Climate changes
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 it's effect on the fruit development of five olive cultivars placed in a region noted for it's 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.
Figure 1 - Dry fruit weight (continuous line) and dry fruit oil percentage (dashed line), of 'Barnea' and 'Koroneiki' fruits grown at the HT site, Tirat Zvi (red line) and the MT site, Tzuba (blue line), during the entire season are presented. The right Y axis is the dry fruit weight, while the left Y axis is the dry fruit oil percentage
Among the effects of high temperatures on chemical parameters characterized in the current study, were total polyphenols and the fatty acid profile of olive oil.Our analysis shows that a high temperature environment caused a decrease in the total polyphenol content of all analyzed cultivars.In accord with other studies on olive oil, in our study all cultivars showed a reduction in their oleic acid content in response to the high temperature environmentRegarding 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.
Figure 2 - Oil quality, represented by polyphenol a. and oleic acid b. levels in the oil extracted from the various cultivars in the two climatic regions, during both years.
We analyzed the transcriptome of two extreme cultivars, 'Barnea', which is tolerant to high temperatures in regard to quantity of oil production, but sensitive regarding it's quality, and 'Souri', which is heat sensitive regarding quantity of oil produced, but relatively tolerant regarding its quality. We focused on the genes involved in the oil biosynthesis pathway. We found that heat-shock protein expression was induced by the high temperature environment, but the degree of induction was cultivar dependent. The 'Barnea', whose oil production showed greater tolerance to high temperatures, exhibited a larger degree of induction than the heat sensitive 'Souri'. On the other hand, many genes involved in olive oil biosynthesis were found to be repressed as a response to high temperatures. OePDCT as well as OeFAD2 genes showed cultivar dependent expression patterns according to their heat tolerance characteristics. The transcription factors OeDof4.3, OeWRI1.1, OeDof4.4 and OeWRI1.2 were identified as key factors in regulating the oil biosynthesis pathway in response to heat stress, based on their co-expression characteristics with other genes involved in this pathway.
Oil biosynthesis
figure - Expression pattern of the genes involved in the olive oil biosynthesis pathway in fruit from the HT site compared to that of fruit grown in the MT environment. Expression levels for each gene are presented in a green to red scale above or beside the name of the enzyme. In each gene, the six left squares represent the expression pattern in the 'Souri' cultivar, whereas the six right squares represent the expression pattern in the 'Barnea' cultivar. Within each cultivar, the three left squares represent the expression level at 83, 104 and 146 DPA in the HT environment, whereas the three right squares represent the expression level at the same sample dates in the MT environment. The various cell components appear in different colors.
We characterized the development of flowers and inflorescences under natural conditions in the cultivars 'Arbequina' and 'Koroneiki', and defined eight phenological stages, beginning with the emergence of reproductive buds through anthesis.
Figure 3 - Stages of normal development of an inflorescence, flower and pollen grains of 'Koroneiki' cultivar. Flowers and inflorescences were photographed with binoculars. Histological sections of spores were stained with Safranin-Fast green and photographed with a light microscope. For each step, five repetitions were performed. a. Inflorescence development b. Flower development c. Sporogenic tissue in anther d. Interphase of first meiosis of mother cells e. Interphase of second meiosis f. Tetrads of microspores surrounded by callose g. Free microspores After callose breakdown h. development and coelom and polarization of the nucleus i. Pollen grains during first mitosis j. mature pollen grains.
We examined the response of olive inflorescence and flowering, to two heat scenarios: prolonged exposure to moderately high temperatures and a two-hours heat shock treatment. Analysis of our data revealed that both treatments had a detrimental effect on the development and functioning of the tapetum, the innermost layer of the anther, which is essential for the development and functioning of pollen.
Figure 4 - . Transverse sections of anther from stage VIII of the cultivars 'Koroneiki' and 'Arbequina', after acute and moderate heat stress treatments, compared to control anthers.
Viability of pollen grains was determined by a viability test, based on Alexander staining. The number of viable pollen grains after acute and moderate heat stress, compared to the control was calculated in the two cultivars 'Koroneiki' and 'Arbequina'. Significant interaction (P<0.0004) between the effect of the cultivar and the effect of heat stress on pollen viability was found. 'Koroneiki' pollen grains were found to be resistant to acute heat stress and their viability after acute stress was not different from the viability of the control pollen grains. However, after moderate heat stress, all pollen grains were non-viable. 'Arbequina' pollen grains were found to be sensitive to both moderate and acute heat stress, and both treatments caused a significant decrease in pollen viability. However, unlike 'Koroneiki', some viable pollen grains were found after moderate heat stress. Pollen grain structure with emphasis on the exine texture was examined by electronic microscopy and showed similar results to those of the Alexander stain. After acute heat stress 'Koroneiki' pollen grain structure was normal. However, many 'Arbequina' pollen grains were defective and exhibited non-symmetric structure. After moderate stress, both, 'Koroneiki' and 'Arbequina' pollen grains were found to be distorted
Figure 5 - 'Arbequina' and 'Koroneiki' pollen viability after acute and prolonged heat stress. a. Alexander's staining of pollen grains from 'Koroneiki' and 'Arbequina' cultivars after acute and moderate heat stress, with control of untreated pollen. b. SEM microscope photograph of pollen grains of the 'Koroniki' and 'Arbequina' cultivars,