The Role of Ultrastructural Organization of Cells in Adaptation of Winter Wheat to Low Temperature

Changes in the ultrastructural organization of mesophyll cells during low-temperature adaptation (4°C, 7 days) were studied on seedlings of a frost-resistant variety of winter wheat (Triticum aestivum L.). It was found that, under the influence of low hardening temperature, wheat leaf cells increased in size, while the area of the cytoplasm increased and the size of the vacuole decreased. The electron density of the cytoplasm visually increased, and numerous vesicles appeared in it. In addition, the size of chloroplasts, as well as the number of chloroplasts, mitochondria, and peroxisomes per unit area of a cell section, increased. Accumulations of mitochondria and peroxisomes in the form of chains were noted near chloroplasts in the cells of hardened seedlings, and the mitochondria themselves changed their shape from round to elongated or dumbbell-shaped. The low temperature also affected the shape of chloroplasts, which became more rounded from lenticular, and outgrowths (stromules) were found in them. Significant changes under the influence of low temperature occurred in the ultrastructure of chloroplasts: the number and size of plastogbules increased; starch inclusions completely disappeared; the number of grana, the average number of thylakoids per grana, the height and area of the grana, the density of photosynthetic membranes in the chloroplast, and the index of grana stacking (ratio length of appressed membranes to the length of non-appressed membranes) decreased. At the same time, in the process of low-temperature hardening, an increased frost resistance of wheat was formed, which was analyzed by the survival rate of seedlings and the release of electrolytes from leaf tissues after testing freezing. The relationship between the found structural transformations in wheat leaf cells and functional and physiological-biochemical changes occurring in cold-tolerant plants in the process of low-temperature adaptation is shown. It is assumed that the observed ultrastructural reorganization of cells is one of the important components in a complex adaptation program as well as a link necessary for the formation of increased resistance of winter cereals to low temperatures.