Herbicides and Plant Physiology. Andrew H. Cobb

typically rapid growth rates and devote most of their resources to reproduction. Because they inhabit recently disturbed environments there is little competition with other plants for resources, which therefore can be obtained without difficulty. They are generally short‐lived ephemeral annuals that occupy the earliest phases of succession. Conversely, biennial and perennial weeds often employ a more competitive growth strategy in relatively undisturbed conditions. They use their resources perhaps less for seed production and more for support tissues, for example, to provide additional height for the interception of light, or more extensive root systems to obtain more water and minerals. Rapid growth rate may still be evident with high rates of leaf turnover. The third growth strategy, exhibited by the stress tolerators, is to reduce resource allocation to vegetative growth and seed production, so that the survival of relatively mature individuals is ensured in high‐stress conditions. Consequently, they have slow growth rates and are commonly found in unproductive environments.

      Many arable weeds have characteristics common to both competitors and ruderals, and are referred to as competitive ruderals. Indeed, most of the annuals listed in The World’s Worst Weeds (Radosevich and Holt, 1984) fit into this category, and are found in productive sites where occasional disturbance is expected. Examples include arable land that is cultivated, and meadows and grassland that are grazed or mowed. Interestingly, most crop plants also adopt a competitive ruderal strategy with their rapid growth rates and relatively large seed production. Competition between crop and weed is then related to their relative abilities to exploit the resources available.

The practice of growing crops in monoculture has exerted a considerable selection pressure in the evolution of weeds. Many characteristics have evolved that contribute to weed success and the main ones are listed in Table 1.8. Fortunately, not all of these features are present in any one weed species, yet each character may give the weed a profound competitive advantage in a given situation. Some of these characteristics are discussed in more detail in the following sections of this chapter.

Table 1.7 Growth strategies of plants.

      Source: Hill, T.A. (1977) The Biology of Weeds. London: Edward Arnold.

Intensity of disturbance	Intensity of stress
High	Low
High	Death	Ruderals
Low	Stress tolerators	Competitors

Table 1.8 The ‘successful’ weed.

      Source: Adapted from Baker, H.G. and Stebbins, G.L. (1965) The Genetics of Colonising Species. New York: Academic Press.

Characteristic	Example species
1. Seed germination requirements fulfilled in many environments	Senecio vulgaris
2. Discontinuous germination (through internal dormancy mechanisms) and considerable longevity of seed	Papaver spp.
3. Rapid growth through the vegetative phase to flowering	Cardamine hirsuta
4. ‘Seed’ production in a wide variety of environmental conditions	Poa annua
5. Continuous seed production for as long as conditions for growth permit	Urtica urens
6. Very high ‘seed’ output in favourable environmental conditions	Chenopodium album
7. Self‐compatible but not completely self‐pollinating	Alopecurus myosuroides
8. Possession of traits for short‐ and long‐distance seed dispersal	Galium aparine
9. When cross‐pollinated, unspecialised pollinator visitors or wind pollinated	Grass weeds in general
10. If a clonal species, has vigorous vegetative growth and regeneration from fragments	Cirsium arvense
11. If a clonal species, has brittleness of leafy parts ensuring survival of main plant	Taraxacum officinale
12. Shows strong inter‐specific competition by special mechanisms (e.g. allelopathic chemicals)	Elytrigia repens
13. Demonstrates resistance to herbicides through a number of resistance mechanisms	Alopecurus myosuroides

Invasive species have received far greater research focus in recent years (see, for instance, Shaw and Tanner, 2008 for a review), and DAISIE (Delivering Alien Invasive Species Inventories for Europe) currently reports 10,822 invasive species in Europe (this figure is for all invasive species, not just plants). Alien species present a real threat to biodiversity, and a number of political drivers have been put into place to combat their spread and reduce the occurrence of a number of alien species, including plants. These measures include the Convention of Biodiversity and the EU 2010 Halting Biodiversity Loss, both of which identify invasive weeds as being a key factor in biodiversity loss.

      Most non‐native plants in the UK were introduced by plant collectors in the last 200 years. They become invasive when they have negative impacts on native species, our economy and even our health. Alien plant species become a problem because they are growing in habitats away from their natural predators and so can spread with ease and do not need to invest valuable energy in biologically expensive chemical defence mechanisms. In addition, the Novel Weapons Hypothesis proposes that in some cases allelopathic chemicals produced by alien species are more effective against native species in invaded areas than they are against species from the alien’s natural habitat (Inderjit et al., 2007). This gives further ecological advantage to some invasive species of plant.

      The Royal Horticultural Society notes 1402 invasive plants in the UK, of which 108 can have negative effects, and its website offers advice and guidance on how they should be dealt with (www.rhs.org.uk/advice/profile?PID=530). The EU Regulation on Invasive Alien Species lists 36 plants.

Observations of the pathogens and predators that affect these invasive plants in their natural habitats may identify mycoherbicides and other biological controls that may prove useful in their management. Research into such controls for Japanese Knotweed (Fallopia japonica), Himalayan Balsam (Impatiens glandulifera) and Giant Hogweed (Heracleum mantegazzianum) have so far produced, at best, limited success (Tanner, 2008).

      Moles et al. (2008) have recently proposed a framework for predicting plant species which may present a risk of becoming invasive weeds. This may prove more useful than Table