Hybrids of Gossypium hirsutum crossed with G. davidsonii, G. gossypioides and certain strains of G. arboreum die prematurely at some point during seed, seedling or later plant development, depending on the species involved and the number of incomparability genes present in G. hirsutum. The dying plants normally exhibit necrotic lesions and vascular necroses similar to those associated with hypersensitive resistance to plant pathogens. Tissues from plants showing lethal symptoms and from sister plants containing compatibility genes that prevent the lethal responses were analyzed for the known cadalene, lacinilene, naphthofuran, and terpenoid aldehyde phytoalexins of Gossypium.

Cotyledons from plants showing symptoms induced by the G. davidsonii lethal gene contained 157 ug of hemigossypol/ g dry weight and 92 ug of lacinilenes and cadalenes combined/ g dry weight, whereas these phytoalexins were not detected in the sister compatible plants. A four fold and thirtyfold increase in gossypol and hemigossypolone concentrations, respectively, also occurred in the cotyledons undergoing lethal reactions. Smaller but significant increases in all of these phytoalexins also occurred in hypocotyls. Tannin concentrations increased from 0.56 to 1.66% and from 1.09 to 2.84% dry weight in cotyledons and hypocotyls, respectively, as a consequence of the lethal gene. Thus, terpenoid phytoalexins and tannins increased markedly in both cotyledons and hypocotyls because of the G. davidsonii lethal gene.

Plants showing symptoms from the G. arboreum and G. gossyipioides lethal genes contained 44-197 ug of terpenoid aldehyde phytoalexins/g dry weight of stele or stem tissue, whereas these compounds were not detected in sister compatible plants. Tannin increased from 0.71 to 1.13% dry weight in stele as a consequence of the G. arboreum lethal reaction and from to 3.91% dry weight in stem as a consequence of the G. gossypioides lethal reaction. In leaves, tannin concentrations were unchanged and terpenoid aldehyde concentrations were decreased in association with the lethal response to the G. gossypioides lethal gene. Therefore, the G. arboreum and G. gossypioides lethal genes elicit synthesis of phytoalexins and tannins in stems, including stele, but not in leaves.

These studies show that deregulation of phytoalexin and tannin synthesis is consistently associated with genetic lethal responses. Thus, it is possible that lethal genes may regulate the ease with which phytoalexin synthesis is triggered. If so, lethal genes, when incorporated with appropriate compabitility genes into cultivated cottons, may be useful sources of resistance to pathogens. The effects of these various lethal genes and compatibility genes on resistance of cotton to pests are being explored.