Morphology of root cells after inducing cell death. (a) 6-d-old seedlings were treated with DMSO (control), 55°C, or 77°C for 10 min or with H₂O₂ for 6 h or NaCl for 16 h. Root hairs were stained with Sytox green 6 h after HS or right after H₂O₂ or NaCl treatments. The appearance of cells was examined using DIC microscopy or fluorescent microscopy. Bars, 50 µm. (b) TEM micrographs of Arabidopsis root cortical cells 6 h after HS: (a–c), root cells from a nonstressed (control) root; (d–f) root cells from a 55°C-treated root, showing cytoplasmic retraction and shrunken, abnormal mitochondria; and (g–i) root cells from a 77°C-treated root, showing mitochondria with a light matrix and very few cristae. V, vacuole; N, nuclei; M, mitochondria; P, plastids; CR, cytoplasmic retraction. (c) Size of mitochondria in roots of plants submitted to HS. Roots were stained with MitoTracker green FM, and mitochondrial images were obtained using confocal microscopy at the times indicated. Quantification of mitochondrial area was performed using the Mito-Morphology macro for ImageJ software (T0, n = 93; 2 h, n = 58; 3 h, n = 33; 6 h, n = 62).