Higher Plant Respiration and Its Relationships to Photosynthesis
77
JHH Williams 1991). That is, in all but the youngest cells, respiration may
be pulled along at a rate appropriate to the rate of growth, transport,
nutrient uptake and assimilation, and maintenance processes, and this is
generally below the rate limitation set by the amount of carbohydrates and
respiratory enzymes. Because respiratory products are used by nearly all
heterotrophic processes, and because the respiratory network is flexible and
contains many branch points (see Figs. 4.2 and 4.3), "it seems inevitable
that control will be found to be extremely complex and to be shared among
a variety of reactions and a range of regulators" (ap Rees 1988).
Even when respiration is coupled to other processes via the use of its
products, it does not follow that those other processes obligatorily use
respiratory products with maximum efficiency. Although it is difficult to
imagine extensive inefficiencies in higher plants following their long evolution, there is no compelling evidence that plant metabolism is optimal, nor
need there be any, for "contrary to a widely held belief, the anatomical and
physiological features of ... extant life forms are not necessarily optimal
solutions from the synchronic point of view, which takes into account only
the present situation .... [But instead,] it appears that evolution used, not
what was theoretically optimal, but whatever happened to be available to it
and could be appropriated to serve a needed function" (Delbriick 1986) (see
also Maynard Smith 1978). Moreover, "consumption of ATP by simple
hydrolysis in which there is no outcome useful to the plant probably occurs
in all cells at a finite rate and would yield a minimal background of idling
respiration" (Beevers 1970). Also, in most plant communities, no more than
a few percent of the energy in solar radiation absorbed by leaves is contained
in the plant mass resulting from growth (Larcher 1983). Nevertheless, the
efficiency of higher plant heterotrophic metabolism is, by many measures,
impressive.
4.2.3 Energy Conservation During Plant Respiration
As many as 30 to 36 ADP can be phosphorylated per hexose oxidized
completely during plant respiration (see Figs. 4.2 and 4.3; Nicholls and
Ferguson 1992; Amthor 1993a). When ATP use is rapid but carbon skeleton
use is slow, the mitochondrial ADP: 0 can be expected to be high (perhaps
near 3) and the ratio of carbon diverted away from the respiratory network to that released as CO2 will be small. That is to say, carbohydrate
metabolism in respiration may yield mostly CO2, heat, and ATP in mature
cells rather than carbon skeletons. Under different circumstances, e.g., in
rapidly growing tissue, NADH produced during carbon skeleton use in
growth may exceed that required for ADP phosphorylation (for ATP use)
via the respiratory chain and oxidative phosphorylation (Penning de Vries et
al. 1974). In such a case, the circumvention of respiratory control by ADP
availability may be required. The rotenone-resistant complex I bypass (item
77
JHH Williams 1991). That is, in all but the youngest cells, respiration may
be pulled along at a rate appropriate to the rate of growth, transport,
nutrient uptake and assimilation, and maintenance processes, and this is
generally below the rate limitation set by the amount of carbohydrates and
respiratory enzymes. Because respiratory products are used by nearly all
heterotrophic processes, and because the respiratory network is flexible and
contains many branch points (see Figs. 4.2 and 4.3), "it seems inevitable
that control will be found to be extremely complex and to be shared among
a variety of reactions and a range of regulators" (ap Rees 1988).
Even when respiration is coupled to other processes via the use of its
products, it does not follow that those other processes obligatorily use
respiratory products with maximum efficiency. Although it is difficult to
imagine extensive inefficiencies in higher plants following their long evolution, there is no compelling evidence that plant metabolism is optimal, nor
need there be any, for "contrary to a widely held belief, the anatomical and
physiological features of ... extant life forms are not necessarily optimal
solutions from the synchronic point of view, which takes into account only
the present situation .... [But instead,] it appears that evolution used, not
what was theoretically optimal, but whatever happened to be available to it
and could be appropriated to serve a needed function" (Delbriick 1986) (see
also Maynard Smith 1978). Moreover, "consumption of ATP by simple
hydrolysis in which there is no outcome useful to the plant probably occurs
in all cells at a finite rate and would yield a minimal background of idling
respiration" (Beevers 1970). Also, in most plant communities, no more than
a few percent of the energy in solar radiation absorbed by leaves is contained
in the plant mass resulting from growth (Larcher 1983). Nevertheless, the
efficiency of higher plant heterotrophic metabolism is, by many measures,
impressive.
4.2.3 Energy Conservation During Plant Respiration
As many as 30 to 36 ADP can be phosphorylated per hexose oxidized
completely during plant respiration (see Figs. 4.2 and 4.3; Nicholls and
Ferguson 1992; Amthor 1993a). When ATP use is rapid but carbon skeleton
use is slow, the mitochondrial ADP: 0 can be expected to be high (perhaps
near 3) and the ratio of carbon diverted away from the respiratory network to that released as CO2 will be small. That is to say, carbohydrate
metabolism in respiration may yield mostly CO2, heat, and ATP in mature
cells rather than carbon skeletons. Under different circumstances, e.g., in
rapidly growing tissue, NADH produced during carbon skeleton use in
growth may exceed that required for ADP phosphorylation (for ATP use)
via the respiratory chain and oxidative phosphorylation (Penning de Vries et
al. 1974). In such a case, the circumvention of respiratory control by ADP
availability may be required. The rotenone-resistant complex I bypass (item