Infrageneric classification of Haworthia (Aloaceae): perspectives from nectar sugar analysis

Species and genus concepts and infrafamilial delimitations in the Aloaceae (subfamily Alooideae of the Asphodelaceae) have often been controversial. Arguments are mostly based on evidence obtained from vegetative and floral morphology, but data from other fields have also been used for speculating on taxonomic affinities within the Aloaceae. For the present study, nectar sugar composition (glucose, fructose, sucrose) was determined for representatives of Astroloba, Chortolirion, the three subgenera of Haworthia and for interspecific haworthioid hybrids and miscellaneous taxa. Two nectar types were distinguished in Haworthia and the two related genera: a Haworthia type (subg. Haworthia only, usually less than 50% sucrose) and a Hexangulares type (subg. Hexangulares, subg. Robustipedunculares, Chortolirion and Astroloba, usually more than 60% sucrose). Results support Uitewaal's subdivision of Haworthia in two main groups, but reveal little substantiation for infra- subgeneric groupings.