Papaver dubium L., Long-headed Poppy
Account Summary
Introduction, archaeophyte, a very rare casual. Eurosiberian southern-temperate, but very widely naturalised in both hemispheres.
1902; Abraham, J.T. & McCullagh, F.R.; around Newtownbutler.
August to September.
Growth form and preferred habitats
As with P. rhoeas (Common Poppy) dealt with above, nowadays the weedy annual P. dubium and its two subspecies only merit the status of rare casuals in the flora of Fermanagh. The same is true in many other areas of Britain and Ireland where this once common plant has markedly declined, being almost eliminated by the combination of modern agricultural seed cleaning and the use of herbicides. The stations of P. dubium now tend to be rare or scarce, and generally the plant is transitory and confined to recently disturbed soils on roadsides, urban or residential areas, disused farmland, lough shores, calcareous screes and gravel pits.
The most recent edition (8th) of the Irish field botanist's An Irish Flora (2012), lists subsp. lecoqii (also regularly spelt lecoquii) on the basis that it differs from P. dubium in containing yellow rather than white latex, and bearing brownish to bluish anthers rather than yellow anthers. However, Parnell & Curtis (2012) remark that "other supposedly distinguishing characteristics appear to be unreliable". Since the BSBI's New Atlas maps P. dubium species and its two subspecies separately, we will follow it and list the record details of the eleven Fermanagh records in that manner. There are sufficient similarities between the subspecies, however, to make it sensible to discuss them together in this one species account.
Soil requirements
The substrate requirements of P. dubium and its subspecies are fairly dry, or at least free-draining, light, sandy or clayey soils, which again, as for P. rhoeas, absolutely must be subject to some degree of disturbance. The brief species accounts in the New Atlas suggest that subsp. dubium favours light calcareous soils and that subsp. lecoquii differs in colonising heavier ones (P.J. Wilson, in: Preston et al. 2002). It is uncertain just how reliable a feature this is, but it might possibly assist identification decisions based on the morphological characters of latex and anther colour mentioned above.
On account of the essential pioneer colonising behaviour of the species, and the necessity of soil disturbance and open ecological conditions for its growth, P. dubium, like Common Poppy, used to occur frequently in arable land before the advent of herbicides, and previously these two Papaver species regularly coexisted.
Variation and identification difficulties
Whatever the ranking of the taxa, the morphological similarity between P. dubium and P. lecoquii is such that most recorders experience great difficulty in distinguishing them, despite the fact that they do have different chromosome numbers – ie subsp. dubium is hexaploid while subsp. lecoquii is tetraploid. Kadereit (1990) advocates their separation mainly by the colour of the latex, "This is white or cream when fresh, and brown to black when dry in subsp. dubium, but mostly yellow or turning yellow when fresh and red when dry in subsp. lecoquii." However, another study undermines this identification character and echoes Stewart's (1888) doubts on the matter in N Ireland. Some forms or races of otherwise P. dubium morphology have been encountered in Europe that contain yellow latex (Koopmans 1970). The separation of P. dubium and related taxa is certainly difficult and, from the above, clearly reliance on the colour of the contained latex colour alone is unwise.
Kadereit (1990) also points out that despite the difference in ploidy levels, hybrids between the two forms can easily be produced. Furthermore, the hybrid form carries out regular but unequal meiosis, indicating that chromosome homology and pairing does occur. It therefore seems likely that subsp. dubium originated as a hexaploid directly from tetraploid subsp. lecoquii without the participation of any other taxonomic form.
Other distributional differences between the forms in C Europe suggested to Kadereit (1990) that subsp. dubium might be a native of SE Central Europe, and subsp. lecoquii of W. Anatolia and SE Europe. This idea is very much built upon his assumption, however, that Papaver species in this taxonomic section of the genus predominantly have vicarious distribution patterns (ie they occupy separate areas that do not overlap to any great extent). On the other hand, it is very clear from the studies of McNaughton & Harper (1964) and of Harper (1966), that the five archaeophyte (ancient, introduced) forms of 'cornfield' poppy which are found in the British Isles, "represent a progression of more and more northerly distribution, but that there is nothing in the distribution of these species which suggests any geographical or a regional displacement of one species by another." (Harper 1966, p. 27).
