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The paper focuses on the seasonal oil accumulation in traditional and super-high density (SHD) olive plantations and its modelling employing image-based linear models. For these purposes, at 7-10-day intervals, fruit samples (cultivar Arbequina, Fasola, Frantoio, Koroneiki, Leccino, Maiatica) were pictured and images segmented to extract the Red (R), Green (G), and Blue (B) mean pixel values which were re-arranged in 35 RGB-derived colorimetric indexes (CIs). After imaging, the samples were crushed and oil concentration was determined (NIR). The analysis of the correlation between oil and CIs revealed a differential hysteretic behavior depending on the covariates (CI and cultivar). The hysteresis area (Hyst) was then quantified and used to rank the CIs under the hypothesis that CIs with the maximum or minimum Hyst had the highest correlation coefficient and were the most suitable predictors within a general linear model. The results show that the predictors selected according to Hyst-based criteria had high accuracy as determined using a Global Performance Indicator (GPI) accounting for various performance metrics (R2, RSME, MAE). The use of a general linear model here presented is a new computational option integrating current methods mostly based on artificial neural networks. RGB-based image phenotyping can effectively predict key quality traits in olive fruit supporting the transition of the olive sector towards a digital agriculture domain.

Considering climate change predictions, it is logical to anticipate detrimental effects on the mandarin tree, an essential citrus crop. Therefore, scientists should promptly focus on developing methods to enhance its resistance to climatic stress effects such as sunscald. This study assesses the strategies employed in ‘Balady’ mandarin trees when covered by shading nets of varying colors and percentages (white 50%, green 50% or 63%, black 50% or 63%), as well as the application of reflective materials (kaolin at 4% and CaCO3 at 3%) on the micro-climate of orchards, leaf, and fruit surface temperatures, fruit sunburn%, productivity, and fruit quality. The results indicated that shade nets effectively reduced temperature and enhanced humidity, especially during the period from June to September, when compared to open-field treatments. Black shade nets, particularly those with a shading level of 63%, demonstrated the most notable decrease in canopy temperature and an elevation in humidity, surpassing the performance of green and white shade nets. The present study found that shade nets and reflecting materials like kaolin and calcium carbonate significantly reduced fruit sunburn. Trees without shade had a sunburn rate of 8.74%, while those with shade treatments suffered no sunburn. Kaolin foliar spray at a concentration of 4% and calcium carbonate at a concentration of 3% reduced sunburn incidence to 3.64% and 7.32%, respectively. These treatments also reduced the intensity of sunburn. All treatments increased fruit yield % compared to the control and yield efficiency (kg/m2), especially the trees covered with white shade net of a 50% shading rate provided the highest values (43.70 and 40.17%) and (5.24 and 5.47 kg/m2) compared to other treatments in both seasons, respectively. Trees covered with a white shade net of a 50% shading rate, followed by a green shade net of 50% and a 63% shading rate, as well as a black shade net of 50% and a 63% shading rate, tended to improve the physical and chemical fruit properties. Therefore, it could be recommended that trees be covered with a white shade net of a 50% shading rate or a green shade net of a 50 and 63% shading rate in summer months due to its beneficial impact on mitigating fruit sunburn damage and enhancing the productivity and quality of ‘‘Balady’’ mandarin trees. Hence, shade nets can be a beneficial technology to protect citrus fruits from sunburn without affecting fruit quality in commercial citrus farms.

Olive growing is undergoing a transition from traditional cultivation systems to a more technological model characterized by increased mechanization and a higher density of plants per hectare. This shift implies the use of less vigorous varieties that can adapt to the new system. Most traditional varieties are highly vigorous, and breeding programs can provide solutions to this challenge. This study investigates the parental effect on different agronomic and olive oil characteristics and its role in breeding programs. The objectives were to evaluate and characterize different agronomic and olive oil traits in the progenies from ‘Arbosana’ × ‘Sikitita’ cross and its reciprocal cross ‘Sikitita’ × ‘Arbosana’. The results showed a high variability of the characters evaluated in the progenitors of the reciprocal crosses. The highest coefficients of variation were observed in traits related to ripening index, phenolic compounds, polyunsaturated fatty acids, and Δ5-avenasterol, with phenolic content exhibiting the greatest variability. No statistically significant maternal effect was detected for any of the evaluated traits, although a slight positive maternal effect was systematically observed in the mean values of the evaluated traits. These results suggest that the maternal effect on olive is quite subtle, although due to a slight tendency of the maternal effect in the descriptive analyses, future studies are suggested to understand in depth the possible maternal effect on olive breeding.

