Problems for cultivation
Origins and evolution
The genetic system
Implications for breeding
Dr Horrobin will be talking to you shortly about the medical and dietary potential of the oil extracted from the seed of the evening primrose. By way of introduction, I would like to tell you something about the plant itself, and the problems which are facing us in its domestication.
To start at the beginning, the plant is called the evening primrose because, firstly, the flowers are primrose yellow, and secondly the flowers open in the evening, with each flower lasting for only one day. Taxonomically speaking, the plants which we are interested in comprise some 10 - 12 closely related species within the subsection Euoenotherae, section Oenotherae, genus Oenotherae, family Onagracea. The nearest commonly known relatives are the willows herbs, the garden plants Clarkia and Godetia and, more distantly, the Fuschias. Let no one therefore say that there is no Fuschia in evening primrose!
Top of page
The evening primrose is effectively a wild plant and, as such, presents a number of problems for cultivation. I would now like to consider some of the more important of these.
In the wild, the evening primrose rows as a primary coloniser. That is to say that it is able to rapidly exploit patches of bare ground, but doesn't compete well in an established sward. It has therefore evolved a system of pronounced seed dormancy so that the seed can remain in the soil for many years if necessary, waiting for the right conditions to germinate. This is however the exact reverse of what is needed in a crop plant. The seed dormancy means that the farmer is unable to control the density of his crop simply by varying his seed rate, since the rate of emergence will be determined by the conditions after sowing - conditions largely outside of his control. There is also, of course, a much greater risk of crop failure due to very low establishment. In addition to this, dormant seed which doesn't germinate immediately, together with further seed shed onto the ground at harvest can remain in the soil and pose problems as a weed in following crops.
The evening primrose plant is a biennial, that is, its life cycle takes two years. In the wild, seed is shed in the autumn and remains dormant over winter. In the following spring/summer a proportion of that seed germinates and the seedlings form into rosettes - plants with greatly foreshortened stems which radiate their leaves flat to the ground from a central point, like plantains or dandelions. It is in this form that the evening primrose plant overwinters and it gains a positive benefit since if there is any snow the plant will be covered and so insulated from low air temperatures. In the following spring the plant bolts and produces a plant some 3-6 feet tall which flowers and produces the seed and then dies. To mimic this cycle in the field would clearly be expensive since the evening primrose would be occupying the ground for two complete growing seasons, and the final yield would have to compete economically with two harvests from other crops. The problem can be circumvented by timing the sowing in late summer/early autumn so that the crop has enough time to produce a rosette large enough to survive the winter, but there is also enough time for the farmer to grow and harvest an early crop of, say, cereals. In this way the evening primrose is effectively occupying the ground for only one growing season.
Unfortunately, life is not that simple, since one can easily run into problems with bolting. There are a number of factors, such as day length and temperature, which influence the timing of the bolting response, and the degree of stimulus varies between different varieties. If the variety bolts too readily, then the crop can bolt in the autumn whereon the plants are killed by the winter frosts. On the other hand, if the variety needs a strong stimulus for bolting then there is the risk that it not bolt at all in the spring and just continue growing in the rosette form. All is not lost, since these plants will usually bolt after the second winter, but it does mean that the crop will have occupied the ground for an extra 12 months, representing the loss of a complete year's production to the farmer. It is possible to overcome the problem by selecting varieties with an intermediate requirement, although the selection of varieties obviously has to be tailored to the environment and latitude in which the crop is to be grown.
When the plant is flowering, flowers open at the rate of 2-4 per day, and there is a `zone of flowering' which gradually moves up the stem as the season progresses. Since flowering can last for two to three months, this means that at one time you can have all stages of development from small buds, through flowers, to mature pods on the same plant. By the time that full maturity is reached, the pods from the first flowers to open will have been mature for a considerable time. This is serious because, as the pods mature, they gradually split and the ends spread apart. The seed is then scattered by the action of the weather and the attentions of numerous small birds. The farmer therefore faces a difficult decision regarding the time of harvest - one of the `heads I win, tails you lose' variety since, if he havests when all the pods are mature, he will lose most of his yield through seed shedding. On the other hand, if he harvests as soon as the first pods start to drop their seed, then he will lose the yield from all the pods which are still immature, not to mention those which have yet to flower. The answer to this is clearly the production of varieties in which the pods remain closed and the flowering period is shortened. Until these are available, farmers will have to continue to accept high seed losses at harvest, using their skill to try and minimise them.
In addition to these problems, there are of course all the aspects of agronomy which are largely taken for granted in established crops but which have to be painstakingly worked out for any new crop plant. For instance, one has to establish the optimum sowing date and the amount of seed to use, how much fertiliser and which compound to use and at which stage of the plants growth cycle particularly with regard to the oil content and quality in the seed. There is also the important question of weed control - which chemical herbicides are safe to use on evening primrose, and whether there are differences between different varieties (there are!). And of course one has to deal with the various pests and diseases, a problem which usually increases as the area devoted to a new crop grows.
