Adoxaceae

 

This small Family of four genera (two of them being woody and with temperate distributions) has been separated from the Caprifoliaceae. The two woody genera, Sambucus and Viburnum, both have species which exhibit unusual and somewhat eccentric dormancy controls which involve, inter alia, the maturation of an immature embryo.

 

Sambucus

 

This genus, particularly, has developed some unusual dormancy patterns - which because they have not been fully understood (and for several species are still not fully elucidated) - have often created uncertainty in achieving reliable germination protocols.

 

The genus Sambucus is a relatively small genus. It is, however, widely distributed in the Northern Hemisphere - in the Southern Hemisphere it is limited to Australasia and eastern South America. It is subtropical and temperate in its distribution. The genus is predominantly woody but there are a couple of herbaceous species.

 

The number of woody species in the genus appears to be a matter of taxonomic debate – some taxonomists taking the view that there are only three species - with two of these having very extensive, geographical distributions. These are then divided, in particular geographical areas, into subspecies or major variants; while others give specific rank to these more local entities.

 

These two portmanteau species contain the following entities which are relevant to temperate climates:-

a) the several variants with black berries (sometimes with a bloom which makes them appear blue or blue-black): if these are determined, in toto, as the species S. nigra – then the entities included are S. australis (eastern South America) S. canadensis (eastern North America) S. caerulea (western North America) and S. nigra (Europe and adjacent Asia) and

b) a complex of red-berried variants which if similarly determined as S. racemosa will include the entities S. callicarpa (western North America), S. chinensis (montane, eastern Asia), S. latipinna (Korea and adjacent Siberia), S. microbotrys (montane, south west North America), S.  pubens (north of North America), S. racemosa (northern Europe and adjacent Asia), S. sieboldiana (Japan and Korea), S. tigrani (Caucasus) and S. williamsii (north eastern Asia).

 

All of these entities are sufficiently distinct to come ‘true from seed’: thus for the purpose of these notes they are regarded, for all intents and purposes, as distinct species.

 

The Elder, Elderberry or Elderflower (the genus Sambucus) is readily recognised by the distinctive smell of the vegetative parts, its rapid and succulent annual growth with a virtually hollow pith and its prolific and dense flowering habit with its particular aroma.  There are many selected variants (chiefly of the foliage) which are grown as ornamental plants.

 

The plants vary from small shrubs up to trees which may reach eight metres in height. They are deciduous and produce fairly large, pinnate leaves with the leaflets having serrated margins.

 

The flowers are produced prolifically in early summer and these are often harvested and used as a flavouring. Each individual flower is small, having five petals and is white or cream (occasionally pink) in colour: the individual flowers are carried in dense clusters of variously structured corymbs. They are capable of self pollination.

 

The fruits, which are fleshy, berry-like drupes, are also produced prolifically in dense clusters. The fruits, of suitable species, can be used for cordial, wine making and flavourings. Each berry contains three to five seeds referred to as ‘stones’.

 

The fruits – which mature in the early autumn - should be collected as soon as they are ripe (or just before); they are quickly taken by birds once they reach a palatable stage. As the fruits are produced in dense clusters, and the shrubs are, usually, easily accessed, they are readily gathered by hand.

 

Extraction of the fruit can be achieved in two ways either:–

a) by macerating and fermenting the fruit clusters in warm water, when the seeds will eventually fall to the bottom of the container and the dross and liquid can be decanted off;

or

b) by drying – by separating the berries from the woody parts of the cluster and placing them on a sheet of paper in a dry sunny place – when completely dry the whole can then be lightly crushed when this dry flesh will turn to dust and can be blown off.

 

The seed coat (endocarp) is reasonably hard but is not impermeable (this provides protection while in passage through the bird’s gut). Normally the seed coat does not develop a ‘hard’ (ie impermeable) condition (however see below) but the latter treatment may however, marginally, induce such a condition if drying is excessive.

 

At seed maturity (ie when it is dispersed) the embryo is only partially developed along the length of the seed and before it can germinate it must develop to complete the entire length of the seed.

