Plant 312

Prunus dulcis (Mill.) D.A.Webb (Rosaceae)


The almond is a small, deciduous tree reaching ten metres in height, with white to pink, insect-pollinated flowers that produce velvety, ovoid, flattened fruits, which are technically called drupes. As almond drupes mature, their leathery flesh (mesocarp) dries and splits to reveal the 'stone' (endocarp), the inside layer of the fruit wall. Inside the endocarp there is usually a single seed, which is the part that is eaten. Drupes are also produced by cherries, plums and peaches; all members of the Rosaceae, like the almond. Almonds are processed into all manner of food products from biscuits and cakes, through flour and syrups to milk substitutes.

There are two sorts of almond, the sweet and the bitter. The bitterness in the seed is caused by the occurrence of benzaldehyde, which is produced by the action of gluconidase enzymes on cyanogenic glycosides. A specific transcription factor attaches to the DNA of two genes involved with glucoside production, making the almonds bitter. However, a mutation in the transcription factor, which prevents it binding, means that the glucoside is not produced and the almonds are sweet.

Another product of the gluconidase reaction is hydrogen cyanide, the odour of which is often likened to that of almonds. Cyanide is probably part of the almond's defence against predators of the oil-rich seed. Consuming a handful of bitter almonds is likely to be fatal to an adult human; sweet almonds contain only a trace of cyanide.

The almond is probably one of the earliest domesticated tree crops in the Old World. Archaeological evidence shows almond domestication occurred more than five millennia ago in the region of the Fertile Crescent. Its economic value, particularly as a food plant, has led to it being spread throughout the world's arid and semi-arid regions. Domestication of the almond has led to the selection of fruits with lower toxicity, thinner endocarps and greater seed size than their wild relatives.

Almonds are self-incompatible; they can only set seed when eggs and pollen combine from trees with different combinations of incompatibility genes. Consequently, the pollination services provided by bees are essential for the economics of global almond production. In 2017, over 2.2 million tonnes of almonds were harvested, of which more than half came from orchards in the United States and Spain. Over the past decade there has been a 35% increase in global almond production, which is today worth several billion US dollars.

Further reading

Henselek Y et al. 2018 Pollination requirements of almond (Prunus dulcis): combining laboratory and field experiments. Journal of Economic Entomology 111: 1006-1013.

Sánchez-Pérez R et al. 2019. Mutation of a bHLH transcription factor allowed almond domestication. Science 364: 1095-1098.

Velasco D et al. 2016. Evolutionary genomics of peach and almond domestication. Genes, Genomes, Genetics 6: 3985-3993.

Stephen Harris