Abstract
BackgroundTable olives (Olea europaea L.), despite their widespread production, are still harvested manually. The low efficiency of manual harvesting and the rising costs of labor have reduced the profitability of this crop. A selective abscission treatment, inducing abscission of fruits but not leaves, is crucial for the adoption of mechanical harvesting of table olives. In the present work we studied the anatomical and molecular differences between the three abscission zones (AZs) of olive fruits and leaves.ResultsThe fruit abscission zone 3 (FAZ3), located between the fruit and the pedicel, was found to be the active AZ in mature fruits and is sensitive to ethephon, whereas FAZ2, between the pedicel and the rachis, is the flower active AZ as well as functioning as the most ethephon induced fruit AZ. We found anatomical differences between the leaf AZ (LAZ) and the two FAZs. Unlike the FAZs, the LAZ is characterized by small cells with less pectin compared to neighboring cells. In an attempt to differentiate between the fruit and leaf AZs, we examined the effect of treating olive-bearing trees with ethephon, an ethylene-releasing compound, with or without antioxidants, on the detachment force (DF) of fruits and leaves 5 days after the treatment. Ethephon treatment enhanced pectinase activity and reduced DF in all the three olive AZs. A transcriptomic analysis of the three olive AZs after ethephon treatment revealed induction of several genes encoding for hormones (ethylene, auxin and ABA), as well as for several cell wall degrading enzymes. However, up-regulation of cellulase genes was found only in the LAZ. Many genes involved in oxidative stress were induced by the ethephon treatment in the LAZ alone. In addition, we found that reactive oxygen species (ROS) mediated abscission in response to ethephon only in leaves. Thus, adding antioxidants such as ascorbic acid or butyric acid to the ethephon inhibited leaf abscission but enhanced fruit abscission.ConclusionOur findings suggest that treating olive-bearing trees with a combination of ethephon and antioxidants reduces the detachment force (DF) of fruit without weakening that of the leaves. Hence, this selective abscission treatment may be used in turn to promote mechanized harvest of olives.
Highlights
Table olives (Olea europaea L.), despite their widespread production, are still harvested manually
The detachment force (DF) of olive fruits increased from fruit set in May (30 days post anthesis (DPA)), until fruits reached their full size in September, the harvest season of table olives
Among the transcripts associated with the gene ontology (GO) term of the auxin biosynthetic process, we found that ANTHRANILATE SYNTHASE BETA SUBUNIT 1 (OeASB1) expression was induced in response to ethephon treatment in all three Abscission zone (AZ)
Summary
Table olives (Olea europaea L.), despite their widespread production, are still harvested manually. A selective abscission treatment, inducing abscission of fruits but not leaves, is crucial for the adoption of mechanical harvesting of table olives. Abscission refers to a set of separation events in which entire plant organs, such as leaves, fruit, or floral organs are shed. During these events, separation usually occurs in specialized, narrow bands of cells termed abscission zones (AZs), which form between the mother plant and each of the above mentioned organs [2]. Abscission of leaves, flowers, young, and mature fruit are predominantly controlled by contracting actions of ethylene acting as an inducer, and auxin (indole-3-acetic acid, IAA) acting as a suppressor. Abscisic acid (ABA) is involved, as ABA levels were shown to increase significantly prior to abscission [10]
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