 

Despite the received wisdom that the coats of Sambucus seeds are not permeable to water, recent evidence suggests that this is not so. Like many Viburnums this scenario was perceived as the cause of delayed germination when the immature embryo condition was not recognised. Nevertheless the seed coat does not allow rapid water uptake and full imbibition requires the availability of water for seven days at above 20˚C before full water status of the seed is achieved.

 

All the relevant species exhibit an immature embryo and have a chilling requirement. The pattern of dormancy characteristics however do not appear to be relative to the particular taxonomic group but are a reflection of the environment in which the particular entity has evolved - regardless of the taxonomic position - with particular species from both groups showing the same responses in particular geographic areas.

 

This particular phenomenon is exemplified, firstly, by S. caerulea (from the S.nigra group) and S. callicarpa (from the S. racemosa complex), which are both native to western North America. Both species have immature embryos @ +/- 75% of the seed length and therefore need to mature the embryo, subsequently they require chilling to overcome physiological dormancy and will then be able to germinate. Unusually all three parts of this sequence is optimal at a single temperature regime at 5˚C. A second scenario is provided by S. canadensis (from the S. nigra group) and S. pubens (from the S. racemosa complex) which are both native to eastern North America. These two species have an immature embryo @ +/- 60% of the seed length and maturity of the embryo is achieved, optimally, by exposing them to an environment of 25˚C - S. canadensis for 84 days and S. pubens for 42 days – this is followed by chilling for 84 days (for both species) to eliminate physiological dormancy and then (at 20˚C) achieve germination.

 

Of the Eurasian species only S. racemosa (sensu stricto) from Sweden has been assessed and this showed more or less the same pattern as the western North American species (see above).

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It would appear from reliable evidence that most (if not all) of the species not only require light for germination but that light might also be beneficial during the stratification period.

 

Viburnum

 

The genus Viburnum is relatively large (about 135 species) and widely distributed group of ornamental plants - both evergreen and deciduous. They are mostly shrubs but some grow into small trees. The Wayfaring Tree (V. lantana) and the Guelder Rose (V. opulus) have a European distribution and both are native to the British Isles. The genus has a distribution throughout the Northern Hemisphere and also below the equator in South East Asia and South America – in the tropics but at higher elevations.

 

The species are diverse in their characteristics - varying from deciduous in cold temperate climates to evergreen in warmer areas, from winter to spring to summer flowering and from smooth leaved to hairy leaved.

 

Flowers are produced in corymbs; the individual flowers having five petals, are white or pink and are often very fragrant; in some species the outer flowers of the corymb are modified as large sterile pollen guides.

 

Individuals of many species are more or less self sterile – or are functionally dioecious (eg V. davidii) and require a pollinator, of a distinct genotype, to produce a reasonable crop of fruits. In general it would seem prudent to grow groups of seedling plants together, in order to develop a significant level of viable crop.

 

The fruit is a single seeded drupe with an external, brightly coloured skin, a soft flesh and an internal, thin stony shell. The seeds are dispersed by a wide range of mammals and birds and the stony seed coat acts as a protection for the seed as it passes through the gut - as the flesh is digested off. This ‘hard’ seed coat is not, generally, impermeable to water.

 

The degree of ripeness and hence the palatability of the fruit is advertised to the predator/vector by colour – the skin of the drupe is initially green and then turns orange and progresses through red to blue, purple or black – the fruits of several species (eg the European V. opulus and the Asiatic V. betulifolium) however remain red. As many species in this genus have an unusual dormancy mechanism, it is prudent to determine the sequential development of the colour of the fruit so that, if necessary, harvesting can be geared to a particular stage.

 

In most species the first sign of fruit maturation – indicating that the embryo has developed as far as it is going to and the seed has isolated from the parent - is when the fruit begins to turn orange. It is this stage which provides the first opportunity for the fruit to be harvested without detriment and so provide the earliest opportunity for extraction of the seed and its transfer into the necessary treatments to overcome the dormancy mechanisms. 

 

The fruits should be hand picked – at the required stage – or before they are taken by birds. The berries are macerated, fermented in warm water for a few days, after which the water and dross is decanted off. The seeds can then be rinsed and cleaned and should not be allowed to dry further.

