Document Type : Full Paper

Authors

• Sakine Faraji ¹
• Soraya Karami ²

¹ Crop and Horticultural Science Research Department, Markazi Agricultural and Natural Resources Research and Education Center, Agricultural Research, Education and Extension Organization (AREEO), Arak, Iran

² Department of Agriculture, Payame Noor University (PNU), Tehran, Iran

Abstract

Keywords

• Anthocyanin biosynthesis
• Heat stress
• Polyphenolic compounds
• Pomegranate orchards

Main Subjects

• Fruit Science

Extended Abstract

Introduction

Secondary metabolites in some plants such as pomegranate generally influence fruit quality (taste, smell, and color), marketability, and therapeutic properties. Pomegranate (Punica granatum L.) plays a remarkable role in food and pharmaceutical industries, due to its substantial antioxidant activity and being rich of secondary metabolites (especially anthocyanin). In the last decade, the emerging of aril-paleness disorder has widely been reported as a novel and prevalent disorder, leading to a reduction in the quality of pomegranate fruit. Therefore, in order to diminish the delirious effect of aril-paleness disorder on pomegranate, this study aimed to identify the most sensitive growth stage (s) of pomegranate fruit to heat stress at which the aril-paleness disorder emerge as well as to assess the substantial changes occurred simultaneously in the content of secondary metabolites and activity of phenylalanine amonialyase (PAL) enzyme during subjecting the plants to two probable and improbable conditions for the occurrence of aril paleness.

Materials and Methods

This study was carried out during two years (2022 and 2023). Based on the factors affecting the emergence of pomegranate aril paleness, such as temperature, irrigation water, and soil salinity, two orchards named 19 and 17 with high and low magnitude of aril paleness, respectively, were selected in the first year. Regarding the value of average temperature during fruit maturation (25-27 ^oC for the orchard 19 and > 38 ^oC for the orchard 17), 30 trees were selected and labeled carefully. At the end of September (2022), the percentage of fruit paleness in each tree was calculated. In the second year (i.e., 2023), ten trees were chosen out of 30 trees selected in the first year, and their fruit (three fruits per tree) was harvested at four stages of growth as follows: stage 1 (fruit set), stage 2 (Hazelnut size), stage 3 (fast fruit growth), and stage 4 (full ripening), and then some phytochemical characteristics were measured, including the content of total phenolic compounds (TP), total flavonoids (TF), and anthocyanins (Cyd) , as well as PAL activity.

Results and Discussion

The results showed a regularly downward status in the changes of TP content after the hazelnut size until the end of the fruit growth under both conditions, although this trend was remarkably severe in probable conditions rather than improbable conditions for the aril paleness occurrence. The change in the content of total flavonoid (TF) in improbable conditions was different, in a way that it initially decreased (from stage 1 to 3) followed by an increasing in TF (from stage 3 to 4), while in probable conditions its change was regularly downward from fruit set to the end of fruit growth and development. Overall, the lowest TF was recorded at stage 4. The content of cyanidin (Cyd) increased from stage 1 to stage 3 in both conditions. However, after the fast fruit growth stage (stage 3), a significant change in Cyd content was observed in both improbable (increasing trend) and probable (decreasing trend) conditions for the occurrence of paleness disorder. Under both condition and up to stage 3, the PAL activity and Cyd content were mounted simultaneously, although the activity of PAL decreased after the fast fruit growth stage. 

Conclusion

    Our findings showed that under improbable conditions for the occurrence of aril paleness, the highest changes in secondary metabolites and most sensitive stage of fruit growth to aril-paleness disorder occurred at the stage of fruit hazelnut size to full ripening. In order to mitigate the harmful effect of heat stress on pomegranate, some issues should be taken into account during establishing its orchard: choosing land with an appropriate height (> 1000 m above sea), possessing a suitable slope for obtaining maximum shade (Northern and Eastern slopes), choosing a suitable tree-planting pattern, especially in tropical and subtropical areas (square planting pattern), using light mulches, maintaining annual weeds, and using light-colored shades. Additionally, the simultaneous changes in PAL activity and anthocyanin content at the first three stages of fruit growth suggest the role of PAL in the synthesis of anthocyanin. However, regarding separately changes in PAL activity and anthocyanin content (under improbable conditions) as well as a reduction in PAL activity from stage 3 to the end of fruit growth under both probable and improbable conditions substantiate the role of other flavonoid-synthesizing enzymes, beside PAL enzyme, in the synthesis of anthocyanin under heat stress.