Wild and crop weed forms
Is it the case that the wild relatives of our cornfield poppies have separate, distinct species ranges, while the several forms closely associated with man and with cultivation typically co-exist and overlap in distribution with one another? As a measure of the distributional overlap and association between poppy species in S and SE England, McNaughton & Harper (1964) tabulated the number of stations in the total sample of 75 in their study, where the species co-existed; P. rhoeas and P. dubium did so in no less than 33 of these stations, while in an additional four stations they were joined by P. lecoquii (McNaughton & Harper 1964, p. 771, Table 3).
Fermanagh occurrence
As the coincidence tetrad map indicates, both poppy species, P. dubium and P. rhoeas, are widely scattered across Fermanagh, but they display a slight concentration in the SE of the county. There are just six records of P. dubium s.l. in the Fermanagh Flora Database, and a total of seven for the two subspecies. The details of the other four records of P. dubium s.l. are: pile of topsoil beside a house, Lisnagole, 2 km S of Lisnaskea, 3 July 1995, RHN & RSF; disused farm to SE of Clonmackan Bridge, 25 August 1995, RHN; roadside at Whitepark, 2 km NW of Lisnaskea, 8 September 1995, RHN & HJN; roadside N of Killymackan, near Teemore, 25 July 2000 and 11 July 2004, RHN.
The details of the three records of Papaver dubium subsp. dubium L. are:
The details of the four records of Papaver dubium subsp. lecoquii (Lamotte) Syme (Yellow-juiced Poppy) are:
Northern Ireland occurrence
In the pre-1940 period, in the 2nd edition of the Flora of the North-east of Ireland which covers Cos Down (H38), Antrim (H39) and Londonderry (H40) – an area of Northern Ireland at that time very much better recorded than Fermanagh – P. dubium and P. lecoquii were regarded as being about equally frequent (Praeger & Megaw 1938). Fifty years previous to this, in 1888, dealing with the same NE region of Ireland, S.A. Stewart wrote of P. dubium occurring, "on sandy cultivated fields, and sandy or gravelly waste ground; abundant in Down, less common in [Cos] Antrim and Derry." Sagely, he also commented, "Our plant seems to be var. lecoquii of Lamotte, but scarcely distinguishable. The colour of the sap does not appear to be a reliable guide." (Stewart & Corry 1888). In the latest, 3rd edition of the same Flora, Hackney et al. (1992) wrote of P. dubium agg., noting that it was, "less frequent than before", and was, "recorded from only eight 10 Km squares [ie hectads] in Co Antrim."
Interestingly, in view of the slight tendency for Fermanagh sites of P. dubium to occur in towns and villages, in their survey of the Urban Flora of Belfast, Beesley & Wilde (1997) found P. dubium was widespread on disturbed and waste ground in the city.
The effect of competition
In the late 1950s, when McNaughton and Harper's pioneering work on plant populations was carried out, it required very detailed experimental study and careful analysis to discover that density dependent self-thinning of poppy populations was more intense within a single species than between species (Harper 1966). This selection pressure, added to the range and degrees of plastic response which poppy species produce in many individual characters (including some normally quite conservative floral characters) when they are reacting to various forms of environmental stress, taken together often enable or allow plant survival in the population, rather than necessarily always leading to thinning mortality. This in turn then helps us to account for the frequency with which mixed stands of poppy species are found in the field.
The broad similarity in range of habitats and the weedy characteristics shared by all poppy species, contrasted with the subtle differences in their requirements, their ways of using the environment and in their breeding systems, is also very striking. This is very clearly reflected in the distributions in Britain and Ireland of P. rhoeas (a strongly self-incompatible outbreeder, extremely plastic in phenotype) and P. dubium subsp. dubium – which is facultatively self-compatible and almost exclusively inbred, but not quite so phenotypically variable as P. rhoeas (Rogers 1971).
