A few sorghums grow in the humid lowland tropics. Although they are not well studied, the guineense and other related groups (roxburghii and conspicuum, for example) could be useful as genetic sources for improvement of genotypes for humid tropical regions.
At least two undomesticated forms show extremely robust growth under the harshest of conditions.
One, the verticiliflorum form (previously known as Sorghum verticiliflorum) is a wild grass, distributed from the Sudan to South Africa. It is often found in damp areas (along stream banks and irrigation ditches, for example) or as a weed in cultivated fields. On the other hand, it is also a dominant climax species in many of the area's dry, tall-grass savannas. It is thought to be a progenitor of the modern bicolor, caudatum, and kafir races of sorghum but has seldom been considered a genetic resource in its own right. Nonetheless, in research now under way, this plant is proving extremely useful in forage-breeding programs. No doubt it contains disease-fighting abilities and pest resistances that could be deployed to help sorghum.
The other (previously known as Sorghum arundinaceum) is a wild and weedy rainforest species that flourishes in Africa's wet tropics, where today's domesticated sorghums are poorly adapted. Although very little information is available, it appears to be more photosynthetically efficient at low light intensities than cultivated sorghum.14 As of now it is not cultivated, but it may have a future as a domesticated crop for humid and forested regions. It is a robust species, very common along roadsides, vacant lots in cities, and other "wastelands."
Sorghum can be crossed with grasses genetically distant enough to be classified in different genera or even in different subfamilies. It is certainly highly speculative to think that these crosses might have any economic merit, but exploratory research efforts seem well worth undertaking. A few possibilities are discussed here.
Crosses between sorghum and certain types of Chrysopogon, Vetiveria, and Parasorghum are possible. Crosses with Pseudosorghum