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 indigenous to 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
