Thermal time (growing degree days) effects on phenology, yield and quality attributes of fennel (Foeniculum vulgare L.)

Abstract

Temperature-driven crop development is a major determinant of phenology, yield and quality in seed spices such as fennel (Foeniculum vulgare L.). The present study evaluated the influence of accumulated Growing Degree Days (GDD) on phenological development, seed yield and quality attributes of fennel under semi-arid field conditions. Field experiments were conducted at the ICAR-National Research Centre on Seed Spices, Ajmer, India, over three consecutive seasons (2019, 2020 and 2021). Umbels were harvested at different maturity stages ranging from 25 to 95 days after anthesis (DAA), corresponding to cumulative thermal accumulation of 1317.25 to 2894.25 GDD. Phenological progression showed a strong association with cumulative GDD rather than calendar days. Panicle emergence, king umbel emergence and anthesis occurred at approximately 853.75, 956.25 and 1034.25 GDD, respectively. Stage-wise thermal requirements (ΔGDD) indicated that early vegetative growth was the most thermally demanding phase (853.8 GDD), whereas reproductive transitions occurred over comparatively shorter thermal intervals, followed by progressively higher ΔGDD during later harvest stages. Harvest stage and associated GDD significantly influenced seed moisture, biochemical composition, essential oil content and yield. Seed moisture declined from 75.15% at 25 DAA to 14.18% at 95 DAA. Total oil and essential oil contents were highest at early harvest stages (4.41% and 2.61%, respectively) and decreased steadily with increasing GDD, while crude fibre content increased from 12.31% to 38.36%. Seed yield increased with cumulative GDD and peaked at 24.15 q ha⁻¹ at 75 DAA (2349.25 GDD), followed by a slight decline at later stages. Correlation analysis revealed strong negative relationships between cumulative GDD and oil, essential oil and protein content, and positive associations with crude fibre, seed index and yield. Overall, the study demonstrates that GDD is a robust agrometeorological index for predicting fennel phenology and optimizing harvest timing, enabling a balance between yield maximization and quality preservation under variable climatic conditions.