Thermal regulation of phenological development in karonda (Carissa carandas L.) based on an extended BBCH scale

This study provides the first integrated phenological and thermal time framework for karonda ( Carissa carandas L.) using the extended BBCH scale. Karonda is an underutilized evergreen fruit crop valued for its adaptability to marginal environments; however, standardized information on its phenological development and temperature requirements remains limited. This study aimed to (i) characterize vegetative and reproductive phenological stages of karonda cv. Pant Manohar using the extended three-digit BBCH scale, and (ii) quantify stage-specific thermal requirements using growing degree days (GDD) under subtropical conditions. Seven principal growth stages were identified and resolved into 37 distinct secondary stages spanning bud development to fruit maturation. The complete phenological cycle extended over 212 days and accumulated 2966.6 GDD. Thermal requirements varied markedly among stages, with fruit development exhibiting the highest demand (918.7 ± 49.7 GDD), followed by shoot development (649.1 ± 58.7 GDD) and fruit maturation (488.7 ± 26.9 GDD), whereas vegetative bud and leaf development required substantially lower thermal input. Phase-wise thermal partitioning showed that fruit development alone accounted for ~ 30% of total accumulated GDD, while shoot development contributed ~ 22%. Analysis of thermal efficiency further indicated that reproductive and fruiting stages were the most temperature-sensitive phases. Sequential phenological progression showed strong temporal alignment with seasonal temperature rise and monsoonal rainfall, with considerable overlap between vegetative and reproductive growth phases. Overall, integration of extended BBCH codification with thermal time analysis provides a standardized, climate-responsive framework for phenology-based orchard management in karonda and offers a baseline for assessing developmental responses of underutilized perennial fruit crops under changing climatic conditions.