Climatic changes and global warming affect the growth, development, and productivity of crops. In this review, we highlight the possible benefits of using innovative breeding techniques like clustered regularly interspaced short palindromic repeats (CRISPRs), exogenous phytohormone-like strigolactones (SLs), nanomaterials (NMs), and beneficial microbial endophytes to address the challenges in sustainable cultivation of horticultural crops. These applications are evaluated by examining how they affect different metabolic, morphological, and biochemical parameters in diverse crops. Endophytes are symbiotic microorganisms and can be used as nematicides for improving crop yield. With an emphasis on quality control, we examined the impacts of applying NMs, a novel family of phytohormones called SLs, and microbial endophytes on horticultural commodities. Furthermore, we reviewed the benefits of CRISPR for the editing of plant genomes, as well as how it affects gene expression and transcription factors to increase crop tolerance and yield. These innovations hold the potential to improve crop yield, quality, and resilience by acting as safe, natural components in biofertilizers and plant protection solutions. Gradually adopting these methods could decrease reliance on agrochemicals, thereby reducing their negative effects on biodiversity, soil fertility, and human health.

Plants are an essential component of our daily diet, and their nutritional value has been thoroughly studied for many years. The ability of plants to adapt to changing environmental conditions through signaling systems is an essential component of their survival. Plants undergo an array of physiological alterations to respond to stress from biotic sources. Secondary compounds frequently accumulate in crops that are sensitive to stress, particularly those with several eliciting agents or signaling molecules. Plants contain various types of bioactive compounds, including phytosterols, alkaloids, glycosides, and polyphenols, which make them valuable for the food and pharmaceutical industries. The increased production of secondary metabolites via elicitation has opened up a new field of study with the potential to provide substantial financial gains for the pharmaceutical and nutraceutical industries. These elicitors are pharmacological compounds that activate specific transcription factors and up-regulate genes to activate metabolic pathways. Thus, the current review discusses the mechanism of biotic elicitation and various elicitation techniques using biotic (proteins, carbohydrates, rhizobacteria, fungi, and hormones) elicitors that may increase the yield of secondary metabolites, particularly in medicinal plants, which is advantageous to the agrochemical and therapeutic industries.

The genetic relationships between Greek wild olive tree populations and cultivars were investigated. A total of 219 wild genotypes and 67 cultivar genotypes were analyzed by employing 10 SSR markers. Data evidenced that the wild populations exhibited high levels of genetic diversity and exclusively host 40% of the total number of alleles detected. Inbreeding was observed within populations, probably as a consequence of their fragmented spatial distribution. The genetic differentiation between cultivars and wild individuals, as well as within wild populations, was low. Nevertheless, three gene pools of wild trees were detected, corresponding to the geographical areas of Northeastern Greece, Peloponnese-Crete and Epirus. Most cultivars clustered in a separate group, while the rest of them formed a heterogenous group with membership coefficients akin to the three wild olive clusters. Regarding the history of olive cultivation in Greece, bidirectional gene flow was detected between populations of Peloponnese-Crete and the gene pool that composes some of Greece’s most important cultivars, such as "Koroneiki” and “Mastoidis”, which is inferred as an indication of a minor domestication event in the area. A strategy for the protection of Greek-oriented olive genetic resources is proposed, along with suggestions for the utilization of the genetically diverse wild resources with regard to the introgression of traits of agronomical interest to cultivars.

