Global warming is predicted to have a negative effect on plant growth due to the damaging effect of high temperatures. In order to address the effect of high temperature environments on olive oil yield and quality, we compared its effect on the fruit development of five olive cultivars placed in a region noted for its high summer temperatures, with trees of the same cultivars placed in a region of relatively mild summers. We found that the effects of a high temperature environment are genotype dependent and in general, high temperatures during fruit development affected three important traits: fruit weight, oil concentration and oil quality. None of the tested cultivars exhibited complete heat stress tolerance. Final dry fruit weight at harvest of the 'Barnea' cultivar was not affected by the high temperature environment, whereas the 'Koroneiki', 'Coratina', 'Souri' and 'Picholine' cultivars exhibited decreased dry fruit weight at harvest in response to higher temperatures by 0.2, 1, 0.4 and 0.2 g respectively. The pattern of final oil concentration was also cultivar dependent, 'Barnea', 'Coratina' and 'Picholine' not being affected by the high temperature environment, whereas the 'Koroneiki' and 'Souri' cultivars showed a decreased dry fruit oil concentration at harvest under the same conditions by 15 and 8% respectively. Regarding the quality of oil produced, the 'Souri' cultivar proved more tolerant to a high temperature environment than any other of the cultivars analyzed in this study. These results suggest that different olive cultivars have developed a variety of mechanisms in dealing with high temperatures. Elucidation of the mechanism of each of these responses may open the way to development of a variety of olives broadly adapted to conditions of high temperatures.

Plants vary in their stomatal sensitivity in response to water stress. Some plants, termed isohydric, keep their water potential constant by rapid stomatal closure. In contrast, anisohydric plants close their stomata only when the plant water potential decreases dramatically. The distinctions between isohydric and anisohydric strategies among different cultivars of the same species are often unclear. This raises the question, whether these modes of behavior can be considered two distinct strategies? In our study, we compared these strategies using two wine grape cultivars: Grenache, which known to be near-isohydric and the anisohydric cultivar Shiraz. The plants were exposed to a prolonged period of water deficit, and their physiological responses were recorded. When water potential was measured in the stem tissue of the two cultivars at mid-day, and in leaf tissue at pre-dawn, a consistently lower value of stomatal conductance was found in the ‘Grenache’ plants compared to the ‘Shiraz’. The ‘Shiraz’ plants exhibited a more vigorous response to water deficit, demonstrating vegetative growth and lower defoliation compared to the ‘Grenache’ plants. An analysis of the anatomical architecture of the two cultivars revealed that ‘Grenache’ plants have a larger xylem vessel diameter, higher hydraulic conductivity and a higher density of stomata than the ‘Shiraz’ plants. A summary of our results and those of other investigators suggests that isohydric and anisohydric strategies may not be a result of distinct mechanisms, but rather of a well defined time-regulated response. Plants defined as isohydric may be in greater danger of developing a possibly fatal xylem embolism, compared to anisohydric plants, and therefore their response to water deficit must be rapid. This accelerated response results in “avoiding” the drought stress, but lowers the plant’s ability to survive the moderate stress of prolonged drought. However, this is their only option to minimize the risk of xylem cavitation.

Table 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. The 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. Our 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.