* ac gene: The "ac" gene is responsible for the production of the enzyme acridinase.

* Acridinase: This enzyme breaks down the compound cyanogenic glucoside, which is a precursor to HCN.

* Homozygous ac: A homozygous ac plant has two copies of the ac gene, meaning it produces abundant acridinase.

* No HCN: With high levels of acridinase, the plant effectively breaks down all the cyanogenic glucoside before it can convert into HCN.

Therefore, a homozygous ac plant cannot produce HCN because it possesses the enzyme necessary to completely degrade the HCN precursor.