Rosaceae (2)

Amygdalae

(The Stone Fruits)

 

This large group of woody plants (currently designated at about 430 soecies) has proved difficult for the taxonomists to delineate with any certainty – even in this DNA age – and currently they are all attributed to the one genus Prunus.

 

They are natives of the Northern Hemisphere with a distribution through the temperate zones but they also occur in the warmer climates of Central and South America and Africa. The Cherries predominate in the species distribution in Eastern Asia and North America while the Plums and Peaches are chiefly from Western Asia and Europe.

 

They vary from prostrate shrubs to medium sized trees; although mainly deciduous, there are several evergreen species. Most flower in the spring often before leaf bud break. The fruit is a fleshy (generally edible) drupe (a prune) which contains a single, hard coated seed which is described as a ‘stone’ or ‘pit’. Many species have been developed and selected from ancient times as edible fruit crops - especially in Europe and Asia. Others have been used, selected and bred as ornamentals – eg the Sato Zakura Cherries of Japan.

 

The genus Prunus, if determined to be one genus, contains in the region of 430 species and is generally divided into the following sub-genera:-

Amygdalus – the Almonds and Peaches;

deciduous;

single flowers;

fruit is grooved on one side;

stone is deeply grooved.

Prunus – the Plums and Apricots;

deciduous;

lack terminal bud,

many flowers per bud;

fruit is grooved on one side;

stone is rough.

Cerasus – all the Cherries;

deciduous;

inflorescence an umbel;

fruit is not grooved;

stone is smooth;

axillary buds are single.

Lithocerasus – the Sand Cherry (P. pumila)

as above but

axillary buds are in threes.

Padus – the Bird Cherries;

deciduous;

racemose inflorescence;

fruit is not grooved;

stone is smooth.

Laurocerasus – the Cherry Laurels;

evergreen;

racemose inflorescence;

fruit is not grooved;

stone is smooth.

 

The fruit should be collected when it is fully ripe – ie when the flesh is softening. The stones are extracted by hand or by maceration and fermentation - depending on individual size and quantity. Subsequently the ‘stones’ should be thoroughly cleaned to remove all flesh as there is the possibility that germination inhibitors may occur in the flesh. The seeds should not be allowed to dry (or surface dry only if to be stored). The seed coat (endocarp) is hard – this condition is to provide protection for the seed in the passage through the gut of frugivorous birds and mammals which eat the fruit and so disperse the seeds. However this hard seed coat is not normally impermeable to water – but it can become impermeable if dried and the condition can then be difficult to overcome.

 

Seed treatment will begin with a warm water soak for 48 hours and in most cases this will achieve imbibition. The seeds should then be warm stratified; this is a necessity and is beneficial insofar as it appears to cause the maturation of an otherwise imbibed, morphologically mature embryo – 28 days at 20˚C would appear to exceed the requirements of those species which have been observed, thus this could be used as a standard treatment in a protocol.

   

Most of the temperate species then exhibit an endogenous physiological dormancy which is mitigated by chilling. However the constraint does not appear to be in the embryo insofar as excised embryos will germinate without treatment although the resulting seedlings usually produce dwarf plants. A chilling treatment of 63 days at below 3̊C will usually overcome the constraint.

 

Germination at 20˚C will be speedy and synchronised.