Higher Plant Respiration and Its Relationships to Photosynthesis
89
existent (Amthor 1991; Baker et al. 1992; Wullschleger et al. 1992b). Photorespiration is inversely related to CO2, so mitochondrial NADH oxidation
during the day may also be inversely related to CO2, Decreased daytime
demand for NADH oxidation with elevated CO2 might lead to decreased
mitochondrial oxidative capacity (diminution of respiratory chain components), resulting in the observed decrease in nighttime respiration rate.
But, if a decrease in mitochondrial oxidative capacity due to slowed
mitochondrial photo respiratory NADH oxidation does lead to a decrease in
oxidative activity, respiration is not normally under tight control by product
use, unless the decreased capacity is also related to a decline in, say, ATP
use for respiratory chain maintenance.
In addition to negative effects of long-term daytime elevated CO2 on
specific respiration rate, nighttime shoot and leaf CO2 efflux is negatively
and instantaneously related to nighttime CO2 level (Decker and Wien 1958;
Gale 1982; Bunce 1990; Amthor et al. 1992; but see Ryle et al. 1992). The
apparent inhibition of respiration by CO2 in the dark is readily reversible
(Amthor et al. 1992). Elevated nighttime CO2 can slow mobilization of leaf
starch and this, too, may be related to slowed respiration (Wullschleger et
al. 1992a). Both past and present CO2 levels affect respiration rate.
4.6 Photosynthesis and Root Respiration
Root respiration supports growth and maintenance in roots and active uptake
of nutrients from the soil solution. It also supports nutrient assimilation,
although a considerable fraction of nitrate assimilation may occur in shoots
(Pate and Layzell 1990). Roots are frequently chosen for respiratory studies
to avoid concurrent photosynthesis and photorespiration, but root respiration is difficult to study in soil because access to roots is limited and soil
microbes also respire. (Soil microbes oxidize primarily dead roots, shoot
litter, and root exudates so that soil respiration is linked to long-term previous photosynthesis and plant growth.) Thus, root respiration and carbon
budgets are often studied in nutrient solution culture (e.g., Farrar and JHH
Williams 1991) whereas little is known of in situ root respiration (but
see, e.g., Mogensen 1977; Holthausen and Caldwell 1980; Wagner and
Buyanovsky 1989). In solution culture, a significant portion of nutrient
uptake may function to replace nutrients lost during efflux from roots
whereas in soil this component of uptake may be small (Macduff and
Jackson 1992). Theoretical estimates of minimal respiratory requirements
for ion uptake, maintenance, and growth have been outlined frequently
(e.g., Amthor 1993a).
The linkages between root and shoot growth and metabolism (Brouwer
1983) and stoichiometry between whole-plant photosynthesis and growth
(Farrar 1985) dictate a linkage between photosynthesis and root respiration
89
existent (Amthor 1991; Baker et al. 1992; Wullschleger et al. 1992b). Photorespiration is inversely related to CO2, so mitochondrial NADH oxidation
during the day may also be inversely related to CO2, Decreased daytime
demand for NADH oxidation with elevated CO2 might lead to decreased
mitochondrial oxidative capacity (diminution of respiratory chain components), resulting in the observed decrease in nighttime respiration rate.
But, if a decrease in mitochondrial oxidative capacity due to slowed
mitochondrial photo respiratory NADH oxidation does lead to a decrease in
oxidative activity, respiration is not normally under tight control by product
use, unless the decreased capacity is also related to a decline in, say, ATP
use for respiratory chain maintenance.
In addition to negative effects of long-term daytime elevated CO2 on
specific respiration rate, nighttime shoot and leaf CO2 efflux is negatively
and instantaneously related to nighttime CO2 level (Decker and Wien 1958;
Gale 1982; Bunce 1990; Amthor et al. 1992; but see Ryle et al. 1992). The
apparent inhibition of respiration by CO2 in the dark is readily reversible
(Amthor et al. 1992). Elevated nighttime CO2 can slow mobilization of leaf
starch and this, too, may be related to slowed respiration (Wullschleger et
al. 1992a). Both past and present CO2 levels affect respiration rate.
4.6 Photosynthesis and Root Respiration
Root respiration supports growth and maintenance in roots and active uptake
of nutrients from the soil solution. It also supports nutrient assimilation,
although a considerable fraction of nitrate assimilation may occur in shoots
(Pate and Layzell 1990). Roots are frequently chosen for respiratory studies
to avoid concurrent photosynthesis and photorespiration, but root respiration is difficult to study in soil because access to roots is limited and soil
microbes also respire. (Soil microbes oxidize primarily dead roots, shoot
litter, and root exudates so that soil respiration is linked to long-term previous photosynthesis and plant growth.) Thus, root respiration and carbon
budgets are often studied in nutrient solution culture (e.g., Farrar and JHH
Williams 1991) whereas little is known of in situ root respiration (but
see, e.g., Mogensen 1977; Holthausen and Caldwell 1980; Wagner and
Buyanovsky 1989). In solution culture, a significant portion of nutrient
uptake may function to replace nutrients lost during efflux from roots
whereas in soil this component of uptake may be small (Macduff and
Jackson 1992). Theoretical estimates of minimal respiratory requirements
for ion uptake, maintenance, and growth have been outlined frequently
(e.g., Amthor 1993a).
The linkages between root and shoot growth and metabolism (Brouwer
1983) and stoichiometry between whole-plant photosynthesis and growth
(Farrar 1985) dictate a linkage between photosynthesis and root respiration