Higher Plant Respiration and Its Relationships to Photosynthesis
83
by light in growing tissue, but perhaps slowed by light in mature tissue.
McCashin et al. (1988) presented evidence that TCA cycle activity was
slowed only 20% (but see Gemel and Randall 1992) by light in growing
Triticum aestivum leaves, but they used nonphysiological, i.e., nonphotorespiratory, conditions. Evidence that the TCA cycle (but not necessarily
mitochondrial electron transport and oxidative phosphorylation) is slowed in
the light comes from the partial inactivation of mitochondrial pyruvate dehydrogenase complex (mtPDC) by light. The light inactivation is apparently
transduced through some product of photosynthesis or photo respiration
resulting in the phosphorylation of the mtPDC (Gemel and Randall 1992).
Graham (1980) surmised that "inhibition of oxidative phosphorylation in
the light seems probable." Conversely, the results of Kromer et al. (1988)
and Kromer and Heldt (1991a) indicate that oxidative phosphorylation is
actually required for rapid photosynthesis (measured as O2 evolution). They
suggested that mitochondria contribute needed ATP to the cytosol in photosynthesizing cells and can oxidize excess photosynthetic redox equivalents
(see also Ebbighausen et al. 1985). Oxidative phosphorylation may be most
important to photosynthesis in leaves not capable of rapidly producing
starch - respiratory ATP is apparently required to support rapid cytosolic
sucrose-P synthesis (Hanson 1992) and the consequent release of Pi for
further use in photosynthetic carbon metabolism (Sharkey 1985), whereas
rapid starch synthesis (and Pi release) can occur without concurrent respiration. Active mitochondrial electron transport might limit photoinhibition
of photosynthesis through the lessening of over-reduction of the photosynthetic apparatus (Saradadevi and Raghavendra 1992). Also, respiratory chain
activity (which will be slowed if oxidative phosphorylation is inhibited as in
the experiments of Kromer and his colleagues) may be needed for continued
operation of photorespiration via the turnover of mitochondrial NADH
required by glycine decarboxylase.
Clearly, photosynthetic metabolism can alter the demands for respiratory
products. Indeed, mutual interactions between photosynthesis and respiration in the light are likely. Quantitative effects of light on respiration
remain largely unknown, however. A diversity of estimates of effects of
photosynthesis on respiration are not necessarily contradictions, for they
might result from differing needs for respiration in the light in different
studies; photosynthesis will not inhibit respiration by some constant fraction irrespective of environmental, physiological, and ontogenetic factors.
Regarding the significance of effects of photosynthesis on respiration, it was
suggested (Amthor 1989) that respiration by mature leaves is not overly
important to the whole-plant carbon balance, especially in crops during seed
growth (see, e.g., Gaastra 1963). In forests, however, where net primary
production may be a smaller fraction of gross primary production (Kira
1975), leaf respiration may account for a majority of whole-plant respiration
(Allen and Lemon 1976; Hagihara and Hozumi 1991), and effects of light on
respiration are more important to productivity. To the extent that photo-
83
by light in growing tissue, but perhaps slowed by light in mature tissue.
McCashin et al. (1988) presented evidence that TCA cycle activity was
slowed only 20% (but see Gemel and Randall 1992) by light in growing
Triticum aestivum leaves, but they used nonphysiological, i.e., nonphotorespiratory, conditions. Evidence that the TCA cycle (but not necessarily
mitochondrial electron transport and oxidative phosphorylation) is slowed in
the light comes from the partial inactivation of mitochondrial pyruvate dehydrogenase complex (mtPDC) by light. The light inactivation is apparently
transduced through some product of photosynthesis or photo respiration
resulting in the phosphorylation of the mtPDC (Gemel and Randall 1992).
Graham (1980) surmised that "inhibition of oxidative phosphorylation in
the light seems probable." Conversely, the results of Kromer et al. (1988)
and Kromer and Heldt (1991a) indicate that oxidative phosphorylation is
actually required for rapid photosynthesis (measured as O2 evolution). They
suggested that mitochondria contribute needed ATP to the cytosol in photosynthesizing cells and can oxidize excess photosynthetic redox equivalents
(see also Ebbighausen et al. 1985). Oxidative phosphorylation may be most
important to photosynthesis in leaves not capable of rapidly producing
starch - respiratory ATP is apparently required to support rapid cytosolic
sucrose-P synthesis (Hanson 1992) and the consequent release of Pi for
further use in photosynthetic carbon metabolism (Sharkey 1985), whereas
rapid starch synthesis (and Pi release) can occur without concurrent respiration. Active mitochondrial electron transport might limit photoinhibition
of photosynthesis through the lessening of over-reduction of the photosynthetic apparatus (Saradadevi and Raghavendra 1992). Also, respiratory chain
activity (which will be slowed if oxidative phosphorylation is inhibited as in
the experiments of Kromer and his colleagues) may be needed for continued
operation of photorespiration via the turnover of mitochondrial NADH
required by glycine decarboxylase.
Clearly, photosynthetic metabolism can alter the demands for respiratory
products. Indeed, mutual interactions between photosynthesis and respiration in the light are likely. Quantitative effects of light on respiration
remain largely unknown, however. A diversity of estimates of effects of
photosynthesis on respiration are not necessarily contradictions, for they
might result from differing needs for respiration in the light in different
studies; photosynthesis will not inhibit respiration by some constant fraction irrespective of environmental, physiological, and ontogenetic factors.
Regarding the significance of effects of photosynthesis on respiration, it was
suggested (Amthor 1989) that respiration by mature leaves is not overly
important to the whole-plant carbon balance, especially in crops during seed
growth (see, e.g., Gaastra 1963). In forests, however, where net primary
production may be a smaller fraction of gross primary production (Kira
1975), leaf respiration may account for a majority of whole-plant respiration
(Allen and Lemon 1976; Hagihara and Hozumi 1991), and effects of light on
respiration are more important to productivity. To the extent that photo-