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7 The photorespiratory pathway, although energy-consuming, may also have a useful function for the plant

7 The photorespiratory pathway, although energy-consuming, may also have a useful function for the plant

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Further reading

Photorespiration, the unavoidable side-reaction of photosynthesis, is thus utilized by the plant for its protection. It can therefore be imagined that lowering

the oxygenase reaction of RubisCO by molecular engineering (Chapter 22)—

as attempted by many researchers, although still without success—may lead

to a plant that uses energy more efficiently, but at the same time may increase

its vulnerability to excessive illumination or shortage of water and thereby losing a feature of protection (see Chapter 8).

Further reading

Christensen, K. E., MacKenzie, R. E. Mitochondrial one-carbon metabolism is adapted

to the specific needs of yeast, plants and mammals. Bioessays 28, 595–605 (2006).

Douce, R., Heldt, H. W. Photorespiration. In R. C. Leegood, T. D. Sharkey, S. von

Caemmerer (eds.). Photosynthesis: Physiology and metabolism, pp. 115–136.

Dordrecht, Niederlande: Kluwer Academic Publishers (2000).

Douce, R., Bourguignon, J., Neuburger, M., Rébeillé, F. The glycine decarboxylase system: A fascinating complex. Trends in Plant Science 6, 167–176 (2001).

Hayashi, M., Nishimura, M. Arabidopsis thaliana—A model organism to study plant

peroxisomes. Biochimica Biophysica Acta 1763, 1382–1391 (2006).

Husic, D. W., Husic, H. D. The oxidative photosynthetic carbon (C2) cycle: An update

of unanswered questions. Reviews Plant Biochemistry and Biotechnology 1, 33–56


Khan, M. S. Engineering photorespiration in chloroplasts: A novel strategy for increasing biomass production. Trends in Biotechnology 25, 437–440 (2007).

Kunze, M., Pracharoenwattana, I., Smith, S. M., Hartig, A. A central role for the peroxisomal membrane in glyoxylate cycle function. Biochimica Biophysica Acta 1763,

1441–1452 (2006).

Linka, M., Weber, A. P. Shuffling ammonia between mitochondria and plastids during

photorespiration. Trends in Plant Science 10, 461–465 (2005).

Reumann, S. The photorespiratory pathway of leaf peroxisomes. In A. Baker & I. A.

Graham (eds.), Plant peroxisomes, pp. 141–189. Dordrecht, Niederlande: Kluwer

Academic Publishers (2002).

Reumann, S., Weber, A. P. Plant peroxisomes respire in the light: Some gaps of the

photorespiratory C2 cycle have become filled, others remain. Biochimica Biophysica

Acta 1783, 1496–1510 (2006).

Visser, W. F., van Roermund, C. W., Ijlst, L., Waterham, H. R., Wanders, R. J.

Metabolite transport across the peroxisomal membrane. Biochemistry Journal 401,

365–375 (2007).


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Photosynthesis implies the

consumption of water

This chapter describes how photosynthesis is unavoidably linked with a

substantial loss of water and therefore is often limited by the lack of water.

Biochemical mechanisms that enable certain plants living in hot and dry

habitats to reduce their water requirement will be described.

8.1 The uptake of CO2 into the leaf is

accompanied by an escape of water


Since CO2 assimilation is linked with a high water demand, plants require

an ample water supply for their growth. A C3 plant growing in temperate

climates requires 700 to 1,300 mol of H2O for the fixation of 1 mol of CO2.

This calculation does not consider the water consumption necessary for

photosynthetic water oxidation since it is negligible in quantitative terms.

Water demand is dictated by the fact that water evaporation from the

leaves has to be replenished by water taken up through the roots. Thus during photosynthesis there is a steady flow of water, termed the transpiration

stream, from the roots via the xylem vessels into the leaves.

The loss of water during photosynthesis is unavoidable, as the uptake of

CO2 into the leaves requires openings in the leaf surface, termed stomata.

The stomata open to allow the diffusion of CO2 from the atmosphere into

the intercellular gas space of the leaf, but at the same time water vapor

escapes through the open stomata (Fig. 8.1). The water vapor concentration in the intercellular gas space of a leaf amounts to 31,000 ppm (at 25°C)



Figure 8.1 Schematic

presentation of a crosssection of a leaf. The

stomata are often located

on the lower surface of

the leaf. CO2 diffuses

through the stomata into

the intercellular gas space

and thus reaches the

mesophyll cells carrying

out photosynthesis. Water

escapes from the cells into

the atmosphere by diffusion

of water vapor. This scheme

is simplified. In reality, a

leaf has several cell layers,

and the intercellular gas

space is much smaller than

shown in the drawing.


Photosynthesis implies the consumption of water












gas space





in equilibrium with the cell water. Since this concentration is two orders of

magnitude higher than the CO2 concentration in the atmosphere (350 ppm),

the escape of a very high amount of vaporized water during the influx of

CO2 is inevitable. To minimize the water loss from the leaves, the opening

of the stomata is regulated. Thus, when there is a rise in the atmospheric

CO2 concentration, plants lose less water and therefore require less water.

Opening and closing of the stomata is caused by biochemical reactions and

will be described in the next section.

When the water supply is adequate, plants open their stomata just

enough to provide CO2 for photosynthesis. During water shortage, plants

prevent dehydration by closing their stomata partially or completely, which

results in the restriction or even cessation of CO2 assimilation. Therefore

water shortage is often a decisive factor limiting plant growth, especially

in the warmer and drier regions of our planet. In those habitats a large

number of plants have evolved a strategy for decreasing water loss during

photosynthesis. In the plants dealt with in the preceding chapter the first

product of CO2 fixation is 3-phosphoglycerate, a compound with three carbon atoms; hence such plants are named C3-plants (see section 6.2). Other

plants save water by first producing the C4 compound oxaloacetate via CO2

fixation and are therefore named C4-plants.

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