Rosaceae (1)

Malineae

(Genera of the Rosaceae with Pome Fruits)

 

This group of genera from the Rosaceae all produce pome fruits ie fleshy (in some degree) fruits with a ‘core’ (ie like an apple) which contains 2 to 5 carpels – these contain the seeds.

 

The section Malineae occurs principally through the cold temperate regions of the Northern Hemisphere (but with a few southern outliers) and includes the following genera which could be found in cultivation in the UK:- Amelanchier – Snowy Mespilus, Aria – Whitebeams, Aronia – Chokeberies, Chamaemespilus, Chaenomeles – Japonicas, Cormus – Service Tree, Cotoneaster, Crataegus – Thorns, Cydonia – Quinces, Docynia, Eriobotrya – Loquat, Heteromeles – Toyon, Malus – Apples, Mespilus – Medlar, Osteomeles, Photinia, Pyracantha – Firethorns, Pyrus – Pears, Raphiolepis, Sorbus – Rowans, Stransvaesia, and Torminaria – Wild Service Tree

 

This particular group of the Rosaceae is characterised by a base chromosome number of 17 (as opposed to the more normal 7, 8 or 9) and represent a particular evolutionary group which expresses many features in common. In several genera the ‘vegetative’ production of seeds (a process described as apomixis or agamospermy) is not an uncommon occurrence – especially in Amelanchier, Aria, Cotoneaster, Crataegus, Malus and Sorbus.

 

The individuals of many of the diploid species exhibit self incompatibility in the pollination/fertilisation process and therefore need another individual of distinct genetic complement from the same species (ie not the same clone) to achieve seed production. Apomictic development of seed also often requires the presence of distinct pollen on the stigma to initiate and stimulate the development of the seed.

 

The main logistical issue, when dealing with this group of plants, is extracting the seeds from the sometimes not inconsiderable amount of the fleshy fruit which surrounds them. This development of a fleshy fruit is part of the strategy associated with seed dispersal. The fruits are attractive to various animals and birds and indeed there is evidence that the passage of the seed through the digestive tract reduces the seed coat and so makes it more readily permeable to water, the thick seed coat having evolved to be coarser initially to protect it through the digestive tract ie it is not part of a strategy to spread germination through time.

 

In those species which have fruits with large quantities of flesh, it is sensible to reduce the fruit down to the core by paring away the flesh before attempting to extract the seeds.

 

It would also be prudent to collect the fruits before they become overly ripe to avoid them being taken by the predators (vectors). However it is also evident that, in some species, the flesh develops significant quantities of chemical germination inhibitors as it ripens and that these chemicals will contaminate the seeds. If they are not removed they will cause the chilling period to be attenuated and so confuse any established time scale within a protocol.

 

The pared down fruits or the complete fruits of the smaller fruited species are macerated to break up the flesh and tissue but not sufficiently to damage the individual seeds. This pulp is then mixed with a relevant volume of warm water and left to ferment in a warm environment. This will cause the flesh to break down and the seeds will fall to the bottom of the container – a period that will take several days. The scum and detritus can be decanted off and the seeds collected and rinsed until they are clean – if there is still some pieces of flesh, skin etc mixed with the seeds this can be separated by swirling them in clean water and separating the seeds from the fruit remains.

 

It is prudent, at this stage – if there is no hindrance from the seed coat and the embryo is imbibed, to proceed directly into the cold stratification treatment - as drying the seeds, more often than not, induces the development of a degree of hard-seededness. A short period of warm moist storage however is not detrimental.

 

However there are a considerable number of species in which there is a sufficiently developed ‘hard’ seed coat to prevent an immediate imbibition. For these species it would be sensible to collect the fruits and extract the seeds at an early stage – preferably at, or just before, the time that the fruits are beginning to change colour from green. Most of the relevant genera belong to the Crataegeae as opposed to the Maleae (in which the seeds are ‘pips’) with a relatively soft seed coat but which nevertheless can harden if dried.

 

If the seed has been dried (or has been harvested after the development of the hard seed coat condition) it is quite possible that it will have developed a sufficiently hard seed coat that it will prevent imbibition. Subjecting the seed to a mild scarification regime will usually overcome the problem. The seeds can then be imbibed.