In order to evaluate the potential of climate change mitigation measures on soil physiochemical properties, an experiment based on the application of five agroecological practices such as the addition of composted olive-mill wastes, recycling pruning residue, cover crops, organic insect manure, and reduced soil tillage, solely or combined, was conducted over two years (2020 to 2022) in a 48-year-old olive plantation. The results showed significant increases in soil water content during the spring and summer periods for the combined treatment (compost + pruning residue + cover crops) (ALL) compared to the control (CONT) by 41.6% and 51.3%, respectively. Also, ALL expressed the highest soil organic matter (4.33%) compared to CONT (1.65%) at 0–10 cm soil depth. When comparing soil nutrient contents, ALL (37.86 mg kg−1) and cover crops (COVER) (37.21 mg kg−1) had significant increases in soil nitrate compared to CONT (22.90 mg kg−1), the lowest one. Concerning exchangeable potassium, ALL (169.7 mg kg−1) and compost (COMP) (168.7 mg kg−1) were higher than CONT (117.93 mg kg−1) at the 0–10 cm soil depth and had, respectively an increase of 100.9% and 60.7% in calcium content compared to CONT. Over the experimental period, the implementation of the five agroecological management practices resulted in enhanced soil fertility. In a long-term Mediterranean context, this study suggests that these sustainable practices would significantly benefit farmers by improving agroecosystem services, reducing reliance on synthetic fertilizers, optimizing irrigation water use, and ultimately contributing towards a circular economy.

Intensive agricultural management practices (i.e., the burning of pruning residues, the absence of organic amendments) lead to a reduction in organic matter and nutrients in the soil resulting in agroecosystem vulnerability. Implementing a cover crop would provide soil organic matter while increasing nutrition levels in the soil. A mixture of cover crop trial in sandy loam soils under Mediterranean climatic conditions was conducted in a rainfed olive grove in Western Crete. In this study, the dry biomass, macro- and micronutrition, and carbon status of the seeded cover crops (legume and grass) were compared to natural plants in an olive grove. Seeded cover crops were conducted in two sustainable management systems (cover crops solely, and a combination of compost, pruning residues, and cover crops); natural plants were in a conventional system involving soil tillage. In combination with conservation tillage practices, the addition of carbon inputs may improve soil fertility. Results indicate that the dry biomass production and C content of cover crops under sustainable management systems was significantly higher than that of the control. The higher dry biomass production and C content found in cover crops compared to the natural vegetation indicates not only that this type of management provides enhanced carbon storage, but can also potentially lead to a future increase in soil organic matter through decomposition. Higher dry biomass is important in the context of carbon sequestration, and cover crops facilitated carbon storage in this study. In addition, this study suggests that sustainable agricultural management practices would provide significant benefits in terms of nutrient retention and CO2 fixation, thus improving ecosystems in Mediterranean countries.

Leaf analysis is essential for diagnosing nutritional status and guiding fertilizer application. The present study aimed to investigate the appropriate time for leaf sampling and the effect of genotype on olive nutrition. We determined leaf nutrient concentrations of calcium (Ca), iron (Fe), and manganese (Mn) in five Greek (‘Amfissis’, ‘Chondrolia Chalkidikis’, ‘Koroneiki’, ‘Mastoidis’, and ‘Kalamon’) and one Spanish (‘Picual’) variety from May 2019 to April 2020. The concentrations of Ca, Fe, and Mn were significantly affected by genotype and season. The highest concentrations for all nutrients were determined in April, while the lowest were in May, June, and October. Leaf Ca concentration significantly increased progressively from May to September for all the varieties. Leaves of ‘Koroneiki’ had the highest Ca concentration. Iron concentrations were within the sufficiency thresholds for all the varieties during the whole experimental period, and ‘Mastoidis’ showed the highest concentration. Leaf Mn concentration for all the varieties increased from May to September with an instant decrease in June, apart from ‘Amfissis’. The varieties ‘Kalamon’ and ‘Chondrolia Chalkidikis’ were found to be above the Mn sufficiency threshold throughout the year. Variations among season and genotype depict the complexity of nutrient transportation in leaf tissues.