As you might imagine, the answer to many of the problems I have outlined lies with plant breeding, and this is the key to increasing the intrinsic seed yield on the plants, the oil content of that seed, and the quality of the oil. However, before I talk about the breeding of the evening primrose, I would like to digress for a while and tell you something of the origins and evolution of the evening primrose.
Top of page
The evening primrose is unusual in that its evolution has been traced in considerable detail, even down as far as its genetic constitution, from its origin in Mexico and Central America some 70,000 years ago. During the intervening years, there have been a succession of ice ages which have swept down across North America, with intervening warm periods. At the end of one of these ice ages, the primitive evening primroses followed the retreating glaciers, taking advantage of the disturbed ground which they left in their wake, and in this way colonised much of the continent. During the next ice age, most of these colonies were wiped out but, as the ice moved northwards again, so a second wave of evening primroses from Mexico moved with them and recolonised the area. In the process, they encountered and hybridised with the remnants of the first wave. This was repeated for four ice ages, with four separate waves of colonisation, each hybridising with the remnants of the previous waves. This generated a present day population which is enormously rich in genetic diversity, spread right across the North American continent.
The process did not stop there, however, since when trade started with Europe, seeds of evening primrose were inadvertently carried with the soil used to ballast the ships returning to Europe. On reaching port the soil was dumped and colonies of evening primrose became established around the major ports. In this way, plants from different parts of North America were brought together and were able to hybridise to form yet more new plant types and a whole new, diverse population.
However, the most remarkable part is that, during the American phase, the evening primrose evolved a completely new and unique genetic system, very different from anything found anywhere else in the plant kingdom. This has profound implications for the improvement of evening primrose through plant breeding.
Top of page
There are three separate aspects to the genetic system of the evening primrose which I would now like to outline.
In normal plants, during the formation of the egg and pollen cells during meiosis, the chromosomes form into pairs and random translocations between them permits the shuffling of genetic material. This is the primary means by which new combinations arise in sexual reproduction. In the evening primrose however, at meiosis, the chromosomes do not pair up but join together end to end to form a large ring structure. Since the chromosomes only come into contact at their tips, translocation is prevented and so there is no exchange between them the genetic content of the chromosomes is 'fixed' and cannot change.
Secondly, in normal plants, when the diploid stem cells split to form the haploid egg or pollen cells, the paired chromosomes split into their component halves and there is no restriction on which half goes to which daughter cell. Since the plant was originally formed from two sets of chromosomes, one from the mother and one from the father, this means that the new pollen or egg cells will each receive a mixture of maternal and paternal chromosomes. In the evening primrose, on the other hand, when the ring of chromosomes breaks the chromosomes move in an ordered pattern so that one of the daughter cells receives only maternal chromosomes and the other receives only paternal chromosomes. No cells receive a mixture.
Thirdly, in normal plants, there is usually no restriction on the possible combination of genes, although some combinations are obviously more favourable than others. The evening primrose however has developed a series of balanced lethal genes of two types. In plants with the first type, the lethal genes kill any pollen which contains the maternal set of chromosomes, and any egg cells which contain the paternal chromosomes. In plants with the second type, both types of pollen and eggs are produced but after fertilisation the lethal genes kill all the embryos which have either two sets of maternal, or two sets of paternal chromosomes. For both of these types of lethal genes the net effect is the same. If the plant is self pollinated then the resulting seed will have a genetic compliment comprising of one set of intact maternal chromosomes and one of paternal chromosomes.
To rephrase all that, the evening primrose plant is produced from one set of chromosomes received from the egg cell at fertilisation, and one set received from the pollen. When it in turn produces its own egg and pollen cells, the ring structure prevents the transfer of genetic material between chromosomes, and the ordered movement of the chromosomes ensures that each new cell receives either a complete set of either the maternal or the paternal chromosomes. On self-fertilisation the lethal gene system ensures that the new embryos contain one set of each and hence that the seed will have the identical genetic constitution to the parent, regardless of the mixture of genes present. In other words, the evening primrose has developed a system which permits it to maintain its genetic constitution constant down the generations without any of the reassortment of genetic material which is the hallmark of sexual reproduction.
If the evening primrose plant is to keep its genetic complement intact, then it needs to ensure self-pollination, and it does this by allowing pollination to take place before the flower opens. In evening primrose the anthers split and the pollen is shed in the bud 1-2 days before the flower opens. However, in primitive races, the stigma (receptive female organ) is held clear of the anthers by the style which is elongated and, since the pollen is sticky, none reaches the stigma until the flower opens and it is carried by the action of insects or the wind. It is then a matter of chance whether the flower is fertilised by its own or foreign pollen. In the more advanced forms the style has been shortened so that the stigma is comp lately surrounded by the anthers. When the pollen is released, it falls directly onto the stigmatic surface and self-pollination is assured.
