Phenolic composition and antioxidant capacity of pistachio seed coats at different tree ages under saline irrigation conditions

Authors: CHIRENJE, TAKUDZWA - California State University; CHAVEZ, REBECCA - California State University; RIJAL, SANDHYA - California State University; Arroyo, Irvin; Banuelos, Gary; SOMMERHALTER, MONIKA - California State University

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2025
Publication Date: 12/7/2025
Citation: Chirenje, T., Chavez, R., Rijal, S., Arroyo, I.S., Banuelos, G.S., Sommerhalter, M. 2025. Phenolic composition and antioxidant capacity of pistachio seed coats at different tree ages under saline irrigation conditions. Agronomy. 15(12). Article 2816. https://doi.org/10.3390/agronomy15122816.
DOI: https://doi.org/10.3390/agronomy15122816

Interpretive Summary: With recurring droughts, water becomes a limiting resource for irrigated agriculture in California. Consequently, alternative water sources of varying quality must be considered to sustain irrigated agriculture with selected crops. In this regard, the San Joaquin Valley is one of the most irrigated, productive agricultural regions in the world, but it is also one of the most vulnerable regions to drought, soil salinity, and water quality problems. To sustain irrigated agriculture in this region of California, crop species that are tolerant to salt and boron (B) in soils and in alternative water sources must be identified. Pistachios are grown in over 4000,000 acres in California and have been demonstrated to tolerate irrigation with salt and B-containing water. The question remains, however, what are the effects of saline water on nut production and nut quality. In this field study with pistachio trees of different ages, we examined the effects of irrigation with either saline or non-saline water on anti-oxidation stress responses in the pistachio seed coat. Seed coat is a major storage site for phenolic compounds that respond to stress. We found that continued irrigation with high-saline water compared to a three-year intervention by irrigation with non-saline water did not influence overall antioxidant capacity within the seed coat for pistachios of any planting age. However, the planting age did influence the biochemical antioxidant response of only two compounds in the pistachio seed coat. Based upon the lack of response of phenolic stress indicators, we conclude that pistachios possess other mechanisms that allow it to tolerate irrigation with high saline water in different aged trees.

Technical Abstract: Sustaining irrigated agriculture under drought conditions with alternative water sources such as saline groundwater requires understanding their effects on salt-tolerant crops like pistachio. During recent California droughts, pistachio trees field-planted in 2002, 2009, and 2011 were irrigated with high-salinity water containing traces of boron (B) and selenium (Se). In 2018, irrigation delivery was divided into saline and non-saline irrigation. Within each planting year, one hundred and five trees were respectively irrigated with low-saline and high saline water. Three years later, nuts were harvested from these trees, respectively, and the phenolic antioxidants in the seed coats were evaluated for each irrigation treatment for each planting year. Seed coat extracts were analyzed for antioxidant capacity (ABTS, DPPH, FRAP assays) and total phenolic content (Folin–Ciocalteu method). Seed coats from nuts of the oldest trees (planted in 2002) had the highest antioxidant capacity and phenolic content, while saline irrigation had no significant effect on antioxidant capacity. High-performance liquid chromatography (HPLC) identified gallic acid and nine flavonoids. Catechin, procyanidin B1, and eriodyctiol were most abundant in the oldest trees but only quercetin and isoquercetin levels were the only compounds significantly influenced by saline irrigation. These compound-specific shifts suggest that tree age and some salinity exposure affected flavonoid metabolism, providing insight into the biochemical mechanisms underlying pistachio’s response to salinity.