Microscopic Characterization of Damage Produced by Frost in Cherry and Apple Ovaries

Abstract

Additional index words. freezing damage, low temperatures, Malus sylvestris, Prunus avium Fifty long branches of 30 cm were cut with most small fruit in Fleckinger’s H and I stages (Fleckinger, 1945), taken to the laboratory and placed in beakers with a sucrose solution (5 g/100 mL) and kept at different temperatures (–2, –4, and –6 °C) controlled by digital process controller (ES100; Omron, Tokyo); 48 h after treatment, fruits were collected. A minimun of 25 samples from the previously described kinds of ovaries were evaluated. The samples evaluated had a diameter between 30–50 mm, and were fixed in FAA [(Formaldehyde (0.4 mL) : Glacial Acetic acid : Alcohol (ethanol, at 0.7 mL); 5 : 5 : 90; v/v/v] After cutting the ovary transversely (30 μm), it was stained with safranine and fast-green. The resulting preparations were studied by optical microscopy (Nikon, mod. Optiphot, Tokyo) at magnification of ×4 and ×10. At –4 oC, 80% of cherry fruit was frost damage; broken ovary tissue was observed, contrasting clearly with healthy tissues. The dehydration produced by frost appeared as radial intercellular spaces. There were also radial areas in which very intense dehydration alternated with broken tissues. Damage in laboratory test and in orchard freezes were similar. The reticular appearance of the dehydration produced by the deterioration from lack of fertilization, contrasts with the radial dehydration observed from freezing. Frost-produced dehydration and broken tissues in apple looked similar to those observed in cherry. It also was observed in the apple fruit that the most damaged area was that of the pericarp. Fig. 1 shows the comparative damage in recently formed cherry and apple fruits. In view of the above observations, we concluded that under severe frost conditions in both cherry and apple fruit, the broken ovary tissue can be observed easily under the microscope, while tissue dehydration caused by frost has a characteristic appearance distinguishable from dehydration for other reasons.