These two poppy taxa are remarkably alike in their ecology, both being opportunist colonisers of relatively dry, bare ground, and prior to the use of selective herbicides, both plants were widespread major weeds of annual arable crops in Britain and Ireland. For these reasons, they extensively overlap in their distribution, but P. dubium is the better represented of the two in Wales and further north, and especially so in Scotland. It is also better able to transcend the altitude limit of arable agriculture (Preston et al. 2002).
Flowering
P. dubium flowers from May to July, and the flowers are fully self-compatible, although in an experimental natural selfing, seed set was measured at only between 20 and 30%. The anthers are borne well below the level of the stigmatic disk in the flower, an arrangement which clearly assists the possibility of outbreeding. Even so, Rogers (1969) found that the seed set possible by assisted self-pollination was 41% of the average set in normal 'open pollination' involving insect visitors which, in view of the relative positions of anthers and stigma already mentioned, is a surprisingly high figure. There was, however, great variation in the numbers of seed set as a result of selfing between trials, and this may indicate population differences in the degree of inbreeding success that are likely to be encountered within this species.
In P. lecoquii, the flower's sexual organs are at the same level and self-pollination, commencing in the dangling unopened flower bud, is more likely to be the norm. In this case, Rogers (1969) found the average number of seed set by self-pollination in P. lecoquii was 61% of the open pollinated average (Rogers 1969, p. 56, Fig. 1 and p. 59, Table 2b).
Reproductive strategies and genetics of colonising weed species
Success as an opportunist coloniser of bare, disturbed or cultivated ground requires a high degree of adaptation, and the gene combinations which confer this on plant species are generally maintained by three inter-related mechanisms: self-compatibility, rigorous inbreeding and low levels of genetic recombination (Baker 1959). Certainly P. rhoeas does not conform to this pattern of breeding behaviour, and to a much lesser extent P. dubium and P. lecoquii do not fit it well either. Rogers (1971, p. 274) argues that while the premium for the opportunist coloniser is on adaptations such as rapid germination and establishment, a short vegetative phase and a high reproductive capacity (ie large seed output), the most fundamental characteristic enabling success in this particular form of weedy lifestyle is flexibility. This especially means the ability to mature successfully and reproduce, irrespective of seasonal, climatic or soil variations, or indeed the effects of interspecific competition. Baker (1974) describes this combination of characteristics as, "releasing the weedy species from restrictions on its range of tolerance of abiotic environmental variation", and he refers to the ideal weed characteristics as the development of, "a general purpose genotype (or genotypes)".
Some degree of genetic heterozygosity permitting the flexibility found in these poppy species (and which is particularly marked in the strongly outbreeding and ecologically diverse P. rhoeas) would also be of great advantage to opportunistic colonising species. Research indicates that the balance between inbreeding and outbreeding and how it is achieved is different in each species. For instance, in the case of P. dubium, the degree of inbreeding has been shown to vary from one population to another (Rogers 1971). It is likely that in some cases, only an occasional outcrossing event is required in order to maintain the required level of heterozygosity that enables adequate phenotypic flexibility. This is particularly so if the species has both a very high seed output and a long-lived dormant soil seed bank as a fallback genetic resource (Snaydon 1980). All poppy species appear to meet these two criteria. P. rhoeas is rigorously outbreeding and so far there is no evidence of any breakdown in its incompatibility mechanism. The fact that it has a high degree of genetic heterozygocity is obvious from the wide range of variation shown in many different morphological characters. The really significant question we must ask is, How does P. rhoeas maintain a genotype, or more likely, a range of genotypes which confer adaptive advantage in open, disturbed habitats typical of crop situations, without having to resort to inbreeding? (Rogers 1971; Baker 1974).
