Paul has a track record in grassland science, agronomy and crop physiology. Over the last decade the main focus of his research activities were on the following topics: Seed systems, especially of vegetatively propagated crops. Currently the focus is on developing quality declared seed systems of root and tuber crops in Eastern Africa, Asia and South America. Agrobiodiversity, including the anthropological aspects of the use of diversity in staple crops and the global conservation of genetic resources and crop wild relatives. Linking ecophysiology and genetics of complex traits through modelling with a focus on resource use efficiency and stress tolerance. Modelling of C3 and C4 photosynthesis at different scales and in different dimensions.