For such an elaborate system to have evolved it must confer some substantial advantage on the plant in its struggle for survival. The advantage for evening primrose comes in the form of a phenomenon known as `hybrid vigour'. Any gardeners among you may have seen this in varieties referred to as 'F1 hybrids', in which the seed is produced by the hybridisation of two carefully selected inbred lines to produce a particularly favourable gene combination and hence more vigorous, higher yielding and completely uniform plants. Some of you, possibly in response to the high price of this type of seed, may have been tempted to save the seed from some of the plants for sowing on in the following season. If so you will have found that the resultant plants were much less vigorous, lower yielding and anything but uniform - the hybrid vigour cannot be maintained through sexual reproduction in normal plants.
Some plants get round this problem by vegetative reproduction, in which the offspring have the identical genetic makeup to the parent. Examples include strawberries which produce runners, couch grass which throws up new plants from its root system, plants which use bulbs or corms, and many, many others. However these plants lose the dispersal possibilities which we provided by sexually produced seeds.
Evening primrose on the other hand is able to have its cake and eat it. With its special genetic system it is able to maintain any particular genetic configuration indefinitely over the generations, and at the same time is able to use seed for its dispersal. Since each plant can produce up to 150,000 seeds, this represents a considerable advantage. (Consider the gardening nightmare if couch or ground elder were able to produce 150,000 offspring each year!) It has to be said however that the evening primrose doesn't get everything its own way since there is a price that has to be paid. By preventing the constant reassortment off its genetic material through sexual recombination, the appearance of new gene combinations will be severely limited. Thus, in a changing environment, the evening primrose could only adapt slowly, if at all. In other words, evolutionarily speaking, it has gone up a blind alley.
Top of page
All this may be very interesting, but what relevance does it have to the evening primrose as an oilseed crop? The primary effect is on the production of improved varieties through plant breeding, and there we three main effects.
Firstly, and this is beneficial, the evolution in North America and later in Europe produced a very large natural pool of variation which breeders can use in their selection and breeding programmes.
Secondly, the genetic system of chromosome rings, ordered chromosome movement and balanced lethal genes has a profound effect on the behaviour of the progenies after hybridisation. In normal species, if you hybridise two different plants then you will get a large number of new forms in the progeny combining characters from both, the number depending on the degree of similarity between the two parents but frequently numbering in thousands. The breeder can then select potential new varieties from amongst them. In evening primrose however, if you hybridise two plants, several things may happen, but in many cases you will produce only two plant types in the progeny which obviously reduces the scope for selection drastically.
It is possible to some extent to circumvent the problem by choosing parents so as to break up the chromosome ring and thus permit some reassortment of genes. The ring then has to be rebuilt by making further crosses. This however requires a detailed knowledge of the genetic constitution of the plants to be used as parents and is very time consuming. It is also limited by a number of other factors including the lethal genes sterility and plastid incompatibility.
Thirdly, the possibility of having both self pollinating and cross pollinating types causes problems. Plant breeders conventionally use completely different methods for dealing with the two types of plant. The evening primrose breeder therefore has to select his techniques from both methodologies according to his particular material. As you can see, breeding for improved varieties of evening primrose is by no means a straightforward matter.
Top of page
Thinking back over this talk, I have spent most of the time telling you of the problems involved with evening primrose both with regard to its cultivation and its improvement. I have dwelt on these since they represent the task which we face in the domestication of this new crop plant. However, I don't want to give you the impression that, because of these problems, evening primrose will never be anything other than a minor, rather unusual, medicinal plant. My company, Hurst Gunson Cooper Taber Ltd., have been researching into and breeding evening primrose for some ten years now and, in our experience, none of these problems have proved to be insuperable. We currently have three finished varieties (Constable, Cossack and Commodore) which are protected by Plant Breeders Rights, with others to follow in the pipeline. Given the impetus of the medical results which Dr Horrobin will outline for you shortly, I am sure that substantial improvements will continue to be made.
I would like to leave you with one final thought. Wild strains of evening primrose are capable of producing anything up to about 15 cwts/acre of seed on the plant (although the harvested yield is of course much lower because of seed shedding), and this seed may contain up to 30% of oil. In comparison with current oilseed crops such as rape or sunflower this is not particularly good. However, if you compare it with the yield obtainable from the wild progenitors of these crops, then the boot is very much on the other foot. Therefore, if it proves to be possible to make anything like the improvements in the evening primrose which have been made during the domestication of other crop plants then evening primrose may well have a future out of all proportion to its current performance.
Top of page
|© Peter Lapinskas 1999-2012||Email Peter Lapinskas||Last updated: 3 July 2012|