Advancing DUS characterization of neem through systematic gap analysis and setting future research priorities

Azadirachta indica A. Juss. (neem) is a multipurpose tree species of agricultural, medicinal, pesticidal and industrial importance. Increasing demand for neem-derived products, particularly neem oil for neem-coated urea, has stimulated genetic improvement programme for development of superior planting material with enhanced production and biochemical traits. In this context, Forest Research Institute (FRI), Dehradun, in collaboration with the Indian Farmers Fertiliser Cooperative Limited (IFFCO), released six elite neem varieties characterized by early flowering and fruiting, with oil content ranging from 35.44 to 43.51% and azadirachtin content from 6047 to 9760 ppm. The development and release of such improved varieties necessitate reliable and standardized distinctness, uniformity and stability (DUS) characterization for their identification, registration and legal protection. However, application of DUS testing in neem is challenging because of its perennial growth habit, extended juvenile phase, phenotypic plasticity and environmental influence on morphological traits. This systematic review synthesizes available information on DUS characterization in neem and critically examines morphological, quantitative, pseudo-qualitative and biochemical descriptors used for varietal identification. It identifies limitations in existing descriptor framework and highlights need for systematic validation across environments and developmental stages. Importantly, review contextualizes neem DUS characterization within recent advances in varietal development and considers its broader utility in breeding, germplasm characterization, conservation, and variety protection. Based on identified gaps, priorities are proposed for refining and validating existing descriptors and, where appropriate, integrating morphological, biochemical and molecular information to develop a robust and standardized DUS framework for future neem improvement and variety management.