Anatomical response of olive (Olea europaea L.) to freezing temperatures

Abstract

Histological preparations of leaves, 1 year-old shoots and bark from olive trees ‘Picual’ and ‘Arbequina’ that had experienced freezing temperatures were examined to characterise any potential damage. The material selected for study, although obtained from exposed plants, showed no external symptoms. Previously reported ruptures surrounding the midvein were not observed in transverse sections of leaves, nor was any other internal tissue damage seen, possibly due to the use of externally unaffected material. Transverse sections of cold-exposed young shoots of both cultivars, when compared to similar shoots from control plants, showed significant tangentially-oriented breaks and fissures in the cortex, in addition to radially-oriented ruptures in the phloem. Similar radial breaks were observed in the xylem and medulla of young ‘Picual’ shoots, but not in ‘Arbequina’. This difference could be due to differential cultivar tolerance, tree size, or temperature perceived. Cold-exposed bark tissues showed mainly tangential breaks in the cortex and phloem, but no damage in the periderm. In control trees, both young shoots and older bark contained small tears caused by microtome sectioning of contiguous hard and delicate tissues. The ruptures observed in bark and young shoots were consistent, regarding which cells or tissues were affected and the orientation of the lesions produced, and suggest possible guidelines for evaluating cold-temperature damage in olive shoots.