from those discussed by Prakash and Rao. 5 The system is thought to be too complex, involving at least two components ยฑ substrate and enzyme ยฑ for us to use it commercially, and single compounds were the basis of the book. 5 The present state of biotechnology may be too primitive to effect the transfer of the genetic systems involved into other plants, in spite of recent wishful thinking. 13 Furthermore, because so many of our food plants are already cyanogenic, it may not be sensible to attempt to transfer cyanogenesis into those few important ones that are not.
The data from a large number of sources show that cyanogenesis appears to have played an essential part in the choice of the major food plants by humans at the time of transition from hunter-gatherer to cultivator and husbandman. 11 The evidence is based on the following observations (see Reference 11 for details):
(1) Our food plants have to be ones we can eat in quantity. Most other potential food plants contain compounds that we cannot detoxify or metabolize in quantity. We have found other uses for some of these compounds. In small doses, many are medicinal drugs, although in larger doses they are metabolically dangerous. (2) Our food plants have to be easy to grow with minimal care and attention, and `good' to eat. (3) The cyanogenesis of the leaves deters many would-be pests. A plant with few pests would be an attractive candidate for domestication by our ancestors. These plants are not difยฎcult to grow as crops and could well have been higher-yielding than other candidates not similarly protected. ( 4) Given sufยฎcient protein we can detoxify the hydrogen cyanide released by raw cyanogenic plants, so long as they are only part of a meal. ( 5) By processing the food before eating we can remove most of the hydrogen cyanide and the cyanogenic compounds and so eat much more. Very few other organisms pre-process food, not even our primate relatives. 10 (6) Our domesticated animals can also detoxify hydrogen cyanide, again given sufยฎcient protein in the diet. (7) It is the leaves of our cereal crops that are cyanogenic, not the grains. We eat the grain, our animals may eat the rest. (8) Cyanogenic glycosides and the b-glycosidases are economical compounds for plants to produce; they are turned over in the plant and are used only when the plant is damaged. (9) Although cyanogenic glycosides are arguably the most widespread of all chemical defences in vascular plants and, when degraded, are remarkably effective herbivore deterrents (pesticides), we have included a disproportionally large number of cyanogenic plants among our staple foods.
REFERENCES