of deposits occurred concurrently with collapse of the cytoplasm against the cell wall distal from the area of fungal penetration. Many cells eventually became occluded with electron-dense material which surrounded hyphae in the invaded cells.