Legumes

 

The Plant Family name Leguminosae - which was once widely accepted - is now more often replaced, in modern systems of nomenclature by the name Faboideae or Fabaceae. This huge Family (700+ genera and 19000+ species) of plants consists of both woody and herbaceous subjects. The seeds - especially in the woody species from temperate climates - are characterised by being, in some degree, impermeable to water when they are dispersed. This situation becoming enhanced as further drying occurs. Most of the species develop the seeds in a longitudinal pod (the legume) - which dehisces and disperses the seeds by an often violent ‘explosive’ action. Relatively few of the woody genera are native to temperate climates although, of those that are, many are prominent in Mediterranean and similar climates and hence will succeed in milder areas of the British Isles. A few have a cold temperate distribution and are hardy (eg Cytisus, Laburnum, Spartium, Ulex).

 

The Family is divided into three sub-families:-

a) the Caesalpinoideae are chiefly trees which are found in the moist tropics; however a very few genera have a distribution in warm temperate climates and the relevant genera (ie which are potentially possible for cultivation in the UK in mild niches) include Cassia, Ceratonia, Cercis, Gleditsia and Gymnocladus. Root nodulation is rare in this group.

b) the Mimosoideae contains the smallest number of genera. These genera all occur in warm climatic areas - and of these only Acacia, Albizzia, Calliandra and Mimosa are relevant for milder niches in the UK climate. They are characterised by the flowers having small petals and large, usually brightly coloured, stamens – which produce the recognisable ‘fluffy’ flowers.

c) the Papilionoideae is the largest sub-family – including both woody and herbaceous species - with a distribution throughout the world - mostly in warm climates but with several genera which have adapted to temperate climates and are suitable for cultivation in relevant areas of the UK:- Adenocarpus, Amorpha, Anthyllis, Caragana, Carmichaelia, Castanospermum, Chorizema, Cladastris, Clianthus, Colutea, Coronilla, Cytisus, Desmodium, Erythrina, Genista, Hardenbergia, Hovea, Indigofera, Kennedya, Laburnum, Lespedeza, Maackia, Ononis, Petteria, Piptanthus, Podalyria, Pueraria, Pultenaea, Robinia, Sophora, Spartium, Ulex, Virgilia, and Wisteria. The flowers are typically pea-like (ie five-petalled flowers arranged as a standard, two wings and two constituting the keel). The symbiotic nitrogen fixing root nodules are well developed and common in this group.

 

Most of these genera are native to areas with warm temperate climates – these have high summer temperatures and often experience dry or arid conditions - hence the hard seed coat condition protects against excessive water loss and in nature its degradation is often also associated with high temperatures or even wild fire.

 

The pods can, in general, be collected just before they are ready to dehisce and disperse the seeds. At this stage further drying in paper bags allows them to open and at the same time entrap the seeds as they are dispersed – this is especially significant if dispersal is as a result of the explosive dehiscing effect. When the process is completed the remains of the pods can be separated by picking over or by sieving - with the result that a very clean sample can be achieved.

 

In the short and medium term, the seeds can be stored dry in an airtight jar.

 

The only condition which will prevent germination in the seeds of this Family is the presence of a water impermeable seed coat – there are no endogenous dormancy conditions.

 

Inevitably there is, in such a large Family, a tremendous range in the shape and size of the seeds. The size may vary from large seeds eg Gymnocladus dioicus (0.8-1.0K seeds per kg) through Wisteria sinensis (1.6K seeds per kg) and Gleditsia triacanthos (7K seeds per kg) to Robinia pseudacacia (60K seeds per kg). Usually the seeds are more or less spherical but can be flattened in a disc shape. The seed coat is generally hard, brittle and bony in some degree but occasionally it is more of a leathery texture.

 

It is not unusual for seeds, in this Family, with a hard seed coat for this feature to become enhanced if the seeds are not processed reasonably quickly and are allowed to dry even marginally. So that seed treatment, for a particular species, will often vary according to the age of the sample and the storage conditions provided - there will be no hard and fast rules. It may be possible to extract the seeds at an earlier stage but this process will usually be tedious and unrewarding.

 

Seed treatments to create water permeability

If seeds are very dry the initial stage of imbibition can be very rapid and may cause injury to microscopic structures in the seed.

 

When the seed coat is made permeable, the seed will imbibe water quickly and swell, indicating that the particular treatment has been successful.

 

In order to achieve a sufficient seed coat degradation for imbibition to proceed, it may be necessary to process through a series of treatments – grading up the scale until success is achieved:-

a) soaking the seeds in warm water (20-30˚C) for up to 48 hours, removing the seeds from the water once it is evident that the seeds are beginning to swell.

b) hot water treatment - pour five times the volume, of the seed sample, of boiling water over the seeds and leave to cool and soak as in a); repeat for a second time if there is no response.

c) percussion treatment (agitate the seeds in a metal drum or glass jar, so that they bounce vigorously, for several minutes), this activity should cause the dislodgement of a strophiolar plug or alternatively may cause the brittle seed coat to crack, follow with a warm water treatment as in a).

d) subject the seeds to a dry heat treatment regime, by placing the seeds on a metal tray mixed with sand and place in an oven at 90-120˚C for  five to ten minutes, this will cause the seed coat to crack. Higher temperatures will cause damage to the embryo but it will be necessary to monitor the time scale to achieve the required result, follow with the warm water soak as in a).

or

e) scarification - agitate the seeds in a drum lined with sand or glass paper (or similar) or in a suitable container with an abrasive material (a very ‘sharp’ angular grit) until the thickness of the seed coat is abraded down to a level at which water can penetrate (only a very small area of the seed coat needs to reach this stage); follow with a warm water treatment as in a).

 

With seeds which are large enough to handle individually the seed coat can be chipped manually either by paring away a piece of the seed coat with a knife or rubbing on a rasp. They can then be imbibed by warm water soaking.

 

The seeds of the majority of the species of temperate climate, leguminous woody plants are big enough to be sown individually. They can be sown in modular containers quite successfully - as viability is usually high and seeds which have not imbibed can be discarded.

 

Once the seeds have imbibed, germination is usually rapid at temperatures of 20˚C+ - often in the order of only seven to ten days.

 

Seedlings of Wisteria are normally produced for use as one year old rootstocks for grafting during the following winter. They are most satisfactorily sown and grown in a conventional ameliorated soil seedbed in the spring. The seeds are sown individually at a density of 400 per square metre. As the viability and size of the seeds can be variable it is prudent to grade out the smallest seeds and to imbibe the remainder of the sample before sowing - so that seeds which do not swell can also be discarded. The top growth produced by a seedling Wisteria in its maiden year is sparse but the hypocotyl and tap root will have made sufficient size for use as a rootstock. Wisteria seeds, at dispersal, have a waxy impregnation in the outer part of the seed coat which protects both against water loss and imbibition in the short term and hence postpones germination until the following spring, this material however is readily removed with the warm water soak.