Sinapis arvensis L., Charlock

Account Summary

Growth form and preferred habitats

A coarsely hairy, summer or more rarely winter annual, previously a widespread and abundant weed of farm cultivation throughout B & I, but which nowadays has hugely declined thanks to its susceptibility to selective herbicides. Together with the almost total absence of arable agriculture in Fermanagh, this means Charlock is now only occasionally found in the area, usually as solitary plants, typically in regularly disturbed, open, fully illuminated, lowland sites. The result of such a solitary or minimal presence is that the once ubiquitous weed is readily overlooked nowadays and is very probably under-recorded.

Having said that, Charlock, throughout B & I, appears to tolerate a wide range of soil types, preferring a medium texture clay of neutral or alkaline reaction, with a high to moderate lime content. At the same time, it can also be found on peaty agricultural soils of low lime content with a pH around 5 or so. What it abhors, in this respect however, are waterlogged conditions and both very light, dry terrain subject to regular drought, or very acid, cold, heavy clay or bog soils (Fogg 1950). In any soil or site, the disturbance associated with the activities of man are of primary importance and Charlock therefore has always been virtually confined to artificial habitats.

Well established S. arvenis individuals not subject to excessive competition will eventually produce a deeply penetrating tap root up to 74 cm deep and the species has a very rapidly developing and extensive fibrous secondary root system. Vegetative growth of the seedling plant is very vigorous in the early stages, so that it often outgrows the seedlings of other species, including crop plants (Mulligan & Bailey 1975).

Competitive ability

In general, S. arvensis is only weakly competitive and prefers open sites with bare, or disturbed soil available for colonisation. The competitive success and size of Charlock plants depends very greatly on the particular cohabiting species (singular or plural) involved, the density of the competing plants and the fertility of the soil, but in general Charlock has a pronounced depressing effect on the growth and yield of cereal crops (Mulligan & Bailey 1975, p. 179).

Identification

Identification features, variation and characters distinguishing the most similar yellow-flowered crucifer species with which S. arvensis might be confused are clearly spelt out and keyed by Mulligan & Bailey (1985), Rich (1991) and also by the latter author in the Plant Crib 1998 (Rich & Jermy 1998).

Variation

The vegetative growth of S. arvensis plants, their reproductive performance and even characters such as the depth of flower colour, all display a very wide degree of plasticity with respect to environmental conditions (phenotypic plasticity). For example, flowering stems frequently range from dwarfs only 8 cm tall, to comparative giants up to 90 cm in height (Fogg 1950).

Fermanagh occurrence

The Fermanagh Flora Database has records from a total of 42 tetrads, representing 8.0% of those in the VC. Thirty-seven tetrads have post-1975 records. The tetrad map shows that S. arvensis is widely but very thinly scattered throughout lowland Fermanagh, being slightly more frequent around Enniskillen. Charlock appears in both urban and rural disturbed waste ground and also on regularly visited, disturbed lakeshores, on field margins, waysides and in quarries.

When considering a local map displaying occurrences of a species like this one, we must remember that due to the scarce, occasional visits of recorders and the solitary nature of most of the plants discovered, the map may well underestimate the plant's true distribution. However, the map symbol size and the length of the period it represents can equally create a greater mental impression of the species presence than the recorder experiences in the field. This is illustrated rather well with S. arvensis by comparing the standard tetrad map and symbol size (2-km square) used in this Flora, with the equivalent 1-km square representation. The scarce and scattered nature of the species and its real decline in frequency is much more readily appreciated from the latter.

Many, or indeed most, of the recent occurrences of S. arvensis in the VC probably derive from buried seed emerging from the species long-persisting seed bank after local soil disturbance, eg in roadworks, by lakeshore jetties, on building sites and on waysides, especially those near sand and gravel quarries. Since so much of Fermanagh's ground is regularly covered with flood water or consists of damp to wet habitats such as acidic pastures and meadows, wet peat or scrub woodland, it is really not terribly surprising that suitable sites are scarce in the county for a ruderal, short-lived plant like S. arvensis which prefers open, unshaded, fertile, regularly but not excessively disturbed ground, with a high proportion of exposed bare soil (Grime et al. 1988).

Flowering reproduction

Individual Charlock plants normally require about two and a half to three months growth to order to mature and flower. The species is phenotypically extremely plastic, however, and almost irrespective of individual size plants flower very freely from March or April through to July. Occasional very late developing individuals can be found flowering even into December, although as with other species which behave in this way, these individuals are not likely to set much seed, if any (Mulligan & Bailey 1975). The yellow flowers form a tight corymb, which together with the sometimes dense clustering of the plants, makes them conspicuous to both man and insect. Additional insect attractants are a slight, sweet perfume, nectar and abundant pollen. The flower reflects not only yellow wavelengths but also UV, so the insect eye sees more detail and pattern in the flower than humans do. Each flower lasts just two days and Fogg (1950) reported numerous insect types visiting to collect nectar and/or pollen foods, including long-tongued bees, butterflies, flies and beetles.

