The impact of irrigation water quantity and quality on olive fruit density throughout the growth period

Introduction:As it has been for several years, drought conditions in various regions of Iran, including the central part of Marvdasht city in Fars province, have been very noticeable and have led to a severe decrease in the quantity and quality of groundwater, resulting in severe restrictions on agricultural production. Therefore, the farmers in the region have spontaneously turned to planting drought-resistant trees such as olives and pistachios. On the other hand, due to the significant reduction in the quantity and quality of groundwater in the study area, the quantitative and qualitative parameters of the fruit of the region's agricultural products, especially olive trees, which are irrigated under these conditions, are effective. It can be said that the quality parameters of olives increase with low irrigation and high salinity (to a tolerable extent). On the other hand, water and salinity stresses will have different effects on the performance of the mentioned plants. In this region, olives are used in two forms, canned and oil production. The amount of olive oil is often dependent on the growing conditions and fruit ripeness, and how the oil accumulates. The amount of oil in the dry matter of the fruit during the fruit ripening period (second fruit growth period) and its accumulation increases until the onset of dark purple. The quantity and quality of irrigation water affect the time and amount of oil accumulation, as well as the quality of olive oil. Also, the shelf life of olives in canned conditions and the shelf life of the aroma and taste of extra virgin olive oil are important features that will be controlled and examined by studying the density of olives during the fruit growth period. Therefore, the main purpose of this study is to investigate the changes in the density of olives during the fruit growth period under conditions of low irrigation and salinity stress.Methods:The study area is located in the central part of Marvdasht city in Fars province, Iran, at geographical coordinates '693196;3303288' with an elevation of 1620 meters above sea level. According to the De Martonne method, the climate of the region is semi-arid and the dominant wind direction is eastern. Two years (2020-2021) of field experiments were conducted to study olive fruit density in a 20-year-old olive orchard (Roghani variety). The experiment utilized a factorial layout in a randomized complete blocks design with three replications. Five irrigation levels (I1-I5) were tested, ranging from 25% to 125% of olive water requirement. Three saline water levels (S1-S3) were also included, with S1 being saline well water with an electrical conductivity of 3.5-7.5 dS/m, S2 being a combination of 50% well water and fresh water, and S3 being fresh water. The weight and volume characteristics of olive fruits were examined and measured at seven different periods, starting from the fruit's appearance in June until its harvest in December for oil production. The I5S3 represented the control treatment.Results:Despite the fact that poor quality and quantity of irrigation water (S1, I1, and I2) will result in a decrease in the weight and volume of olive fruit, it is beneficial for improving the quality of suitable oil and will have an impact on the density of olive fruit. Oil production in olive fruit, which occurs during the second period of fruit growth, has a significant role in the density of the fruit. If the volume of fruit does not change (in the case of low-quality and low-quantity irrigation water), due to the increase in oil in a specific volume, it will lead to an increase in fruit weight and consequently an increase in olive density. The increase in density during the first growth period is accompanied by the production of dry matter in the fruit. A decreasing trend in the density of olive fruit can be observed in all irrigation treatments and different salinity levels in both consecutive years. The study found that normal irrigation water (I4 and I5) and appropriate quality (S2 and S3) increase the weight and volume of olive fruit and alter the density of olive fruit. The interaction effect of salinity and irrigation on the density of olive fruit was significant (5%) in both years. In 2020, changes in fruit density during the second period of fruit growth at salinity level (S1) were more pronounced in different irrigation treatments from I2 to I5 compared to other salinity levels (S2 and S3). The highest and lowest density values in 2020 were I2 and I3, respectively. The highest fruit density (1.1356 gr/cm3) was observed in the high salinity level (S1) and low irrigation treatment (I2). In 2021, the highest fruit density (1.1105 gr/cm3) in the first period of growth is related to S1, and in the second period of growth, it decreases compared to S2 and S3. In the same year, I1S3 had the highest value (1.0753 gr/cm3) and I5S3 had the lowest value (0.9899 gr/cm3) in the second stage of olive fruit growth. Although low quality and quantity of irrigation water (S1, I1 and I2) reduces the weight and volume of olive fruit, it is suitable for improving the quality of oil and will affect the density of olive fruit.