Ristic Z., Ashworth E. N.
Center for Plant Environmental Stress Physiology, Department of Horticulture, Purdue University, West Lafayette, Indiana 47907.
Plant Physiol. 1994 Feb;104(2):737-746. doi: 10.1104/pp.104.2.737.
Freezing behavior of wood tissue of red osier dogwood (Cornus sericea L.) cannot be explained by current concepts of freezing resistance. Previous studies indicated that water in wood tissue presumably froze extracellularly. However, it was observed that xylem ray parenchyma cells within these tissues could survive temperatures as low as -80[deg]C and the walls of these cells did not collapse during freezing (S.R. Malone and E.N. Ashworth [1991] Plant Physiol 95: 871-881). This observation was unexpected and is inconsistent with the current hypothesis of cell response during freezing. Hence, the objective of our study was to further examine the mechanism of freezing resistance of wood tissue of red osier dogwood. We studied freezing stress response of xylem ray parenchyma cells of red osier dogwood using freeze substitution and transmission electron microscopy. Wood samples were collected in winter, spring, and summer of 1992. Specimens were cooled from 0[deg]C to -60[deg]C at 5[deg]C/h. Freezing stress did not affect the structural organization of wood tissue. However, the xylem ray parenchyma cells showed two unique responses to a freezing stress: protoplasm contraction and protoplasm fragmentation. Protoplasm contraction was evident at all freezing temperatures and in tissues collected at different times of the year. Cells with fragmented protoplasm, however, were noticed only in tissues collected in spring and summer. Protoplasm contraction in winter tissue occurred without apparent damage to the protoplasm. In contrast, protoplasm contraction in spring and summer tissues was accompanied by substantial damage. No evidence of intracellular ice formation was observed in parenchyma cells exposed to freezing stress. Differences in protoplasm contraction and appearance of cells with fragmented protoplasm likely indicated seasonal changes in cold hardiness of the wood tissue of red osier dogwood. We speculate that the appearance of fragmented protoplasm may indicate that cells are being injured by an alternative mechanism in spring and summer.
红枝山茱萸（Cornus sericea L.）木材组织的冷冻行为无法用当前的抗冻性概念来解释。先前的研究表明，木材组织中的水可能在细胞外结冰。然而，观察到这些组织中的木质部射线薄壁细胞能够在低至-80℃的温度下存活，并且这些细胞的细胞壁在冷冻过程中不会塌陷（S.R. Malone和E.N. Ashworth [1991]《植物生理学》95: 871 - 881）。这一观察结果出乎意料，并且与当前关于冷冻期间细胞反应的假设不一致。因此，我们研究的目的是进一步探究红枝山茱萸木材组织的抗冻机制。我们使用冷冻置换和透射电子显微镜研究了红枝山茱萸木质部射线薄壁细胞的冷冻应激反应。木材样本于1992年冬、春、夏三季采集。标本以5℃/小时的速度从0℃冷却至-60℃。冷冻应激并未影响木材组织的结构组织。然而，木质部射线薄壁细胞对冷冻应激表现出两种独特的反应：原生质收缩和原生质碎片化。原生质收缩在所有冷冻温度下以及在一年中不同时间采集的组织中都很明显。然而，只有在春季和夏季采集的组织中才注意到有原生质碎片化的细胞。冬季组织中的原生质收缩并未对原生质造成明显损伤。相比之下，春季和夏季组织中的原生质收缩伴随着大量损伤。在遭受冷冻应激的薄壁细胞中未观察到细胞内结冰的迹象。原生质收缩和原生质碎片化细胞外观的差异可能表明红枝山茱萸木材组织的抗寒能力存在季节性变化。我们推测，原生质碎片化的出现可能表明细胞在春季和夏季正受到另一种机制的损伤。