 

The chilling requirement within this group can be very variable not just between the genera but also between species – according to their evolutionary position. In many ‘primitive’ species the presence of inhibitor is only in the endosperm – demonstrable by the fact that excised embryos will germinate without chilling – more evolutionarily advanced species have developed endogenous embryo dormancy.

 

The great majority of the species, which have an endogenous dormancy, require a significant period of chilling – in the order of 60 to 84 days below 3ºC – this is especially true of those species from northerly (colder) latitudes; areas with continental winter climates and higher elevations in warmer areas. Any period of chilling which requires significantly more than 84 days suggests that there is some other factor complicating the process – possibly a hard seed coat which delays imbibition and hence the beginning of chilling or the presence of inhibitors derived from the fruit which have not been fully eliminated prior to chilling.

 

Particular and relevant characteristics of individual genera and species relating to their propagation by seed.

 

The various Quinces and Medlars (Chaenomeles, Cydonia, Mespilus) have a hard flesh until well on into the ripening season. This requires considerable effort to pare away and it will be practical to leave the fruits to ripen until the flesh softens. By this stage there may well be a build up of inhibitor and so care should be taken to clean and rinse the seeds to remove this factor. The amount of chilling required is, in most species, in the order of 84 days below 3ºC.

 

Seeds of plants in the Crataegeae have a seed coat with a nut-like, somewhat woody structure - eg Crataegus, and Cotoneaster and will probably best be dealt with by collecting the seed when the fruit is immature (ie the fruit is still green and has not developed any colour) at this stage the seed coat has not fully developed the ‘woody’ or ‘tough’ consistency. Then extract the seeds from the fruit but do not allow them to dry out.

 

Crataegus and Cotoneaster are stratified for 84 days at 3ºC.

 

Heteromeles arbutifolia (which occupies a warm temperate niche) seeds are virtually non-dormant but germination may be delayed by a marginally tough seed coat (which is a protective device against digestion when the fruits are taken by birds) but a short warm water soak is usually enough to cause permeability.  As the plant does not flower until after mid-summer it is important not to harvest the berries until they are fully ripe as the embryo and food reserve are not fully developed and matured until well into the late autumn. A short stratification of 21 days at less than 7ºC will speed and synchronise germination. Osteomeles (also from warmer provenances) has a hard seed coat but this is readily overcome by a reasonably prolonged warm water soak.

 

Pyrus is an example of a genus exhibiting an endogenous dormancy condition in which there is a wide spectrum of chilling requirement. The genus (which contains deciduous and semi-evergreen species) occurs from the sub-arctic to the sub-tropical. The chilling requirement thus is directly related to the severity of the temperature of the winter climate experienced by a particular species. Species from northerly latitudes (eg P. betulifolius and P. ussuriensis) tend to experience severe winter conditions and exhibit conventional chilling requirement of 84 days at 3ºC. Species from milder climates (eg P. amygdaliformis, P. calleryana and P. koehnii) respond below 7ºC and will generally need 63 days of exposure. Species from warmer climates (P. pashia) need only a chill below 10ºC for around 35 to 42 days. After stratification the seeds of all species should be transferred to at least 15ºC to achieve a rapid and synchronised germination.

 

The genus Aria contains a relatively large number of apomictic, so-called, micro-species which often have a very local geographic distribution eg A. devoniensis – French Hales - in West Devon and East Cornwall and A. bristoliensis in the Cheddar Gorge. Understanding the parameters of these microspecies is a difficult task and expert advice should be sought on the correct naming of each individual and its provenance.

 

There are also a significant number of apomictic microspecies among the Himalayan species of Sorbus - however these appear to be more clearly defined. The advantage with all of these species is that they do come true from seed – they are effectively clones.

 

The individuals of the diploid species of Sorbus are mostly self incompatible and have a high tendency to hybridise if the opportunity is provided. This is as much true of the native S. aucuparia as it is of the exotic species. The chilling requirement of the cold temperate species of Sorbus is fairly typical ie in the order of 84 days below 3ºC but those from milder climates will probably respond at higher threshold temperatures and for shorter periods (eg S. harroviana).

 

Most Malus species are individually self incompatible and require a distinct (separate genotype) pollen source to induce fertilisation – hence there is a high possibility of hybridisation if the opportunity is provided. Care should therefore be exercised in selecting parents for seed propagation in order to ensure ‘trueness-to-type’ – unless hybrid offspring are sought.

