Phenological characterization and thermal requirements of prominent cultivars of litchi in the Indo-Gangetic Plain

Aim: Litchi (Litchi chinensis Sonn.) exhibits marked phenological variability among cultivars, which influences productivity and climate adaptability. This study aimed to characterize and compare the phenological behavior of ‘Shahi’ and ‘China’ using the extended BBCH (Biologische Bundesanstalt, Bundessortenamt and Chemische Industrie) scale and to examine the influence of thermal indices on phenophase progression. Methodology: The experiment was conducted at the ICAR–National Research Centre on Litchi, Muzaffarpur (Bihar) during 2024–2025. Phenological observations were recorded from bud development to fruit maturity following the BBCH scale. Growing degree days (GDD) and chilling days were computed to assess temperature-driven responses. Results: Clear cultivar-specific differences were observed in the timing and duration of phenophases. ‘Shahi’ exhibited early bud break, shoot growth and fruit maturity, whereas ‘China’ showed delayed but prolonged development. Although flowering intensity was comparable, ‘China’ recorded higher fruit set (89%) and retention (78%) than ‘Shahi’ (35% and 22%). Thermal accumulation was also greater in ‘China’ (1162.9 GDD) compared to ‘Shahi’ (955.5 GDD). A novel Chill-Triggered Transition and Synchrony (CTTS) model was developed to integrate cultivar-specific chilling thresholds, flowering transition dynamics, and orchard-level flowering synchrony into a unified phenological framework. Interpretation: ‘China’ demonstrated superior reproductive efficiency despite its delayed phenology. The BBCH scale together with the newly developed CTTS model provides a robust framework for predicting flowering behaviour and improving climate-resilient orchard management. Key words: BBCH scale, Chilling days, Growing degree days, Litchi, Phenology