Herbicides and Plant Physiology. Andrew H. Cobb. Читать онлайн. Newlib. NEWLIB.NET

Автор: Andrew H. Cobb
Издательство: John Wiley & Sons Limited
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Жанр произведения: Биология
Год издания: 0
isbn: 9781119157700
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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.

Schematic illustration of 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.

      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.

      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