Total and Percent Atropisomers of Gossypol and Gossypol-6-methyl Ether in Seeds from Pima Cottons and Accessions from the Gossypium barbadense L. Germplasm Collection

Gossypol occurs naturally in the seed, foliage and roots of the cotton plant (Gossypium) as atropisomers due to restricted rotation around the binaphthyl bond.  The atropisomers differ in their biological activity.  That is, (R)-(‑)‑gossypol is more toxic than (S)-(+)-gossypol.  Most commercial Upland (Gossypium hirsutum) cottonseed have a (R)- to (S)-gossypol ratio of ~2:3, but some Pima (G. barbadense) seed have an excess of (R)-gossypol.  The high ratio of (R)- to (S)-gossypol that is generally thought to prevail among Pima cottonseed has led the dairy industry to use caution when feeding Pima cottonseed to their herds because cottonseed with a high total gossypol and a high (R)- to (S)-gossypol ratio can be toxic to cows if fed in large amounts.  Some of the accessions of G. barbadense in the U.S. Cotton Germplasm Collection have been characterized with respect to their (R)- to (S)-gossypol ratios.   We now report the analysis of all 671 accessions that were available in the Collection.  Approximately half of the accessions have an excess of (R)-gossypol (i.e., the toxic atropisomer) and 52 accessions have essentially a 1:1 ratio.  The highest percentage of (R)-gossypol was found in accessions GB26 (68.2%) and GB283 (67.3%).  Five accessions had 5% or less of (R)-gossypol: GB516 (5.0%), GB761 (4.5%), GB577 (4.3%), GB719 (3.7%) and GB476 (2.3%).  These accessions may be useful in a breeding program to reduce (R)-gossypol in Pima type seed.  Also, GB710 was devoid of gossypol.  The more recent Pima releases show a shift toward a smaller (R)- to (S)‑gossypol ratio except for Pima S7.  The ratios in Pima S5 and S6 are in line with that observed in most commercial Upland cottonseed.  However, total gossypol in the Pima accessions were relatively high compared to other G. barbadense accessions.  That is, Pima S1, S2, S5, S6 and S7 ranked in the top 6% of accessions with the highest levels of total gossypol.