How can two true breeding acyanogenetic (not able to produce cyanide) clover plants produce offspring that are all cyanogentic (able to produce cyanide)?
The successful formation of cyanide requires two enzymes to be present. The two enzymes are under control of alleles that are dominant to their mutant alleles.
The ability of a plant to release cyanide when injured is known as cyanogenesis. This ability is determined by two different genes, known as Ac and Li. Plants with the Ac gene can produce glucosides capable of releasing cyanide in the presence of linamarase. Plants with the Li gene can produce linamarase. Consequently a plant must have copies of both the Ac gene and the Li gene in order to perform cyanogenesis.
In your example, one of the parent plants is AcAc lili, and contributes the dominant Ac gene to the offspring. The other parent is acac LiLi and contributes the Li gene. The genotype of the offspring is Acac Lili; because it has a copy of the Ac gene and a copy of the Li gene, it is cyanogenic.