Firstly, it should be clear that the characteristics of the ideal weedy species, as delineated by Baker (1974), do not require that the weed population should contain only a single genotype, and individuals certainly do not need to be completely homozygous, even when selfing and inbreeding prevails and selection pressure is at its most severe (Baker 1974). If this is the case, then the same thing may apply in reverse, ie outbreeding, and the associated chromosomal heterozygosity, need not imply absolute gene heterozygosity. We can then postulate that over time, selection pressure would gradually remove less well adapted recessive homozygotes, reducing the frequency of recessive alleles in the populations of the particular weed species. In turn, this would also favour a low mutation rate in the dominant habitat-adapted alleles. Most phenotypes would then as a consequence approximate to the optimum for strongly selected characters, while still maintaining a higher level of heterozygosity in other characters that are subjected to less rigorous selection pressure (Rogers 1971).
Survival of buried seed
In an experiment studying seedling emergence and seed survival of four poppy species (P. rhoeas, P.dubium, P. lecoquii and P. argemone), seedling numbers of all four were greatest in the first year following their production and burial in the soil. Thereafter, germination decreased exponentially from year-to-year and while all species had viable seed after five years, P. dubium showed the slowest rate of decline in seedling emergence. The numbers of seed which remained dormant and viable tended to be greatest for P. lecoquii and P. dubium (Roberts & Boddrell 1984). It is probably safe to conclude from this, that seeds of P. dubium and P. lecoquii are capable of remaining viable in the soil seed bank for a long period, certainly for more than five years, and possibly as long as has been shown for P. rhoeas, ie 80 years or more.
Genetics, numbers and survival
The seed output of poppies such as P. rhoeas and P. dubium is so vast, and their seed longevity in the soil is sufficiently long, that these poppy species generally have an enormous surplus of genotypes available to provide the next generation. The proportion of the seed which produces the next generation is correspondingly small, and a small proportion of ill-adapted phenotypes, even if they were somehow to survive and breed, would be unlikely to have any significant effect on the ultimate population size, or on the success of the species as a weed (Rogers 1971).
Pollination and seed dispersal
The flowers of P. dubium and P. lecoquii attract mainly bees and hover-flies which feed on the plentiful pollen mostly in the early morning. After pollination, the capsule requires five to six weeks to ripen before the pores open to shake out the seed. The pores are quite large in relation to seed size, but even so dispersal distances are likely to be only a few metres even under optimum wind conditions for release and transport. The seed show marked dormancy, with few or none germinating within the first months after shedding (McNaughton & Harper 1964).
Toxicity
As with other poppy species, plants of P. dubium and P. lecoquii are undoubtedly able to deter some browsing herbivores by means of their toxic or distasteful alkaloid content.
European occurrence
In Europe, P. dubium is widespread over much the same area as P. rhoeas, from the Mediterranean, through W & C regions and with more of a presence in the north, extending on both shores of the Baltic to 60N and slightly higher than this in on the Swedish shore (Jalas & Suominen 1991, Map 1989). According to McNaughton & Harper (1964), the northerly spread of P. dubium in Scandinavia is due to the discharge of ships' ballast containing the seed.
World occurrence
Beyond Europe, again like P. rhoeas, P. dubium s.l. stretches from N Africa, through SE Asia and eastwards to reach Nepal. As a previously very common crop seed weed contaminant, it has also been widely introduced by agriculture to both N and S America, Fennoscandia, Australia, New Zealand and Tasmania (Hultén & Fries 1986, Map 888; Jonsell et al. 2001).
Names
The Latin specific epithet 'dubium' means 'doubtful' or 'uncertain' which in view of the identification problems associated with it and P. lecoquii, is all too apt (Gledhill 1985). The name 'lecoquii' is clearly called after someone, and the most likely candidate appears to be Henri Lecoq (1802-1871), a French botanist or plant geographer. There is a genus of the Apiaceae (= Umbelliferae) called 'Lecoqia', 'Lecoquia', or 'Lecokia' found in the eastern Mediterranean, which perhaps is another such memorial (Willis 1973).
Threats
While both P. rhoeas and P. dubium have clearly declined over their entire synanthropic range following the advent of modern seed cleaning procedures and the use of selective herbicides, it still remains possible that these poppy species may produce herbicide resistant forms, as has already happened in over one hundred other crop weeds (Warwick 1991; Briggs & Walters 1997).