However, information about these metabolic and physiological processes is very
limited.
1.8.1 Carbon Metabolism (Fig. 1.20)
• 1st step: Solar radiation energy (electron) is transported through photosystem II
(PSII) and photosystem I (PSI) in chloroplasts, and high-energy compounds, ATP
and NADPH, are produced.
• 2nd step: Solar radiation energy stimulates the K
+ pump to open stomata with
osmotic pressure, and CO 2 and H 2 O diffuse into and out of the cell, respectively. As stomata are porous structures, when H2O evaporates as H 2 O gas due
to solar radiation energy, the vaporization heat (latent heat of vaporization) is
removed, and the leaf is cooled. As CO 2 and H 2 O diffusion are physical
processes powered by solar radiation, CO 2 and H 2 O diffusion are coupled.
This relationship is called water-use efficiency (WUE), defined as the amount
of carbon assimilated as biomass or grain produced per unit of water used by the
plant (WUE ¼ P/E ¼ dry matter (DM) production g/water g), where P and E are
the photosynthetic rate and the evaporation rate, respectively. The WUE
(g DM/liter water) (https://photosyn.jp/pwiki/index.php?%E6%B0%B4%E5%
88%A9%E7%94%A8%E5%8A%B9%E7%8E%87) is 2.5–5 in trees and
1.4–2.5 in grasses, which are equivalent in g water/g DM to 200–400 in trees
and 400–700 in grasses. These values indicate that photosynthetic processes
require large amounts of water and that the WUE of trees is higher than that of
grasses.
• 3rd step: CO 2 is reduced to triose-P through the Calvin cycle using energy from
ATP and NADPH
• 4th step: Triose-P is metabolized into sucrose (carbohydrate-oriented metabolism) and PEP-OAA (organic acid- and amino acid-oriented metabolism), the
regulation of which is mainly by SPS and PECP. PECP is activated by light (solar
radiation), NO 3
À , and Pi (indirectly).
• 5th step: PEP is retranslocated into chloroplasts regulated by PPT (PPT: PEP/Pi
translocator), and PEP is metabolized into precursors of high molecular weight
compounds such as pigments, lignin, aromatic amino acids, and fatty acids. As
trees produce a large amount of lignin, this metabolic pathway is very important.
The production of precursors to high molecular weight compounds is assumed to
require large amounts of energy; however, solar energy is abundant in tropical
areas, and chemical construction energy is provided inside the chloroplasts,
contributing to the consumption of excess energy.
30
M. Osaki et al.
limited.
1.8.1 Carbon Metabolism (Fig. 1.20)
• 1st step: Solar radiation energy (electron) is transported through photosystem II
(PSII) and photosystem I (PSI) in chloroplasts, and high-energy compounds, ATP
and NADPH, are produced.
• 2nd step: Solar radiation energy stimulates the K
+ pump to open stomata with
osmotic pressure, and CO 2 and H 2 O diffuse into and out of the cell, respectively. As stomata are porous structures, when H2O evaporates as H 2 O gas due
to solar radiation energy, the vaporization heat (latent heat of vaporization) is
removed, and the leaf is cooled. As CO 2 and H 2 O diffusion are physical
processes powered by solar radiation, CO 2 and H 2 O diffusion are coupled.
This relationship is called water-use efficiency (WUE), defined as the amount
of carbon assimilated as biomass or grain produced per unit of water used by the
plant (WUE ¼ P/E ¼ dry matter (DM) production g/water g), where P and E are
the photosynthetic rate and the evaporation rate, respectively. The WUE
(g DM/liter water) (https://photosyn.jp/pwiki/index.php?%E6%B0%B4%E5%
88%A9%E7%94%A8%E5%8A%B9%E7%8E%87) is 2.5–5 in trees and
1.4–2.5 in grasses, which are equivalent in g water/g DM to 200–400 in trees
and 400–700 in grasses. These values indicate that photosynthetic processes
require large amounts of water and that the WUE of trees is higher than that of
grasses.
• 3rd step: CO 2 is reduced to triose-P through the Calvin cycle using energy from
ATP and NADPH
• 4th step: Triose-P is metabolized into sucrose (carbohydrate-oriented metabolism) and PEP-OAA (organic acid- and amino acid-oriented metabolism), the
regulation of which is mainly by SPS and PECP. PECP is activated by light (solar
radiation), NO 3
À , and Pi (indirectly).
• 5th step: PEP is retranslocated into chloroplasts regulated by PPT (PPT: PEP/Pi
translocator), and PEP is metabolized into precursors of high molecular weight
compounds such as pigments, lignin, aromatic amino acids, and fatty acids. As
trees produce a large amount of lignin, this metabolic pathway is very important.
The production of precursors to high molecular weight compounds is assumed to
require large amounts of energy; however, solar energy is abundant in tropical
areas, and chemical construction energy is provided inside the chloroplasts,
contributing to the consumption of excess energy.
30
M. Osaki et al.