The Biology of Arable Plants

Annuals

The majority of arable plants are annuals, which means that they complete their life-cycle from germination to death within one year. Many species can have a much shorter life-span: Shepherd's-purse and Groundsel can produce seeds as little as six weeks after germination, and can germinate at almost any time of the year given enough warmth and moisture. These are ephemeral plants, which can exploit a much wider range of habitats than just arable land. Most of the wild plants more strictly confined to arable land have lifecycles closely synchronised with the timing of traditional arable farming. The characteristics of these two groups are summarised below, although relatively few species in each group fulfil them all.

Perennials

Biennials and other short-lived perennials can sometimes flourish in areas that are occasionally, but not annually, disturbed by cultivation. Such species include Perennial Sow-thistle, Creeping Thistle and Common Couch. Other plants like Common Nettle, Colt's-foot and even Bramble can become established where cultivations are shallow. The only uncommon

perennial arable plant is Tuberous Pea, which may have been a 19th century introduction, and is confined to a small area of Essex. Perennials can often pose problems even in high-input arable systems, as they are very difficult to eradicate with herbicides. In areas managed for conservation, they can be problematic, as they rapidly form dense stands in areas without crop cover. These perennials have solved the problem of re-establishment after ploughing in two ways. Some have creeping rhizomes that can either grow beneath plough-depth, or which give rise to new plants from each fragment when broken up by the plough. Others produce small tubers, which remain in the soil after ploughing.

Seed production

All annual plants rely upon regular seed production to maintain their populations. Most annuals can form a persistent bank of seed in the soil. The number of seeds produced varies greatly between species and between plants of the same species. This variation depends upon the availability of nutrients and water in the soil and the amount of light available. This plasticity is typical of annual plants. A plant of Common Poppy may produce only a single seed-pod under conditions of extreme competition from the crop, whereas a nearby plant growing in a gap where the crop has failed may produce more than 100 seed-pods. Common Poppy produces particularly large numbers of seed, on average around 16,500 per plant. The now extinct Thorow-wax, on the other hand, produced only 170 seeds on average per plant. The amount of seed that is returned to the seed-bank each year is important in determining the survival of a population.

Seed-banks and dormancy

In a cultivated field, seeds do not remain on the soil surface for long after being shed from the parent plant and they are usually incorporated into the soil by ploughing or other cultivations to a maximum depth of approximately 20 cm. This can prevent immediate germination, and dormant seeds in the soil are referred to as the seed-bank. Long-lived seed is a characteristic of most arable plants, and gives obvious advantages to plants living in a habitat which may be unsuitable for long periods. Germination is stimulated by exposure to light, and in a regularly cultivated field, a fresh batch of buried seed will be exhumed every year. Weed control by bare-fallowing takes advantage of this by attempting to stimulate germination by repeated cultivations during which the already-germinated seedlings are destroyed.

The longevity of dormant seed is primarily an inherent property of each species. Some are very long-lived. Poppies, for example, are thought to be able to survive for hundreds of years. At the other extreme, the seed of Corncockle, and other species which rely upon being harvested along with the crop and being re-sown with the next season's crop, survive for just a few months. Seed longevity can be reduced by predation by birds, small mammals or soil organisms. Seeds can also be lost to fungal decomposition. Longevity can be increased, however, by deep-burial in anaerobic conditions, and species such as Corncockle and Cornflower sometimes appear after the ground has been disturbed for foundations, pipelines or new roads in areas where they have not been seen for many years.

A question of timing

Annual arable plants are suited to growing in conditions of regular, predictable disturbance, provided that their germination period occurs after cultivation and sowing of the crop. The majority of species that are confined to arable habitats have very restricted germination periods, either predominantly in the late autumn and early winter or in the spring. This germination time period is partly a result of a simple response to external conditions; a seed will not germinate if it is too dry or too cold. It is also a result of the internal programming of the seed interacting with external conditions, chiefly temperature. The combination of these factors means that a seed near the soil surface goes through a cycle of dormancy in which it is dormant at certain periods of the year when conditions are unsuitable for germination and can germinate during those periods when conditions are suitable.

