Freezing tolerance and cyanogenesis in white clover (Trifolium repens L. Fabaceae)

White clover isnaturally polymorphic for cyanogenesis,and the ecological genetics of this polymorphismhas been studied for more than 60 yr. Temperature-associated cyanogenesis clines occur in both native and introduced populations. Whereas the selective factors favoring cyanide-producing plants in warm climates are well established (differential protection against generalist herbivores), those favoring acyanogenic plants in colder regions are less well understood. We tested the hypothesis originally proposed to explain this pattern: that cyanogenic plants suffer increased tissue death following freezing as a result of frost-induced cyanide autotoxicity. Tissue damage was assessed in laboratory freeze treatments at three temperatures: -10°, -11.5°, and -13̇C. We examined cyanogenic and acyanogenic genotypes sampled from across the species range, as well as from polymorphic populations sampled in North Carolina. Cyanogenic genotypes exhibited lower freezing tolerance in the species-wide sample but showed no similar trend in comparisons of plants from polymorphic populations; statistical power was reduced in the latter analysis and may be a contributing factor. These results indicate that expanded genotype sampling of polymorphic populations is warranted, and they tentatively suggest that factors such as herbivore abundance and resource allocation trade-offs may contribute to the occurrence of cyanogenesis clines in white clover.