8 Water2-19

8 Water2-19 - Plant adaptation to water stress We now turn...

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    Plant adaptation to water stress
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    We now turn to the question of how plants living in dry  environments function efficiently.  In particular, we examine the concept of photosynthetic water-use  efficiency in plants.
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    *Photosynthetic water-use efficiency is: amount of CO 2  fixed by photosynthesis amount of H 2 O lost by transpiration  And the units are: umol CO2 cm -2 s -1 mmol H 2 O cm -1 s -1 Where cm -2  means per unit leaf surface area
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    Remember that the pathway for diffusion of CO 2  into leaves is  similar to the pathway for diffusion of H 2 O out of leaves. One factor that has a major influence on photosynthetic water-use  efficiency is the biochemical pathway by which CO 2  is fixed during  photosynthesis.
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    C 4 photosynthesis In most plants, the primary fixation product of photosynthesis is the  three-carbon compound  3-phosphoglycerate (C 3 ) (as we discussed earlier). However, in some plants, the primary fixation product of  photosynthesis is the four-carbon compound oxaloacetate (C 4 ), as  illustrated in  Figure 8.23.
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    *In these C 4  plants, the primary reaction by which CO 2  is  fixed is: Phsophoenol pyruvate + CO 2  ---> oxaloacetate (C 3 ) (C 4 ) The reaction is catalyzed by the enzyme phosphoenol pyruvate  carboxylase (or PEP carboxylase)
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8 Water2-19 - Plant adaptation to water stress We now turn...

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