Cyanogenesis and climatic relationships in U.S. white clover germplasm collection and core subset

Cyanogenesis, the production of hydrocyanic acid, is a trait in white clover (Trifolium repens L.) controlled by alleles at two unlinked loci, Ac and Li. The objectives of this study were (i) to determine the proportion of cyanogenic plants in the accessions of the U.S. white clover germplasm collection, (ii) to determine how representative the core subset was of the entire collection for cyanogenesis, and (iii) to determine the relationship of cyanogenesis, in accessions collected in situ, to collection site and climate. Twenty plants from each of 548 white clover accessions were grown in the greenhouse from 1991 to 1993. Two to three leaves per plant were harvested and the presence or absence of hydrocyanic acid was determined by the picric acid test. The U.S. germplasm collection was predominantly acyanogenic with a third of the plants evaluated being cyanogenic. Accessions with the most cyanogenic plants were collected in Morocco, Spain, Portugal, France, and Turkey while the least cyanogenic plants were collected in China, Romania, Russia, Kazakhstan, Croatia, Bosnia-Herzegovina, and Slovenia. Accessions collected in Morocco, Portugal, and Spain had the greatest intensity of cyanogenic reaction. Geographic stratification with random sampling was an effective method of developing a core subset, as the white clover core subset was representative of the entire collection for total cyanogenesis and distribution of cyanogenic plants. Cyanogenic frequency was greater in accessions collected at low altitudes and at sites with high winter temperatures, lower summer precipitation, greater spring cloudiness (% sunshine), and less snow cover. Latitude of collection was not associated with cyanogenic frequency in white clover.