The urticalean rosids belong to the order Rosales, which are eudicots (The Angiosperm Phylogeny Group, 2016). Though the Rosales comprise some 7700 species (Zhang et al., 2011), they contain relatively few well-characterized model plants. The flowering plant super-models Arabidopsis thaliana (thale cress, Brassicales) and Oryza sativa (rice, monocots) are only distantly related to Cannabis, the lineages leading to Arabidopsis and Cannabis separated some 120 million years ago, those leading to rice and Cannabis some 130 to 140 million years ago (Figure 3) (Magallón et al., 2015). Among the relatively well-characterized plants that are more closely related to Cannabis are many Rosaceae species (rose family, apple, peach and relatives), for which several well assembled and annotated genomes exist (Aranzana et al., 2019; Zhang et al., 2019), the Cucurbitaceae (cucumber, pumpkin and relatives), which serve as an important model for sex determination and sex expression (Li et al., 2019; Schilling et al., 2020a; Zheng et al., 2019) and Fabaceae (bean family) for flowering time regulation (Cao et al., 2017; Schmutz et al., 2010) .

Cannabis sativa itself is phenotypically extremely diverse. Cannabis plants vary in numerous traits including height, leaf shape, photoperiod response, tetrahydrocannabinol (THC) and cannabidiol (CBD) content, plant architecture and sex expression (Clarke and Merlin, 2016; Grassi and McPartland, 2017; Raman et al., 2017; Schilling et al., 2020b). The dioecy of many Cannabis lines and thus the relatively high levels of heterozygosity further contribute to the fact that even within one cultivar the phenotypic diversity can be substantial (our unpublished observations).

For breeders and farmers, the high level of genetic and phenotypic diversity can be problematic, as a crop is usually best to handle when it possesses a high degree of uniformity in the field. However, at the same time, the existing diversity can be harnessed by breeders to produce new lines for a multitude of different purposes. For plant genetics research, the phenotypic and genetic diversity is a gold mine, as it provides the possibility to study the genetic basis of many traits in Cannabis . Some developments in this arena are outlined in the subsequent chapters, but many more are sure to come.

One of the commercially most interesting and valuable products that can be generated from Cannabis plants are phytocannabinoids. We use the term phytocannabinoids here for plant-derived cannabinoids, and to distinguish them from synthetic cannabinoids or those produced by the human endocannabinoid system. Phytocannabinoids are of great interest for medical applications (see chapter 8 for a detailed discussion) as well as commercial exploitations for recreational use. Hence, one of the major breeding goals involves the accurate prediction and targeted manipulation of phytocannabinoid profiles to ensure the optimal combination of active components in plant extracts (see entourage effect chapter 8) or legal compliance for non-psychoactive products.

While there are over 100 different phytocannabinoids described (Pertwee, 2014), three phytocannabinoids are usually at the centre of attention from a medical and commercial perspective: cannabigerol (CBG), cannabidiol (CBD) and tetrahydrocannabinol acid (THC) (Figure 4). Cannabis itself synthesizes phytocannabinoids in the carboxylated form with a carboxylic acid group, i.e. as CBGA, CBDA and THCA. However, to be active in the human endocannabinoid system, phytocannabinoids need to be consumed in their decarboxylated forms, which are usually generated by high-temperature treatment (for example during smoking) (Moreno-Sanz, 2016). Phytocannabinoids are predominantly produced in female inflorescences, more precisely they are secreted from trichomes of perigonal bracts, subtending flowers, and leaves (‘sugar leaves’) within inflorescences. However, in lower concentrations, phytocannabinoids can also be detected in vegetative leaves at certain times during the growth period (Aizpurua-Olaizola et al., 2016).