 

The mechanism of germination in Viburnum seeds is a more complex process than is found in most plants. In practice the seedlings of the cold temperate species do not usually germinate (emerge) until the second spring after an autumn dispersal. A characteristic usually associated with the need to degrade a hard seed coat before imbibition will occur and for germination to proceed. However in Viburnum the delay is usually caused by the time required (at a suitable temperature) for the maturation of an underdeveloped embryo which then proceeds, without further check, into the germination phase to the stage of radicle emergence.

 

Subsequently the embryo exhibits an epicotyl dormancy which requires a period of chilling before germination can continue when temperatures are again suitable for growth. Thus the natural sequence of temperature regimes (seasons) and the time scales involved dictate the time of emergence. Depending on their local environment, the great majority of species from warm temperate climates do not require chilling.

 

When the seed has completed its development (to the dispersal stage) it has an ‘underdeveloped linear embryo’ which initially has to grow (mature both physiologically and morphologically)) to the stage at which radicle development and emergence can occur. This phase is reached after a period of exposure to warmth (warm stratification) and takes place in two stages – a period to complete a physiological conditioning and then a period during which the embryo will develop morphologically to a level of maturity at which the radicle will develop. The radicle will then develop, provided that the temperature regime remains suitable and will normally emerge from the seed and potentially develop a root system.

 

The epicotyl/plumule, at the stage of radicle emergence, will also begin to develop but is arrested until the embryo has experienced a specific period of cold temperature and will then develop further only when temperatures rise and reach a particular threshold.

 

Under natural conditions this strategy means, in practice, that the seed does not germinate to complete emergence until the second spring after dispersal.  This time scale can be foreshortened sufficiently for germination to be achieved in the first spring after dispersal if the fruits are collected well before they ripen.

 

The period necessary for the maturation of the underdeveloped embryo is not particularly extensive. Thus if the fruits are collected and the seeds are extracted at an early stage of fruit development (ie after the seed has reached the stage in its normal pattern of development and has become isolated from the parent) the seeds can be warm stratified to mature the embryo. This maturation period will normally be between 56 and 84 days at 20˚C – according to the particular species – and this will be sufficient to achieve the maturation of the embryo to the stage of radicle emergence. Because not many of the species have been assessed it will be prudent, in this range of time scale, to monitor the sample regularly so that when radicle emergence does occur the sample can then be transferred to the chilling regime.

 

The period required to mitigate the chilling requirement is also variable – according to the provenance of the species – in northern species it will be about 84 days at 3˚C.

 

Subsequently germination will not occur until temperatures rise to a threshold temperature. Most species will respond most satisfactorily at c20˚C. This implies that the chilling can overrun - as further development will not occur at low temperatures.

 

However those Viburnum species with a warm temperate or warmer distribution do not appear to exhibit an epicotyl dormancy condition – which requires a period of chilling (even at warmer temperatures) - as they will proceed to seedling emergence without chilling as long as temperatures remain above the threshold for normal growth patterns to proceed. There is however still a delay between radicle emergence and the emergence of the plumule – again this is species specific and can be as much as 42 days (viz V. treleasii – an endemic to the Azores). It would appear that there are no species (of these provenances) in which a chilling period is necessary before the warm period.

 

Some species have an extensive north-south distribution and the responses depend on the particular provenance of the sample. V. dentatum, for example, has a distribution from Massachusetts to Florida and east into Texas – in the north the requirements are conventional and the partially developed embryo requires chilling to achieve emergence – whereas the populations in Florida and Eastern Texas do not require a chill.

 

During the process of seed coat maturation, while the fruit is ripening, some species (eg V. lantana) will develop a hard seed coat, which although marginal and limited in degree, will need to be addressed. If, however the seed is collected at an early stage of fruit maturity (and then warm stratified), this condition will be avoided and hence does not become an issue. The evolution of this condition presumably reflects the greater corrosive effect of the digestive processes experienced in the gut of the particular vector of the seed.

 

The seeds, when ready to sow (after chilling), already have an emerged radicle but are big enough to sow individually and this should be done with due care.