4 Higher Plant Respiration and Its Relationships
to Photosynthesis
1.S. Amthor
4.1 Introduction
The heat produced by a respiring cell is an inescapable
component of cellular metabolism, the cost which
Nature has to pay for creating biological order out
of physical chaos in the environment of plants and
animals.
1.L. Monteith (1972)
Respiration is the complement of photosynthesis in higher plants. 1 The
primary function of photosynthesis is to assimilate CO2 and radiant energy
in the formation of carbohydrates. A significant portion of those carbohydrates become the main substrates of respiration (James 1953; Krotkov
1960; ap Rees 1980), but often after some period of storage or distance of
transport. The function of respiration is to convert photo assimilate into
substances usable by growth, maintenance, transport, and nutrient assimilation processes (Beevers 1961). Respiration does this by breaking down sugars
into smaller molecules (carbon skeleton intermediates), phosphorylating
ADP and other nucleosides, and reducing nucleotides - respiration does not
only generate A TP. Some of the carbon skeleton intermediates become the
precursors of growth and are diverted away from respiratory metabolism
and used in biosynthetic reactions, whereas the ATP and NAD(P)H formed
during respiration are used in all heterotrophic energy-requiring processes
(Fig. 4.1).
During respiration and growth, CO2 is released as a byproduct. Indeed, it
is commonly surmised (Kira 1975; Amthor 1989; Ryan 1991) that up to half,
or even more, of the carbon assimilated in photosynthesis (less photorespiratory decarboxylations) is eventually released during plant respiration,
albeit "accurate and relevant estimates of [the ratio of respiration to photosynthesis] are rare because plant physiologists have seldom tried to measure
the respiration rate of whole plants" (Monteith 1972). Heat is another
important byproduct of respiration and growth, with perhaps half of the
energy contained in photosynthate released as heat during plant heterotrophic metabolism. The fraction of carbon and energy in photo assimilate
that is "lost" during subsequent metabolism depends on the pathways of
respiration and mitrochondrial ADP: 0, relative rates of growth and main1 The focus herein is on terrestrial higher plants. Moreover, except where specifically
noted, the discussion is limited to C3 plants, which comprise 95% of known plant species.
Gardestrom and Edwards (1985) and Dry et al. (1987) have discussed various aspects of
mitochondria and respiration in C4 and CAM plants. Geider (1992) has reviewed respiration in phytoplankton.
to Photosynthesis
1.S. Amthor
4.1 Introduction
The heat produced by a respiring cell is an inescapable
component of cellular metabolism, the cost which
Nature has to pay for creating biological order out
of physical chaos in the environment of plants and
animals.
1.L. Monteith (1972)
Respiration is the complement of photosynthesis in higher plants. 1 The
primary function of photosynthesis is to assimilate CO2 and radiant energy
in the formation of carbohydrates. A significant portion of those carbohydrates become the main substrates of respiration (James 1953; Krotkov
1960; ap Rees 1980), but often after some period of storage or distance of
transport. The function of respiration is to convert photo assimilate into
substances usable by growth, maintenance, transport, and nutrient assimilation processes (Beevers 1961). Respiration does this by breaking down sugars
into smaller molecules (carbon skeleton intermediates), phosphorylating
ADP and other nucleosides, and reducing nucleotides - respiration does not
only generate A TP. Some of the carbon skeleton intermediates become the
precursors of growth and are diverted away from respiratory metabolism
and used in biosynthetic reactions, whereas the ATP and NAD(P)H formed
during respiration are used in all heterotrophic energy-requiring processes
(Fig. 4.1).
During respiration and growth, CO2 is released as a byproduct. Indeed, it
is commonly surmised (Kira 1975; Amthor 1989; Ryan 1991) that up to half,
or even more, of the carbon assimilated in photosynthesis (less photorespiratory decarboxylations) is eventually released during plant respiration,
albeit "accurate and relevant estimates of [the ratio of respiration to photosynthesis] are rare because plant physiologists have seldom tried to measure
the respiration rate of whole plants" (Monteith 1972). Heat is another
important byproduct of respiration and growth, with perhaps half of the
energy contained in photosynthate released as heat during plant heterotrophic metabolism. The fraction of carbon and energy in photo assimilate
that is "lost" during subsequent metabolism depends on the pathways of
respiration and mitrochondrial ADP: 0, relative rates of growth and main1 The focus herein is on terrestrial higher plants. Moreover, except where specifically
noted, the discussion is limited to C3 plants, which comprise 95% of known plant species.
Gardestrom and Edwards (1985) and Dry et al. (1987) have discussed various aspects of
mitochondria and respiration in C4 and CAM plants. Geider (1992) has reviewed respiration in phytoplankton.