 

The recommendations for stratification vary dramatically, from reference to reference, in terms of the length of the chilling period required to eliminate the dormancy condition for Malus species. Virtually all appear to respond most satisfactorily at or below 3ºC and although periods of exposure for as much as 120 days (or even longer) are cited - satisfactory germination is usually achieved with an 84 day period – provided that the seed is not contaminated with inhibitor and that imbibition was effectively completed prior to the commencement of treatment. To avoid the impact of ‘external’ inhibitor, the fruits should be collected at an immature stage (before the development of the inhibitor) and the seed extracted just as it is complete and effectively isolated from the parent - or if the ripe fruit only is available then the extracted seeds should be thoroughly soaked, washed and rinsed to remove any polluting inhibitor. If fresh seed is available then there should be little or no effect of the seed coat on imbibition, however sometimes when seed is dried there can be the development of a sufficiently hard seed coat to impair imbibition – this can be treated by a mild scarification.

 

The following two genera produce relatively small sized seeds which resemble the pips of Sorbus.

 

The genus Amelanchier (probably of about 20 species), described as the Snowy Mespilus in Europe, produces good fruit crops in the UK - and these contain acceptable quantities of seed. However in Amelanchier the species are very close to one another both physically and genetically which causes confusion with naming – taxonomists recognising as few as 6 or 8 species or as many as 33! If grown together the species hybridise readily. It is therefore important to collect fruits from isolated individuals or where groups of the same species are planted together. Some of the species and hybrids (A. x lamarckii) are apomictic and this, as in other related genera, tends to create confusion.

 

The genus is native to the Northern Hemisphere - typically in the United States and Canada, there are two Chinese species (A. asiatica and A. sinica) and one European (A. ovalis). The generalised name for the species in North America (where they occur in all the States except Hawaii) is Shadbush. They are ornamental shrubs or small trees with spectacular and prolific flowering in the spring and often exhibit brilliant autumnal hues – especially reds.

 

The fruits, which are small fleshy pomes, are edible and are harvested locally and used to make pies and jams - one species is used to flavour Pemmican.

 

Because some of the species develop hard seed coats it is prudent to collect the fruits at an immature stage before any such condition has become advanced. The fruits should be macerated and fermented to extract the seeds. As many of the species also produce their fruits early in the year (more than one species is sometimes referred to as Juneberry) it means that season of collection will be relatively early in the year.

 

Assuming that there is no problem created by a hard seed coat then the only germination pre-treatment required is chilling – for the northern species a period of 84 days at 3ºC is adequate but for more southerly species it is not clear what the requirements are. Germination in some northern species will occur below 5ºC ie beneath snow melt but in nursery practice the most uniform results are obtained by germinating at 20ºC. Hard seed coat conditions do develop in a few species (A. canadensis) and can be satisfactorily treated by scarification.

 

Aronia is a small genus of three species (A. arbutifolia, A. melanocarpa & A. prunifolia) of low growing, suckering, shrubs of moist soil conditions from Eastern North America. They are extremely decorative in the autumn with their vivid fall colours (which are not dependent on an acid soil for their expression) and highly coloured fruits. Their common name - the chokeberries - refers to the edible fruits which are astringent to the taste in the raw state. The fruits are now recognised for their medicinal value insofar as they contain high levels of valuable antioxidants.

 

The fruits, which are generally produced prolifically in the UK, are small fleshy pomes which are produced in erect clusters during the late summer and autumn. The fruits of A. arbutifolia and A. prunifolia persist well into the winter and are eventually taken by birds while those of A. melanocarpa drop very soon after ripening. It is just as well to collect the fruits early in the autumn and extract the seeds before there is any possibility that inhibitors developed in the ripening flesh can become significant. The fruit contains five carpels but in practice only two or three develop into mature seeds – the remainder appear as aborted ‘chat’ seeds. The seeds are extracted by macerating the fruits and leaving them for about forty eight hours to ferment – further agitation will see the seeds drop to the bottom and the detritus and scum can be decanted off – the seeds can be cleaned of remaining skin and flesh by successively swirling the seeds in water and pouring off the detritus. The clean seed sample can then be air dried to dry the surface. The seeds can be stored in the short term by maintaining the water status and maintaining a warm temperature. Germination is delayed by an endogenous embryo dormancy. In the case of A. arbutifolia this control is the deepest and will require stratification for 84 days at below 3ºC. The other two species will require in the region of 56 days.