The consequence of these restricted germination periods is that the plants germinating in a field after cultivation on one date may be very different from those germinating after cultivation on another date. Quite minor variations in the timing of cultivation and crop drilling can have major effects on the plant community. The earlier drilling of cereals in autumn and winter has had a dramatic effect on arable plant communities.

Life-cycles

The length of time between germination and seed production also varies between species. As mentioned previously, some annual plants can have a lifecycle of as little as five weeks. A longer life-cycle is more normal, however, and a very few species have life-cycles that are longer than those of the crops with which they grow.

As an example, in a winter Wheat crop drilled at the beginning of October, the bulk of the arable plants germinate before the beginning of January. Provided that the winter has not been too hard, some will start to flower by mid-May. The great majority of plants will be in flower by the middle of June and will have set seed by the end of July, before harvest. The length of the flowering period will depend to some extent upon the weather, and will be extended in a cool, wet summer.

Main germination periods of some annual arable plants

Nitrogen inputs and competition with the crop

Modern crop varieties have been bred to be highly responsive to nitrogen fertiliser. They rapidly form a dense canopy of leaves after germination that shades out the seedlings of other slower-growing arable plants. The majority of the arable plants that have decreased in recent years are relatively slow-growing, and most have a relatively low stature, rendering them vulnerable to the effects of competition with the crop. The taller-growing plants such as Cornflower, Common Poppy and mayweeds eventually reach the crop canopy, but even these cannot compete with the selectively-bred crop plants to take up the fertiliser. Where such species appear to flourish, it is usually because the crop has failed for some reason.

Herbicides and herbicide resistance

Modern herbicides operate on specific biochemical reactions within plants. The earliest-developed compounds such as MCPA work by imitating the actions of natural plant hormones. Many of the most recently developed compounds operate on single steps in photosynthesis. Arable plants vary greatly in their susceptibility to herbicides. This is partly due to internal biochemical differences, such as those that separate grasses from broad-leaved plants. Physical factors can also be important in preventing the uptake of herbicides by plants. In general, the broader a leaf is, the more easily the herbicide is absorbed. Features that can hinder absorption include

hairy leaves and the presence of a waxy leaf cuticle. Corn Marigold, for example, has very waxy leaves, and most water-based herbicides simply run off. The decline of this plant can be partly explained by the development of efficient wetting agents during the 1970s which helped in the retention of herbicide spray on the leaves.

The prolonged use of a single herbicide or group of herbicides can lead to the development of herbicide-resistant strains of some plants. Herbicide-resistant Blackgrass is a particular problem in winter cereals in southern England.

Biotechnology has sought to introduce genes for herbicideresistance into crop plants. The herbicides that are being investigated are very broad-spectrum compounds including Glyphosate. The widespread use of such chemicals in growing crops could pose a very serious threat to the arable flora, and could lead to the evolution of strains of some problem species that are more herbicide-resistant.

The good, the bad and the ugly

The decline of most arable plants, the continued persistence of others and the rise to prominence of a few can be explained by their biology. The ability to form persistent seed-banks is an obvious advantage to an arable plant, and can provide a buffer against the effects of changes in arable farming practice. Many of those species that have declined so dramatically have relatively short-lived seed-banks, conferring little resilience on populations of these species. Corncockle is an extreme example, and other species with short-lived seed, such as Cornflower, Corn Buttercup and Shepherd's-needle, declined rapidly during the 1950s and 1960s as herbicides became more effective and crops more competitive. Susceptibility to herbicides has been the feature that has rendered species most liable to rapid decline, although their inability to compete with a highly fertilised crop has also been very important.

Arable plants such as Common Poppy, knotgrasses and Charlock that have persisted, even if in much reduced quantities, have been those which,

Some reasons for the decline of arable plants of conservation concern

although highly sensitive to herbicides, have long-lived seed and a buffering seed-bank, replenished by occasional chance events that allow seed production. Those species which have become the modern weeds, such as Barren Brome, Cleavers and Black-grass, are opportunists, pre-adapted to the conditions of contemporary cereal farming. Although they have little seed-dormancy they are resistant to many herbicides and are highly responsive to nitrogen. They germinate early in the autumn and produce seed early in the summer, synchronising well with modern cropping cycles.

An understanding of some of these features of the basic biology of arable plants can help us understand the factors that have led to their decline or in some cases to their increase. More importantly, it may tell us how we can approach their conservation.