Herbicides and Plant Physiology. Andrew H. Cobb

warmer regions (Table 1.10). It will be of both interest and commercial significance if the C₄ weeds become more abundant in regions currently termed temperate (e.g. northern Europe), with the development of of climate change.

      Of the 435,000 plant species on Earth, C₄ photosynthesis is present in fewer than 2%, but accounts for about 25% of plant productivity. It is a carbon dioxide‐concentrating mechanism that evolved relatively recently, that makes photosynthesis more efficient. It is noteworthy that many major weeds are C₄ plants (Table 1.10). In the C4 Rice Project (www.c4rice.com), gene editing is being used to introduce C4 genes into crops such as rice. Rice currently accounts for 19% of all calories consumed globally. If successful, GE rice could be 50% more productive than the current C3 rice.

Figure 1.4 Expected rates of photosynthesis (PS) by C₃ and C₄ plants at (a) varying temperature and (b) varying photosynthetic photon flux density (PPFD).

      Source: Andy Cobb, 1992.

Table 1.10 Photosynthetic pathway of the world’s 10 worst weeds.

      Source: Holm, L.G., Plucknett, D.L., Pancho, J.V. and Herberger, J.B. (1977) The World’s Worst Weeds. Distribution and Biology. Hawaii: University Press.

Latin name	Common name	Photosynthetic pathway	Number of countries where plant is known as a weed
1. Cyperus rotundus (L.)	Purple nutsedge	C4	91
2. Cynodon dactylon (L.) Pers.	Bermuda grass	C4	90
3. Echinochloa crus‐galli (L.) Beauv.	Barnyard grass	C4	65
4. Echinochloa colonum (L.) Link.	Jungle rice	C4	67
5. Eleusine indica (L.) Gaertn.	Goose grass	C4	64
6. Sorghum halepense (L.) Pers.	Johnson grass	C4	51
7. Imperata cylindrica (L.) Beauv.	Cogon grass	C4	49
8. Eichornia crassipes (Mart.) Solms.	Water hyacinth	C3	50
9. Portulaca oleracea (L.)	Purslane	C4	78
10. Chenopodium album (L.)	Fat hen	C3	58

       1.5.10 Vegetative reproduction

      Not all weeds classified as competitive ruderals are annuals. The exceptions are the herbaceous perennials, which have a high capacity for vegetative growth and include many of the most important weeds in the world. The vegetative production of new individuals can often be a very successful means of weed establishment. This is because the vegetative structures can rely on the parent plant for nutrients, which can confer a competitive advantage, especially at the start of the growth season. There are, however, disadvantages to vegetative reproduction. The principal ones are that since daughter plants are genetically identical to their parents, they may not be well adapted to a changing environment and that widespread dispersal cannot occur by vegetative means alone, unlike with seeds. The vegetative structures themselves include stolons, rhizomes, tubers, bulbs, corms, roots and turions.

Stolons are long, slender stems that grow along the soil surface to produce adventitious roots and shoots; examples include the perennial bermuda grass (Cynodon dactylon), the annual crabgrass (Digitaria sanguinalis) and creeping buttercup (Ranunculus repens). Rhizomes are underground stems from which adventitious roots and shoots arise. Major examples include the perennials Johnson grass (Sorghum halepense), couch grass (Elytrigia repens), perennial sedges such as purple and yellow nutsedge (Cyperus rotundus and C. esculentus) and ground elder (Aegopodium podagraria). Purple nutsedge (Cyperus rotundus) has an extensive underground system of rhizomes and tubers. The rhizomes can penetrate and pass completely through vegetable root crops, and the tubers can remain dormant and carry the plant through very extreme conditions of drought, flooding or lack of aeration. Cyperus rotundus is a major weed of tropical and warm temperate regions of sugar cane, rice, cotton, maize and vegetables, groundnuts, soybeans and sorghum. Japanese knotweed (Fallopia japonica), a highly invasive weed that has become a particular problem in many parts of the world, propagates largely by means of rhizomes (both locally and in moved soil; Figueroa, 1989), as colonies rarely result from seed.

      Tubers are enlarged terminal portions of rhizomes that possess storage tissues and axillary buds. Examples include the perennial sedges mentioned above, Jerusalem artichoke (Helianthus tuberosus) and the common white potato (Solanum tuberosum). Another particularly troublesome weed that produces tubers is the horsetail (Equisetum arvense). In this case, aerial shoots can be easily controlled, but deep‐seated tubers will produce new shoots when conditions permit.

      Bulbs are also underground organs that are modified buds surrounded by scale leaves, which contain the stored nutrients for growth, an example being wild onion (Allium vineale). Corms are swollen, vertical underground stems covered by leaf bases, for example, bulbous buttercup (Ranunculus bulbosus).

      Many species produce long, creeping horizontal roots that give rise to new individuals, including perennial sow thistle (Sonchus arvensis), field bindweed (C. arvensis) and creeping or Canada thistle (Cirsium arvense). Some biennials and perennials form swollen, non‐creeping taproots capable of regenerating whole plants. Common examples are dandelion (Taraxacum officinale) and curled and broad‐leaved docks (Rumex crispus and R. obtusifolius). Several aquatic weeds produce vegetative buds or turions that have specialised nutrient‐storing leaves or scales. These separate from the parent plant in unfavourable conditions, or are released after the decay of the parent, to remain dormant until favourable conditions return. Examples include Canadian pondweed (Elodea canadensis) and Ceratophyllum demersum.

      Cultivation and soil disturbance will promote the fragmentation of all these vegetative structures. Propagation will then occur when the vegetative structure is separated from