The Effect of Foliar Spraying of Iron, Zinc and Calcium in the Stages of Fruit Maturity on the Incidence and Severity of Pomegranate (Punica granatum L.) Aril Paleness

Introduction Mineral nutrients (macro and micro) and polyphenolic compounds are natural components of many fruits and play an important role in maintaining the quality and nutritional value of the fruit. Therefore, optimal management of plant nutrition in order to increase the quantity and quality of the product and improve the synthesis of secondary metabolites, especially during fruit growth, is necessary and unavoidable. Nevertheless, in the last decade, the occurrence of pomegranate (Punica granatum L.) aril paleness has been reported as a new and pervasive factor in reducing the quality of pomegranate fruit in many countries, including Iran. The present research was conducted with the aim of comparing the mineral concentrations (iron, zinc, and calcium) and biochemical characteristics (anthocyanin content) in pomegranates affected by aril paleness and healthy pomegranates of the ‘Malase Saveh’cultivar (2022). Additionally, the study aimed to investigate the effect of foliar application of the mentioned mineral elements on the incidence and severity of the aril paleness condition (2023).   Materials and Methods This study was carried out during two years (2022 and 2023) and two independent trials. First, based on the introduced factors affecting the occurrence of pomegranate paleness (temperature, irrigation water and soil salinity), two orchards with medium and high percentage of pomegranate aril paleness (orchard number 9 and 17, respectively) were selected based on the results of Faraji & Karami (2024a). The first trial in 2022 (orchard No. 9): At harvest and after splitting fruits, twenty healthy fruits and affected fruits by the aril paleness disorder were randomly selected and were used for determination of mineral nutrient (Fe, Zn and Ca) and anthocyanin content in the laboratory. The second trial in 2023 (orchard No. 17): In the first phase, 70 trees were selected and labeled, then at the end of September (2022), percent and severity aril paleness of each tree was calculated. In the second phase (2023), based on the results of the first year, 54 trees as experimental unit (with aril paleness percentage>85 and paleness severity of high/very high) were selected from previous trees. Afterwards, a factorial experiment (3×3×2) based on a randomized complete block design with three replications and two stages (the beginning of fruit set and the beginning of fruit ripening) were implemented. Experimental treatments included foliar spraying with three concentrations of iron sulfate (0, 2 and 4 per thousand respectively F0, F2 and F4), three concentrations of zinc sulfate (0, 3 and 6 per thousand respectively Zn0, Zn3 and Zn6) and two concentrations of calcium chloride (0 and 4 per thousand respectively Ca0 and Ca4). Then at harvest, the percentage and severity of aril paleness each treatment was calculated.     Results and Discussion Data analysis using the independent t-test (α=0.05, df=38) for the first trial (2022) showed that, the nutritional value of the affected fruit by aril paleness is anticipated to be far less than that of the healthy fruit; so that except for the Zn concentration, the concentration of Fe, Ca and anthocyanin content in affected fruits were lower than in healthy fruits. The results of the variance analysis for the second trial (2023) indicated that foliar spraying with iron sulfate, zinc sulfate, and calcium chloride, either individually or in combination, was effective in reducing the occurrence and severity of aril paleness. A comparison of the mean percentage of aril paleness between the two groups (before and after foliar spraying, regardless of the treatment combination) using a dependent t-test (α=0.05, df=16) also showed that foliar spraying was effective in reducing the percentage of aril paleness. Furthermore, the involvement of calcium in reducing the incidence and severity of aril paleness was found to be significant. In the absence of calcium element, the incidence of paleness was recorded in the range of 60.17-75.00%, and with the inclusion of calcium in the treatment compounds, the percentage of paleness was recorded in the range of 21.83-0.53%. Also, the mean comparison of the double interaction and main effects of elements indicated the synergistic effect of calcium element with Fe and Zn element in reducing the aril paleness disorder. So that, the combination of Fe-Ca and Zn-Ca has been more successful in reducing the aril paleness disorder than the pure application of each element of iron, zinc and calcium.   Conclusions Overall, the simultaneous application of Zn-Fe in combination with Ca was more effective in reducing of the mentioned traits than other treatments. Therefore, in order to simultaneously reduce the occurrence and severity of aril paleness, spraying with Fe4Zn6Ca4 and Fe4Zn3Ca4 is recommended in two stages of pomegranate fruit development, including the beginning of fruit set and the beginning of fruit ripening.