SHORT-TERM CHILLING OF WHEAT SEEDLINGS OR ROOTS AFFECTS THE ULTRASTRUCTURE OF MESOPHYLL CELLS
The effects of short-term chilling (2 °C, 10 min) of seedlings or roots of wheat (Triticum aestivum L.) on the cold tolerance and ultrastructure of mesophyll cells were studied under controlled conditions. The results indicated that the cold tolerance of leaf cells started to increase respectively 1 and 5 h after short-term chilling of seedling or roots, reaching a maximum after 24 h. In addition, a number of ultrastructural rearrangements in mesophyll cells were detected by electron microscopy during the first minutes and hours after short-term chilling. These included changes in the shape and density of chloroplasts, mitochondria and peroxisomes, enlargement of mitochondria and chloroplasts, emergence of protuberances and invaginations in chloroplasts, accumulation of mitochondria and peroxisomes near plastids. At the same time, short-term chilling of wheat seedlings or roots caused changes in the thylakoid system of leaf chloroplasts. Thus, the extent of photosynthetic membranes and the index of grana stacking of chloroplasts increased 24 h after short-term chilling of seedlings (due to the extent of appressed membranes). Local chilling of roots, on the contrary, caused a reduction of the length of photosynthetic membranes, but the index of grana stacking increased as a result of decreasing length of non-appressed membranes. A comparison of ultrastructural changes occurring in leaf cells at short-term and long-term chilling of wheat seedlings or only the root system suggested that a number of different adaptive programs depending on the character (type) of low-temperature treatment are realized in the plants. They can include both similar (of the same type) and different changes in the ultrastructure of leaf cells.