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7 The photorespiratory pathway, although energy-consuming, may also have a useful function for the plant
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).
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Caemmerer (eds.). Photosynthesis: Physiology and metabolism, pp. 115–136.
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peroxisomes. Biochimica Biophysica Acta 1763, 1382–1391 (2006).
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of unanswered questions. Reviews Plant Biochemistry and Biotechnology 1, 33–56
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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
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.