Ultrastructural Changes during Swelling and Contraction of Mitochondria from Cold-hardened and Non-hardened Winter Wheat

Mitochondria isolated from both 2 and 24 C grown winter wheat (Triticum aestivum L.) undergo spontaneous swelling in isomolar KCI solutions, but only 24 C mitochondria exhibit a substrate-induced contraction response. Electron microscopic examination revealed that 24 C mitochondria have more clearly defined cristae, less matrix material, and are generally more electron-dense than 2 C mitochondria. During swelling, the matrix material of both 2 and 24 C mitochondria expands and the mitochondria become less electron-dense. After partial swelling, 24 C mitochondria contract upon addition of succinate, and regain structural characteristics similar to those of untreated mitochondria. In contrast, mitochondria from 2 C seedlings continue to swell after addition of substrate, and many of the mitochondria become irregular in shape and lose much of their matrix material. A comparison of results obtained from absorbancy measurements, electron microscopy, and a Coulter Counter indicate that swelling and contraction involve changes both in over-all volume, and internal structural characteristics of mitochondria from 2 and 24 C grown seedlings. Electron microscopic examination of shoot cells showed that mitochondria in 24 C grown seedlings possessed more recognizable cristae and greater internal organization than mitochondria in 2 C seedlings.