The olive tree (Olea europaea L.) is the main fruit tree in most of the arid and semi-arid regions of Tunisia, which is where the problem of salinity is more pronounced. Salinity is one of the main factors that affects the productivity of olive trees, so the objective of this experiment was to study the effects of salinity on the photosynthesis, water relations, mineral status, and enzymatic activity of two cultivars of Olea europaea L., ‘Chemlali’ and ‘Koroneiki’. The trial was conducted under controlled conditions in a greenhouse for a period of 49 days and included two treatments: T0 control and T100 (irrigation with 100 mM of NaCl solution). Under salinity stress, the photosynthesis, stomatal conductance, and leaves of both cultivars were negatively affected. ‘Chemlali’ showed greater tolerance to NaCl salinity, based on a progressive decrease in osmotic potential (Ψπ) followed by a progressive and synchronous decrease in gs, without a comparable decrease in photosynthesis. The water use efficiency (WUE) improved as a result. In addition, the K+/Na+ ratio in ‘Chemlali’ rose. This appears to be crucial for managing stress. Conversely, enzymatic activity showed an accumulation of glutathione peroxidase (GPX) in stressed plants. The catalase (CAT) and ascorbate peroxidase (APX) content decreased in both stressed varieties. It can be concluded that the cultivar ‘Koroneiki’ is more susceptible to salt stress than the cultivar ‘Chemlali’, because the accumulation of GPX and the decreases in CAT and APX were more pronounced in this cultivar.

The intensity of ultraviolet (UV) radiation affects the yield and quality of red lettuce. The current study aimed to develop a UV management system in a greenhouse to achieve high yield and quality in red lettuce production. The study consisted of two experiments. In the first experiment, the effects of the different UV transparencies of the plastic materials covering the greenhouse on plant growth and the concentration of antioxidants in red lettuce were studied. For this purpose, two greenhouses were covered with polyethene of different transparencies to UV radiation. One greenhouse was covered with a common type of polyethene transparent in a large spectrum of UV radiation (UV-open), while the second greenhouse was covered with polyethene untransparent to ultraviolet radiation (UV-block). The plants were grown in a deep flotation hydroponic system. At the end of the cultivation, plant growth measurements, leaf colour measurements, and the determination of antioxidant components’ concentration were carried out. Red lettuce plants harvested 42 days after planting had an average head weight 42% greater in the UV-block greenhouse compared to plants grown in the UV-open greenhouse. However, the red leaf colour of plants in the UV-block greenhouse lagged significantly compared to that in the UV-open greenhouse. Moreover, the total phenolic content, the total flavonoid content, and the antioxidant capacity of the lettuce leaves in the UV-block greenhouse were significantly lower compared to the corresponding values of the plants in the UV-open greenhouse. During the second experiment, a new cultivation system of red lettuce, which combined a UV-block polyethene film as a greenhouse cover and a pre-harvested supplemental UV light, was tested. For this purpose, various doses of supplemental UV lighting were tested in the UV-block greenhouse for ten days prior to harvest. From these tests, it emerged that applying supplemental UV lighting with a dose of 425 kJ m−2 d−1 for ten days before harvest produces red lettuces of the same quality as those produced in a UV-open greenhouse. This technique of growing red lettuce increases its yield by 30% without a negative effect on the quality of the product.

Morphological characterization of olive (Olea europaea L.) varieties to detect desirable traits has been based on the training of expert panels and implementation of laborious multiyear measurements with limitations in accuracy and throughput of measurements. The present study compares two- and three-dimensional imaging systems for phenotyping a large dataset of 50 olive varieties maintained in the National Germplasm Depository of Greece, employing this technology for the first time in olive fruit and endocarps. The olive varieties employed for the present study exhibited high phenotypic variation, particularly for the endocarp shadow area, which ranged from 0.17–3.34 cm2 as evaluated via 2D and 0.32–2.59 cm2 as determined by 3D scanning. We found significant positive correlations (p < 0.001) between the two methods for eight quantitative morphological traits using the Pearson correlation coefficient. The highest correlation between the two methods was detected for the endocarp length (r = 1) and width (r = 1) followed by the fruit length (r = 0.9865), mucro length (r = 0.9631), fruit shadow area (r = 0.9573), fruit width (r = 0.9480), nipple length (r = 0.9441), and endocarp area (r = 0.9184). The present study unraveled novel morphological indicators of olive fruits and endocarps such as volume, total area, up- and down-skin area, and center of gravity using 3D scanning. The highest volume and area regarding both endocarp and fruit were observed for ‘Gaidourelia’. This methodology could be integrated into existing olive breeding programs, especially when the speed of scanning increases. Another potential future application could be assessing olive fruit quality on the trees or in the processing facilities.

