Study On Histological Characterization Of Rind Disease In Citrus

The development of rind disease from light to serious in citrus was studied on four varieties including 'Fengjie'navel orange, 'Newhall'navel orange,'Hongju'tangerine and 'Ponkan' tangerine by technique of paraffin section in order to find out histological characterization of all stages of rind disease in the citrus and compare the histological characterization of rind in easy suffering and uneasy suffering citrus. The results were as follows:1. Physiological disorders1.1. Initial symptoms of rind oil stain of citrus appeared in the forth to seventh cell layers of the flavedo. Affected cells became flattening and reduced amounts of cytoplasm located in a central position, however, the epidermal cells still arranged regularly and densely. Affected cells were distorted at the middle stage. The cytoplasm of these cells was disarrayed and restricted to a thin peripheral layer around the cell walls. Affected cells whose cytoplasm degraded reached the epidermis across the flavedo at the late stage. Meanwhile the oil glands kept intact. Some of the oil glands depressed at the terminal stage due to that the apical region of which was affected.1.2. Rind brown spot of citrus occurred at the cells around the oil glands initially. Affected cells reduced the cytoplasm in the center of the cells. The oil glands became flattened. The epidermal cells and sub-epidermal cells were distorted at the first stage. Then the oil glands were depressed. The flavedo and albedo were infolded at the middle stage. They showed an undulation appearance. The oil glands were distorted. The cells degraded and the oil glands ruptured. Outside of the flavedo was infected.2. Mechanical- damage-likeInitial symptoms of mechanical-damaged-like fruits of citrus appeared in the transitional zone of the flavedo-albedo extending towards outer of the flavedo and deeper of the albedo.3. Albedo breakdownThe flavedo showed an undulation depression due to that the oil glands collapsed and the albedo ruptured.4. Rusty-spots-likeInitial symptoms of rusty-spots-like in citrus appeared in the epidermis. The damaged part gradually became small local lesion. The parenchymal cells located under epidermis and the cells under the oil glands were damaged. Then out side of the flavedo were damaged. All of the flavedo were damaged at the terminal stage. The adjacent ofthe healthy and the lesion were evident. The cells around the lesion which became oblate or polygomal arranged regularly longitudinally and densely.5. The comparison of healthy rind in easy suffering and uneasy suffering citrus5.1. Epidermal cells in the flavedo of Tengjie'navel orange which was thick and easy suffering were rectangular or polygonal in shape with no intercellular spaces covered by cuticle. These cells had dense cytoplasm and small vacuoles arranged regularly and orderly. The parenchymal cells located under epidermal cells were spherical to slightly oval in profile. Cell size gradually increased deeper in the rind with small intercellular spaces. The oil glands located in these layers. The albedo had large air spaces.5.2. The rind of 'Newhall' navel orange was thinner than the rind of Tengjie' navel orange. Epidermal cells in the flavedo of 'Newhall' navel orange were polygomal and isodiametric in shape. The parenchymal cells located under epidermis almost were spherical in shape. The oil glands and the distance between adjacent oil glands were big.5.2. The rind of 'Hongju' tangerine was thin. The oil glands were irregular in size with large space between two oil glands. The rind had one sratum of oil glands. The albedo was thin and loose.5.3. The rind of 'Ponkan'mandarin was thin. Two statums of oil glands located in the flavedo and albedo. The upper stratum of the oil glands was dense and irregular in size. Space of the adjacent oil glands was small. The lower stratum of the oil glands was rare and big. The two statums of oil glands arranged interphasely. The albedo was thin and loose.The histological characterization of the occurrence and development of rind oil spots, rind brown spots, mechanical-damage-like, crease and rusty-spots-like in citrus as well as the reasons why 'Fengjie' navel orange were easier suffering than other citrus was discussed.