Herbicides and Plant Physiology. Andrew H. Cobb

Scentless mayweed 15,000–19,000 Echinochloa crus‐galli Barnyard grass 2000–40,000 Chamaenerion angustifolium Rosebay willowherb 80,000 Eleusine indica Goose grass 50,000–135,000 Digitaria sanguinalis Large crabgrass 2000–150,000 Chenopodium album Fat hen 13,000–500,000 Triticum aestivum Wheat 90–100

Figure 1.2 Some methods of weed seed dispersal with their estimated range in metres.

      Source: Liebman, M., Mohler, C.L. and Staver, C.P. (2001) Ecological Management of Agricultural Weeds. Cambridge University Press. Reproduced with permission of Cambridge University Press.

       1.5.7 Dormancy and duration of viability

Although the seed production figures of an individual plant are impressive (Table 1.9), the total seed population in a given area is of greater significance. The soil seed reservoir reflects both past and present seed production, in addition to those imported from elsewhere, and is reduced by germination, senescence and the activity of herbivores (Figure 1.3). Estimates of up to 100,000 viable seeds per square metre of arable soil represent a massive competition potential to both existing and succeeding crops, especially since the seed rate for spring barley, for instance, is only approximately 400 m⁻²! Under long term grassland, weed seed numbers in soil are in the region of 15,000–20,000 m⁻², so conversion of arable land to long‐term grassland offers growers a means of reducing soil weed‐seed burden.

      The length of time that seeds of individual species of weed remain viable in soil varies considerably. The nature of the research involved in collecting such data means that few comprehensive studies have been carried out, but those that have (see Toole and Brown, 1946, for a 39 year study!) show that although seeds of many species are viable for less than a decade, some species can survive for in excess of 80 years (examples include poppy and fat hen). Evidence from soils collected during archaeological excavations reveals seeds of certain species germinating after burial for 100–600 (and maybe even up to 1700!) years (Ødum, 1965).

      Dormancy in weed seeds allows for germination to be delayed until conditions are favourable. This dormancy may be innate and contributes to the periodicity of germination, as illustrated in Figure 1.1. In addition, dormancy may be induced or enforced in non‐dormant seeds if environmental conditions are unfavourable. This ensures that the weed seed germinates when conditions are most conducive to seedling survival.

Figure 1.3 Factors affecting the soil seed population.

      Source: Grundy, A.C. and Jones, N.E. (2002) What is the weed seed bank? In: Naylor, R.E.L. (ed.) Weed Management Handbook, 9th edn. Oxford: Blackwell Publishing/BCPC. Reproduced with permission of John Wiley & Sons.

       1.5.8 Plasticity of weed growth

      The ability of a weed species to make rapid phenotypic adjustment to environmental change (acclimation) may offer a considerable strategic advantage to the weed in an arable context. An example of the consequence of such plasticity is environmental sensing by fat hen (Chenopodium album). This important weed can respond to canopy shade by undergoing rapid stem (internode) elongation, although the plant is invariably shorter if growing in full sun. Similarly, many species can undergo sun–shade leaf transitions for maximum light interception (Patterson, 1985).

       1.5.9 Photosynthetic pathways

Photosynthesis, the process by which plants are able to convert solar energy into chemical energy, is adapted for plant growth in almost every environment on Earth. For most weeds and crops photosynthetic carbon reduction follows either the C₃ or the C₄ pathway, depending on the choice of primary carboxylating enzyme. In C₃ plants this is ribulose 1,5‐bisphosphate carboxylase/oxygenase (RuBisCo) and the first stable product of carbon reduction is the three‐carbon acid, 3‐phosphoglycerate. Alternatively, in C₄ plants the primary carboxylator is phosphoenolpyruvate carboxylase (PEPC) and the initial detectable products are the four‐carbon acids, oxaloacetate, malate and aspartate. These acids are transferred from the leaf mesophyll cells to the adjacent bundle sheath cells where they are decarboxylated and the CO₂ so generated is recaptured by RuBisCo. Since PEPC is a far more efficient carboxylator than RuBisCo, it serves to trap CO₂ from low ambient concentrations (micromolar in air) and to provide an effectively high CO₂ concentration (millimolar) in the vicinity of the less efficient carboxylase, RuBisCo. In this way, C₄ plants can reduce CO₂ at higher rates and are often perceived as being more efficient than C₃ plants. In addition, because of their more effective reduction of CO₂, they can operate at much lower CO₂ concentrations, such that stomatal apertures may be reduced and so water is conserved.

The C₄ pathway is often regarded as an ‘optional extra’ to the C₃ system, and offers a clear photosynthetic advantage under conditions of relatively high photon flux density, temperature and limited water availability, that is in tropical and mainly subtropical environments. Conversely, plants solely possessing the C₃ pathway are more advantaged in relatively temperate conditions of lower temperatures and photon flux density, and an assumed less limiting water supply (Figure 1.4).

      Returning to the interaction between crop and weed, it is therefore apparent that, depending on climate, light to severe competition may be predicted. For example, a temperate C₃ crop may not compete well with a C₄ weed (e.g. sugar beet, Beta vulgaris, and redroot pigweed, Amaranthus retroflexus) and a C₄ crop might be predicted to outgrow some C₃ weeds (e.g. maize, Zea mays, and fat hen, Chenopodium album). Less competition is then predicted between C₃ crop and C₃ weeds in temperate conditions, with respect to photosynthesis alone.

In reality, C₄ weeds are absent in the UK but widespread in continental, especially Mediterranean, Europe. In the cereal belt of North America, however, C₄ weeds pose a considerable problem and it is notable that eight of the world’s top 10 worst weeds are C₄ plants