Adaptations of Malus domestica Borkh. (Rosaceae) Fruits Grown at Different Altitudes

Cytophysiological adaptive features of apple fruits (Malus domestica Borkh.) were examined as a function of growth altitude-300, 500, 700, and 1200 m above sea level-in the mountains of the North Caucasus. The altitude-related conditions of apple tree cultivation differed by a range of growth-limiting factors, high irradiance and low temperature in particular. With the use of fruit skins of M. domestica, it was found for the first time that the number of cells containing phenolic substances in the outer cell layers and the amount of accumulated phenols increase with the altitude of tree growth. At higher elevation belts, the content of polyunsaturated fatty acids also increased, which accounted for the rise of the unsaturation index. Phenolic substances protect the fruit peripheral tissues by screening the cells from high doses of UV radiation and by exerting their powerful antioxidant effects. The increased degree of fatty acid unsaturation helps to retain the fluidity of cell membranes in the physiological range at low temperatures. Thus, apple fruits grown at high altitudes adapt to mountain conditions by a suite of structural and functional changes in the peripheral tissues, which facilitates the maintenance of homeostasis in the inner cell layers. The complex rearrangements in fruit tissues of M. domestica trees enable them to grow at high elevation belts under frequently changing conditions and to remain fully productive at various altitudes.