Although Greece is the world’s third largest olive production country, information about the presence of olive viruses is limited. A survey for the presence of virus infections in the ten most important Greek cultivars was conducted in a germplasm collection olive grove located in Chania, Crete. Samples were RT-PCR assayed for the presence of Arabis mosaic virus (ArMV), Cherry leafroll virus (CLRV), Strawberry latent ring spot virus (SLRSV), and Olive leaf yellowing-associated virus (OLYaV), amplifying part of the capsid protein (ArMV), the 3΄UTR (CLSRV, SLRSV) or the HSP70h (OLYaV) gene. Total RNAs were purified using the Trizol method, yielding good quality and purity, thereby confirming application of the method as a rapid economic extraction protocol for detection of olive viruses. SLRSV was the most predominant virus, with an infection rate of 55%, followed by CLRV and OLYaV in 5% of the tested samples. ArMV was detected only in one sample. Mixed virus infections were also commonly detected. The DNA amplicons of the obtained viruses from the infected samples were sequenced. The partial sequences of ArMV, CLRV and SLRSV from olives, which are reported for the first time, showed 74-100% nucleotide similarity with available homologous sequences from other crops, whereas OLYaV isolates showed high sequence variability of 25%. The phylogenetic analysis based on olive-OLYaV HSP70h partial-nucleotide sequences grouped the olive isolate sequences according to the geographical origins of the host germplasm collection. This is the first official report of the occurrence of olive viruses in Greece, emphasizing the need to implement a certification programme for production and distribution of high-quality (virus-free) olive propagation material, in Greece and more generally in the Mediterranean basin.

The olive tree of Vouves in Crete, is considered the oldest producing olive tree in the world with an estimated age exceeding 4000 years. In the present study, we sequenced two samples (from the bottom and the top of the tree) to elucidate the genetic relation of this ancient tree with other olive cvs as well as to gain some insights about its origin. Our results showed that both samples have different genetic origins, proving that this ancient tree has been grafted at least one time. On the basis of whole genome sequences the sample from the top of the Vouves tree showed relation of the same order than half-siblings to one accession corresponding to the present-day Greek cv ‘Mastoidis’. Nevertheless, in the framework of a microsatellite analysis it was found to cluster with the ‘Mastoidis’ samples. The Vouves rootstock (bottom sample) showed a clear grouping with the oleaster samples in a similar way to that of ‘Megaritiki’ Greek cv although it does not show any signal of introgression from them. The genomic analyses did not show a strong relation of this sample with the present-day Greek cvs analyzed in this study so it cannot be proved that it has been used as a source for cultivated olive tree populations represented by available genome sequences. Nevertheless, on the basis of microsatellite analyses, the Vouves rootstock showed affinity with two present-day Greek cvs, one “ancient” rootstock from continental Greece as well as monumental trees from Cyprus. The analysis of the impact of the variants in the gene space revealed an enrichment of genes associated to pathways related with carbohydrate and amino acid metabolism. This is in agreement with what has been found before in the sweep regions related with the process of domestication. The absence of oleaster gene flow, its old age and its variant profile, similar to other cultivated populations, makes it an excellent reference point for domestication studies.

The ‘pollen test’ and ‘fruit set test’ following controlled crossing combinations of parents are the most commonly used methods for pollination incompatibility studies in Olea europaea L. Self-incompatibility (SI), with diagnoses based on the pollen test and pollen germination, indicating self-compatibility, is not always followed by fruit set in this species. To solve this dispute, we have reconciled all observations into a new model. Mismatches between field and laboratory data and between methods are resolved by the dual-successive-screen model (DSSM) supposing two different loci for the expression of the two SI mechanisms. Pollen/stigma is controlled by diallelic SI, or DSI, inferring two G1 and G2 compatibility/incompatibility (C/I) groups for varieties, whereas pollen tubes in ovaries are controlled by poly-allelic SI or PASI with twenty C/I groups. To explain the selfing of varieties, we have suggested that some determinants in the pollen tube and stigma are unstable and degrade (DS-D for degradation of S-determinant) after three to five days, enabling some pollen tubes to avoid being rejected, hence reaching ovules. DSI and PASI in the DSSM and DS-D mechanisms, plus the andromonoecy of the olive tree, complexify SI studies. Inferences from DSSM and DS-D mechanisms in olive orchard practice are detailed.