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 carboxylator. 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-phosphogly cerate. 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 (millimo-lar) in the vicinity of the less efficient carboxylase, RuBisCo. In this way, C4 plants can reduce CO2 at high 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.

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). © Andy Cobb, 1992.
Table 1.10 Photosynthetic pathway of the world's ten worst weeds (from Holm et al., 1977).

Latin name

Common name

Photosynthetic pathway

Number of countries where 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. I mperata 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

In reality, C4 weeds are absent in the UK but widespread in continental, especially Mediterranean, Europe. In the cereal belt of North America however, Ci weeds pose a considerable problem and it is notable that eight of the world's top ten 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 predictions of climate change.

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