Whole genome scanning of a Mediterranean basin hotspot collection provides new insights into olive tree biodiversity and biology.

Olive tree (Olea europaea L. subsp. europaea var. europaea) is one of the most important species of the Mediterranean region and one of the most ancient species domesticated. The availability of whole genome assemblies and annotations of olive tree cultivars and oleaster (O. europaea subsp. europaea var. sylvestris) has contributed to a better understanding of genetic and genomic differences between olive tree cultivars. However, compared to other plant species there is still a lack of genomic resources for olive tree populations that span the entire Mediterranean region. In the present study we developed the most complete genomic variation map and the most comprehensive catalog/resource of molecular variation to date for 89 olive tree genotypes originating from the entire Mediterranean basin, revealing the genetic diversity of this commercially significant crop tree and explaining the divergence/similarity among different variants. Additionally, the monumental ancient tree 'Throuba Naxos' was studied to characterize the potential origin or routes of olive tree domestication. Several candidate genes known to be associated with key agronomic traits, including olive oil quality and fruit yield, were uncovered by a selective sweep scan to be under selection pressure on all olive tree chromosomes. To further exploit the genomic and phenotypic resources obtained from the current work, genome-wide association analyses were performed for 23 morphological and two agronomic traits. Significant associations were detected for eight traits that provide valuable candidates for fruit tree breeding and for deeper understanding of olive tree biology.

Effect of biochar produced from sewage sludge on tomato (Solanum lycopersicum L.) growth, soil chemical properties and heavy metal concentrations.

The addition of biochar, as shown in the literature, improves significantly the chemical and physical soil properties and plant growth. This study examined the effect of biochar, compost and the combination of them on growth, nutrient and heavy metal concentrations of tomato. Biochar (BC) was produced from sewage sludge by pyrolysis at the temperature of 300 °C. The pot trials were carried out under an open-side greenhouse for a total of four months and under four treatments. The treatments applied were: Untreated soil (Control); soil with 2% w/w biochar (BC-SS); soil with 2% w/w compost (Compost); a mixture of biochar and compost at a 2% w/w level (BC-SS + Compost). The application of biochar exhibited substantial improvement on several soil properties. Total organic carbon (TOC) of soil increased (67%-85%), as did the nitrate nitrogen (55%) and ammonium nitrogen (145%). Additionally, available phosphorus significantly increased (45.5%-54.5%) by the application of biochar with/without compost. Dry weight of the aboveground (stems) and belowground (roots) plant tissues increased as well, although tomato yield was not increased significantly. Concentration of heavy metals and trace elements in tomato tissues was quite low. Traces of chromium (Cr), nickel (Ni), and cobalt (Co) were found only in roots of those treated, while silicon (Si) was present in the roots and stems. Arsenic (As), molybdenum (Mo) and lead (Pb) were detected in all plant tissues, but their concentrations did not exceed the permissible levels established for vegetables. Furthermore, the concentration of arsenic (As) and lead (Pb) in fruits decreased by the addition of the amendments (12%-65%). In conclusion, the addition of sewage sludge biochar improved soil characteristics and plant growth. Yet, prior to its general use, factors such as the type of biomass, soil, rate of application and crop must always be taken into consideration.

Agronomic potential of biochar prepared from brewery byproducts.

This work investigated the effect of biochar produced from brewery byproducts, spent grain and surplus yeast on the physicochemical characteristics of a calcareous loam soil and plant growth of maize (Zea mays L.). Maize is a plant which needs high nitrogen fertilization, while the effect of acidic or neutral biochars on alkaline calcareous soils has only been assessed in a few studies. The effect of biochars on dry weight, as well as the level of macro- and micronutrients in soil, and above- and belowground plant tissues, were investigated, in a 30 day experiment after seedling emergence of maize (Zea mays L.), in the presence and absence of nitrogen fertilization. The results indicated that biochar from organic brewery by-products significantly increased the dry weight of the aboveground part of the plant by 59-186%, relative to the control, without the addition of inorganic N fertilization, and by 46-157% with the addition of inorganic N fertilization. The dry weight of the belowground plant tissues significantly increased by 83-92% and 46-106%, relative to the control, with or without the addition of inorganic N fertilization, respectively. Biochar addition, especially at 5% application rate individually or in a mixture, significantly increased the phosphorus content of plant tissues. The content of potassium in the plants was affected mainly by the addition of biochar derived from surplus yeast, while the concentration of calcium and magnesium in plant tissues was positively affected by spent grain biochar, in absence of inorganic nitrogen fertilization. Addition of biochars produced from brewery byproducts improved soil fertility parameters, particularly the contents of total organic carbon (by 133% and 118% with or without fertilization, respectively), total nitrogen (by 120% and 81% respectively) and available phosphorus in the studied loam calcareous soil. Overall, biochar from brewery wastes showed the potential to enhance plant growth and nutrient availability, thus it is a promising organic fertilizer for sustainable agriculture.

The impact of biochars prepared from agricultural residues on phosphorus release and availability in two fertile soils.

Biochars have a high variability in chemical composition, which is influenced by pyrolysis conditions and type of biomass. Essential macronutrient P retained in biochar could be released and made available to plants, enhancing plant growth. This study was conducted in order to evaluate whether biochar, produced from agricultural residues, could release P in water, as well as study its potential effect on plant growth and P uptake. Biochar samples were prepared from rice husks, grape pomace and olive tree prunings by pyrolysis at 300 °C and 500 °C. These samples were used for P batch successive leaching experiments in order to determine P release in water. Subsequently, rice husk and grape pomace biochars, produced by pyrolysis at 300 °C, were applied to two temperate soils with highly different pH. A three-month cultivation period of ryegrass (Lolium perenne L.) was studied in threefold replication, while three harvests were accomplished. Treatments comprised control soils (without amendment) and soils amended only with biochar. Results of P leaching tests showed a continuous release of P from all biochars as compared to raw biomass samples, for which the highest P concentrations were detected during the first extraction. Grape pomace and rice husk biochars pyrolyzed at 500 °C showed higher levels of water-extractable P, as compared to their corresponding raw biomass. Biochars, at 500 °C, leached more P in all four extractions, compared to biochars at 300 °C, apart from olive tree prunings biochars, where both pyrolysis temperatures presented a similar trend. Concerning plant yield of ryegrass, rice husk and grape pomace biochars showed positive statistically significant effects on plant yield only in slightly acidic soil in second and third harvests. In terms of P uptake of ryegrass, grape pomace biochars depicted positive significant differences (P < 0.05) in third harvest, in slightly acidic soil, while in first and second harvests positive significant differences were observed in alkaline soil. These results suggest that biochars derived from agricultural residues may act as a source of P in agronomic applications and improve plant growth, although soil conditions may play a significant role.

Optimizing Agroecological Measures for Climate-Resilient Olive Farming in the Mediterranean.

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.

Introducing Three-Dimensional Scanning for Phenotyping of Olive Fruits Based on an Extensive Germplasm Survey.

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.