Phospholipid Degradation in Frozen Plant Cells Associated with Freezing Injury

Abstract

A striking degradation of phosphatidylcholine into phos-phatidic acid was observed in the cortical tissues of less hardy poplar (Poplus euramericani cv. gelrica), when the tissues were frozen below a lethal temperature. No change in phos-pholipids was detected during freezing or even after thawing in the cortical tissues of hardy poplar which survived slow freezing to-30 C or even immersion in liquid N2 after pre-freezing to-50 C. The degradation of phosphatidylcholine during freezing appears to be intimately associated with freezing injury. There is evidence to show that the primary site of freezing injury in plant cells is the cellular membranes (5, 9, 11). In the electric low frequency resistance studies of alfalfa, Greenham (3) obtained evidence to show that the intactness of the plasma membranes was lost during freezing as a result of injury. In the electrophoretic measurements to study the patterns of freezing in various plant materials, Olien (12) and Sukumaran and Weiser (15) also obtained essentially the same results. These facts suggest that some deleterious changes occur in cellular membranes of plants frozen below the critical temperatures. Thus, it may be postulated that some compositional changes in phospholipids, which are known as the essential components of cellular membranes, occur in frozen plants sustaining injury. In this study with cortical tissues of poplar, phospholipid compositions were determined during freezing, either during or after thawing from various temperatures. Special attention was focussed on the correlation between compositional changes in phospholipids during freezing and injurious effect of freezing on plant cells. MATERIALS AND METHODS Materials. As experimental materials, the cortical tissues of poplar (Poplus euramericana cv. gelrica) from the campus of our institute were used. The cortical tissues from current twigs of poplar were cut into small pieces (0.5 X 3 cm, about 2 mm Ministry of Education. in thickness) and then frozen in a test tube (1.3 X 18 cm) at-5 C. After exposure to-5 C for 2 hr, the frozen tissues were cooled in 5 C steps at hourly intervals to succesively lower temperatures to-30 C. They were held at selected test temperatures for 20 hr. Some tissues which were frozen to-30 C were further cooled to-50 C and then immersed in liquid N2. The frozen tissues were thawed in air at 0 C. After standing at 0 C for 1 hr, they were placed at room temperature for more than 3 days. Freezing injury was then evaluated visually.