CYANOGENESIS IN PLANTS
Authors: ROBINSON MURIEL ELAINE 
Published: January 2008

Prussic acid, identified during the early part of the nineteenth century as a constituent of many members of the Rosaceae, is now known to be present in plants from about fifty natural orders. It is generally considered to exist in the living plant exclusively in glucosidal combination, but certain workers, including Treub and his colleagues, have postulated the existence of free cyanide in plant tissues.

Ten cyanophoric glucosides have been isolated in crystalline form. Of these, seven are derivatives of benzaldehyde cyanhydrin; linamarin and gynocardin contain ketone groupings, and lotusin, the glucoside of Lotus arabicus is a derivative of γ‐pyrone combined with a sugar cyanhydrin. The glucosides prunasin, sambunigrin, prulaurasin, linamarin and amygdalin have been prepared synthetically.

Amygdalin, prunasin and prulaurasin seem to be restricted to the order Rosaceae, sambunigrin to the Caprifoliaceae, and dhurrin to the Graminaceae; while vicianin has so far been found only in a few species of the genus Vicia. Linamarin, on the other hand, has been found in several natural orders, widely separated in morphological classification.

In the plant, cyanophoric glucosides are, with few exceptions, accompanied by active, cyanide‐liberating, hydrolytic enzymes. The conception as to the specificity of such enzymes, as postulated by many authors, may need some modification in the light of the recent work of Willstätter and his pupils.

The concentration and seasonal variation of prussic acid in the plant show considerable differences in the several cases known. In general, the concentration is greatest in young, growing organs; Sambucus nigra, on the other hand, shows little seasonal variation in cyanide content. In other cases, cyanide has been shown to exist in quantity in such tissues as the bark. In certain of the Rosaceae, cyanophoric glucosides appear to be constant constituents of the plant throughout its life cycle, while in the Graminaceae and in Lotus arabicus, they disappear at maturity and are absent in the seed.

In some cases, the concentration of prussic acid in the plant has been shown to be diminished by cultivation (Amydalis communis, Vicia angustifolia, Phaseolus lunatus). Climatic conditions, especially drought, have furthermore been stated to cause variations in the content of dhurrin in Sorghum vulgare and of the cyanophoric glucoside of Lotus corniculatus. Temperatures below freezing point have been observed to cause a rapid and conspicuous increase in the glucoside content of Sorghum vulgare and in Prunus laurocerasus.

The function of prussic acid in the plant is uncertain. Since the researches of Treub on the localisation of cyanophoric substances on Pangium edule, many workers have considered that cyanides may represent the first stage in the synthesis of organic nitrogen compounds by the plant. The rapid disappearance of cyanide under conditions of starvation, as observed in Prunus laurocerasus, supports the view that such nitrogen is readily utilizable by the plant. Other workers regard cyanophoric glucosides as excretory products, or, in virtue of their poisonous properties, as protective agencies.

Quantitative investigations as to nitrogen partition in cyanophoric plants at various stages of growth might help to elucidate the important question as to the participation of prussic acid in protein synthesis by the plant. At the present time, adequate data on this subject are not available.