Until recent years, it was generally but incorrectly assumed that S. arvensis was both cross- and self-pollinated and was self-compatible (Knuth 1906-9; Fogg 1950; Salisbury 1964, p. 144). During most of the last half of the 20th century, however, it has been realized that the species is completely self-incompatible (Bateman 1955; Mulligan & Bailey 1975). The tissue recognition mechanism central to the incompatibility was shown by Ford & Kay (1985) to involve sporophyte rather than gametophyte tissue − ie the proteins on the coat of the pollen grain are sampled by the female stigma (Proctor et al. 1996, pp. 324-5). In fact, S. arvensis has a single-locus, multi-allelic sporophytic incompatibility system similar to that found in almost all other self-incompatible members of the Cabbage family. Ford & Kay (1985) reckoned that there were as many as 24 allele forms of the one genetic locus controlling the incompatibility in Charlock. The efficiency and benefit of this type of breeding system is that it allows any individual plant to breed successfully with most other members of the population, but not with itself or with some of its nearest neighbours (Proctor et al. 1996; Richards 1997a, pp. 224-30). Since cross-pollinated plants rely on external agents for pollination and thus cannot actively choose their sexual partner, this form of last-minute mating selection carried out on the stigma gives the plants which possess it a considerable evolutionary advantage in the long term.

One important consequence of flower self-incompatibility in S. arvensis is that when the plant becomes rare and appears either solitary or in very small numbers as it currently does in Fermanagh, then seed set likewise becomes impossible, or less frequent and of a lesser order. Inevitably, any significant decline in seed production will hasten local population extinction and this is particularly the case with annual species which, by their very definition, are completely dependent on seed for their survival.

Seed production and dispersal

After fertilization the elongated, beaked fruit pods develop between 1-24 seeds, the average plant producing 1,000-4,000 seeds (Salisbury 1964, p. 186). Seed is released by the splitting of the pod and, in addition to the consequent adjacent scatter, numerous birds including Greenfinch and Bullfinch as well as stock animals such as cows dine on the pods and subsequently disperse the still viable seed in their dung (Ridley 1930, p. 440; Salisbury 1964, pp. 102, 104). In the past, seed was also accidentally long-range dispersed by man, as a crop seed contaminant.

Seed survival and germination

It has been appreciated for many years that Charlock seed have a remarkable capacity to survive for long periods in the soil seed bank (Roberts & Boddrell 1983), and viable seeds up to 60 years old have been recovered at depths down to 30 cm (Brenchley 1918). This pronounced seed longevity permits the species' survival in sites which are cultivated or otherwise disturbed, on both a regular and a very infrequent basis. It has been shown that some S. arvensis seed is ready to germinate immediately upon ripening in the autumn after it is formed, but the remainder possess innate dormancy, which is triggered, enhanced or enforced by even very shallow burial. Continuous variation was found in the germination progeny of individual plants, and segregation of single genotypes with reduced dormancy occurs, indicating that dormancy in Charlock is very much under genetic control (Witcombe & Whittington 1972).

The majority of seeds remain dormant in the soil for about 2 years, but there are also seasonal changes in the dormancy status of buried seeds. The capacity for germination is lowest in summer and early autumn, and it reaches a peak in the spring. Young plantlets of S. arvensis are sensitive to frost and except in areas of mild climate and in sheltered sites that are slightly disturbed, it appears that few seedlings of summer or autumn germinating Charlock survive overwinter in most parts of B & I (Fogg 1950; Edwards 1980). Their mortality is due either to frost, or to excessive disturbance, e.g., early winter or spring cultivation, or competition with more vigorous species. Thus it appears that the autumn generation of S. arvensis appears to have little influence on the capacity for survival of the species (Edwards 1980).

Population dynamics

In a population study in cereal fields in Leicestershire, climate was the major factor influencing Charlock population dynamics and reproductive capacity. While the persistent soil seed bank permits species survival in sites which are cultivated or otherwise disturbed only on an infrequent basis, Edwards (1980) estimated that at the particular sites she studied, local population maintenance required a minimum of one successful reproductive cycle every eleven years in order to top up the seed bank. Edwards also analysed a survey of published results from cultivated ground elsewhere and comparing this to her own findings, found the half-life of Charlock seed is usually around three years. However, there is considerable variation within S. arvensis from year-to-year in seed germination, dormancy and longevity.

Population outcome is often determined by factors such as competition from the crop, or from other weeds, in which case the frequency and degree of soil disturbance is important in favouring established, but not very competitive, S. arvensis plants. The effect of competition on Charlock plant size and reproductive capacity was shown to be more important in controlling the population than individual plant survival. This is undoubtedly due to the exceptionally prolonged seed longevity of S. arvensis and the large scale of the buried seed population − numerically much larger than the actively growing plant population; in this case, species survival depends mainly on the seed population (Edwards 1980).

Even more frequently than the influence of competition and soil disturbance, however, Charlock populations in B & I are limited by climatic effects including winter frost, early- or mid-summer drought, or unfavourable conditions for germination such as a late spring. Edwards found that the primary effect of climate was on the timing of germination and seedling emergence and establishment in the springtime. In her study, seedlings began to emerge when the mean weekly temperature at 10 cm soil depth was above 4.4°C and emergence always coincided with rainfall (Edwards 1980, p. 157; Roberts & Boddrell 1983, p. 306).

