Comparative Freezing Patterns in Stems of Cherry and Azalea

Abstract

Ice formation in stems, as determined by means of an electrophoretic mobility technique, occurs much more rapidly in azalea than in sour cherry. The difference is more marked in the bark than in the wood. Disrupting the structure of the tissues completely eliminates differences in freezing patterns, although gross anatomical differences do not appear to account for differences in species response. Microscopic examination of frozen stems indicated that little redistribution of water occurred during freezing in azalea, and the tissues were disrupted as these crystals developed. In cherry, on the other hand, water diffused to nucleating centers where crystal growth was not opposed, giving rise to "glaciers." At maximum hardiness, sour cherry (Prunus cerasus L.) tissues generally are hardier than comparable tissues of azalea (Rhododendron sp.), critical temperatures for severe injury to the phloem being about-35 and-25 C, respectively ((2) and B. Stergios, personal communication). Differences in survival can result from differences in (a) freezing stress arising from various plant characteristics which interact with the test environment , (b) tissue resistance to stress, or (c) ability to recover. During a study of water redistribution and freezing stresses in woody plants, a marked contrast was noted between these two species. A much greater shift in freezing point occurs in cherry than in azalea as liquid along the cell wall diminishes. Cell wall liquid water content was evaluated by electrophoretic mobility of amaranth dye and electrolytes. Such freezing patterns describe the association of water with cell wall substances, which is a characteristic of the genotype. Interaction of freezing pattern with environmental variables during freezing tests determines the types and intensities of injurious stresses (8). This paper describes the freezing patterns observed in azalea and sour cherry. MATERIALS AND METHODS Plant Materials. Six-year-old plants of azalea, cv.