Histological and physiological studies on the cause of the Jonathan spot in apples. I

Abstract

1. Many investigators published their findings on the cause of the Jonathan spots which develop on the skin of apples. However, no definite conclu-sion nor established method of prevention have been published to this date. Therefore since 1956 the author has tried to solve this problem from the histological and physiological viewpoint, experimen-ting with such varieties as Jonathan, McIntosh, Ralls and Indo apples. This paper contains the results of these experiments on the spot develop-ment as influenced by the varietal difference in the structure of apple skin. 2. In the Jonathan apple, which seems to be most susceptible variety, cuticula formation on the skin was very slow as compared to the other varieties of apple, and the arrangement of epidermal cells remained brick wall like until about the middle of August. In the spot resistant variety, namely, Rails and Indo apples, cutin sediments were found on the side walls of epidermal cells early in June, and except for the bottom wall which touches the hypodermal cells, cuticule was formed on the walls of the epidermal cells about the middle of July. In McIntosh, a medium resistant variety, the struc-ture of the fruit skin, at first, seemed to change almost like that of Jonathan, but then became like those of Ralls and Indo. 3. In every variety, as the fruit grew some of the stomata showed signs of cracking in the middle of June, and the stomata gradually collapsed, dis-coloring their surrounding tissue and finally develop-ed into the lenticels. The Jonathan apple skin was very slow in this process, so that many stomata were still normal even late in August, when Mc-Intosh and Ralls already had well-developed lenti-cels. 4. In order to clarify the morphological property of the lenticels, some ripe fruits were separately treated with sudan III solution, methylene blue solution and formalin gas, and six hours later the reaction was observed. In Ralls and Indo, dye-sediments were detected only on the surface of the projected lenticels, showing no trace of dye-infiltra-tion in the inner tissue, while in Jonathan some of lenticels allowed dyes to infiltrate into the inner tissues. With formalin gas, the inner tissues of the skin of all varieties tested were damaged more or less through lenticels. 5. When the Jonathan spots were observed under the microscope, it became clear that the lenticels located in the center of the spot were either 1) those in which cork cambium had developed poorly, 2) or the stomata of which were still at the early stage of crack, 3) or in which the skin tissue around the stomata was about to crack. There fore, the author gave these lenticels the name of “openlenticels” 6. In the storage of the fruits which were treated with dye solution, Jonathan spots developed only on the fruits which allowed dyes to infiltrate. The fruits which suffered from Jonathan spots, afterward, were found to have transpired more water than the healthy apples. This fact suggested to us that for the development of Jonathan spots the existence of open lenticels should be considered as an important causal factor. 7. In addition, it was noticeable that when the fruits were stored at different temperatures and hu-midities after harvesting, Jonathan spots developed more rapidly on the fruits kept in a moist block than on those in a dry block, regardless of the stor-age temperature, and also they developed more rapidly on the fruits stored at a temperature of 20°C±5°C than on those at a constant temperature of 35°C or 0°C.