British and Irish occurrence

On the basis of the BSBI Atlas (Perring & Walters 1962), Rich (1991) suggested that Charlock is "virtually ubiquitous" in B & I. However, this overstates the real position. Somewhat surprisingly, the same major underlying habitat limitations seen in Fermanagh are well displayed on the New Atlas hectad map of these islands, which indicates (even at such a large scale) that S. arvensis is unrecorded in many western and upland areas of B & I where wet acidic soils supporting pasture grasslands, bog, heath or woods are the predominant forms of natural or semi-natural vegetation (Preston et al. 2002).

European and world occurrence

S. arvensis has been a common weed of cultivation in Europe including B & I since earliest times (Fogg 1950). Godwin (1975) lists three fossil sites in B & I from the Neolithic, Roman and Mediaeval periods. As with many such cereal weeds, the species is so closely associated with man and with disturbed ground, that it is not really possible to be certain about its geographical centre of origin or its native habitats. It is considered 'probably native' throughout most of Europe, especially in the Mediterranean basin, but it becomes rarer and more coastal further north in Scandinavia (Jalas et al. 1996, Map 2874). It has a similar status in N Africa, Asia Minor, SW Asia to the Himalaya and eastwards to Siberia. It is also introduced and widely naturalised in N & S America, S Africa, Australia and New Zealand (Clapham et al. 1962; Tutin et al. 1993). S. arvensis is also a casual in many of the Atlantic islands, including Iceland, the Faeröes, Madeira and the Canary Isles (Mulligan & Bailey 1985).

Historic uses

Young Charlock plants are palatable and were boiled and eaten as a green vegetable by poor people in the past from Classical times onwards. This practice continued in B & I until the early decades of the 20th century (Fogg 1950). Vickery (1995) quotes several Irish sources which describe the use of this 'famine food', one of which explicitly testifies just how nauseous the plant tasted. In Scotland, likewise, seeds were eaten in hungry times, being ground and used to make bread. The mustard oil was also sometimes used as a fuel or 'burning oil' (Grieve 1931, p. 570).

Toxicity

The plant is toxic, but only becomes so when it is mature and forms fruit pods. There are three poisonous principles, a volatile mustard oil, Allyl-iothiocyanate, which is released from Sinigrin, plus the alkaloid, Sinapine and an alkaloidal glycoside called Sinalbin (Fogg 1950; Cooper & Johnson 1998). Toxicity has been shown in a number of animals to which seed has in the past been fed, including chickens and, in pastures where mature Charlock was present, sheep, cattle and horses all suffered. When large amounts of either the plant or the seed were eaten the animals died (Cooper & Johnson 1998).

Names

The genus name 'Sinapis' is an ancient classical appellation given by Theophrastus to the mustard plant. Collaterally, it is sometimes spelt 'sinapi' and 'sinape' (Gilbert-Carter 1964). There is no apparent derivation of the name. The Latin specific epithet 'arvensis' is derived from 'arvum solum' meaning 'arable land' and thus conveys 'growing in or pertaining to cultivated fields' (Stearn 1992).

The plant has a plethora of English common names, of which Grigson (1987) lists 31 and Britten & Holland (1886) 56! The most widely known name 'Charlock' is also applied to Rhaphanus rhaphanistrum, although it is more often qualified as 'White Charlock', or 'Jointed Charlock' (Watts 2000). The name 'Charlock' comes from the Old English 'cerlic', or 'cearloc', which Grigson (1974) says is, "an old name, as befits a weed of farm land, but of unknown significance". It must be a name that has been around a long time since it has accumulated a very large number of dialect forms ranging from 'Carlock' and 'Chedlock', through 'Harlock' and 'Skedlock' and 'Skellocks' to 'Warlock'.

In addition, there are a number of forms beginning with 'k', such as 'Kecklock', 'Keblock' and 'Kerleck' (Grigson 1987; Watts 2000). 'Keck' or its variants refers to the hollow dry stems of members of the Umbelliferae, so these names do not meet this requirement and therefore must simply be variations of 'Charlock'. The names 'Runch', 'Runches', 'Runch-balls', 'Runchik', 'Runchie' and 'Rungy', from various parts of the country north of Yorkshire, also appear to refer to dried flower stems (Britten & Holland 1886).

Other more local names include 'Bread-and-Marmalade' from Somerset, a double name which perhaps suggests the fact that Sinapis arvensis seed was used in some districts to make bread in times of famine, plus a reference to the yellow colour of the flower. Four other common names listed by Grigson (1987) include 'yellow' as a word element.

Threats

Once a serious noxious weed of both cereals and broadleaved crops in lowland arable farmland, Charlock is so sensitive to modern herbicides that their widespread use since the 1970s has eliminated the species from cultivated ground throughout B & I. Plants are also sensitive to mechanical damage such as provided by grazing and trampling and it persists on wayside and waste ground sites that are subject to only moderate levels of disturbance. Otherwise plants appear, usually in small numbers, from long buried seed in sites undisturbed for many years, eg roadworks and building plots.

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