PHENOLOGICAL CHARACTERISTICS AND VARIABILITY OF TOTAL PHENOLIC CONTENT IN GREEN HUSKS OF HUNGARIAN-BRED PERSIAN WALNUTS

Walnut is an important temperate tree crop treasured for its nutrient-dense kernels. All fruit parts, including the green husks, are rich in bioactive phenolic compounds with antimicrobial and antioxidant properties. The study examined total phenolic content (TPC) in green husks of twelve cultivars, namely, ‘Alsószentiváni 117’, ‘Alsószentiváni kései’, ‘Eszterházy kései’, ‘Milotai kései’, ‘Milotai intenzív’, ‘Milotai 10’, ‘BD6’, ‘Köpcös’, ‘Tiszacsécsi 83’, ‘Eszterházy II’, ‘Chandler’, and ‘Bonifác1’. Sampling was performed fortnightly, from the lignification of the nuts (late June) to September at harvest time in 2022, 2023, and 2024.

Using the Folin–Ciocalteu spectrophotometric method, this study showed that the biosynthesis of TPC significantly varies (p < 0.001) among cultivars as the fruit develops to maturity across the years. Total phenolic content in the green husks ranged between 27.9 and 67.7 mgGAE/gDM, between 7.1 and 98.6 mgGAE/gDM, and between 37.0 and 91.3 mgGAE/gDM in 2022, 2023, and 2024 respectively. ‘Milotai kései’ recorded the least TPC at 7.1 mgGAE/gDM and ‘Alsószentiváni kései’ the highest amount at 98.6 mgGAE/gDM in 2023. This study also shows that all cultivars achieved the highest phenolic content either at the lignification stage or harvest time in 2022 and 2024. However, in 2023, the TPC content in a number of these cultivars such as ‘Milotai intenzív’, ‘Milotai 10’, ‘BD6’, ‘Köpcös’, ‘Eszterházy II’, and ‘Bonifác1’ was the highest in mid-July and August. These findings suggest that the biosynthesis of phenolic compounds in walnuts is dependent on inherent cultivar characteristics and other exogenous factors such as sampling period.