90
l.S. Amthor
to support nutrient acquisition for the whole plant and growth and maintenance of the root. In herbs, a change in photosynthesis caused by higher
or lower light results in a respective increased or decrease in root respiration
within a few hours (Hansen 1977; Massimino et al. 1981). Carbohydrate reserves allow some temporal delays in the linkage. At constant temperature,
root respiration may be most rapid during the day (Huck et al. 1962). The
relationship between photosynthesis and root respiration is paralleled by
changes in nutrient uptake (Huck et al. 1962; Massimino et al. 1981; Gastal
and Saugier 1989) and perhaps nitrate reduction (Hansen 1980) and root
growth. Daily root respiration may be linearly related to translocation of
sugar to roots (Hatrick and Bowling 1973), which is likely to depend to daytime photosynthesis. Respiration rate of nodulated Trifolium repens roots is
sensitive to changes in shoot photosynthesis in as short a time as 10 min,
which may reflect the time taken to translocate photosynthate from leaves
to roots and indicates that "current photosynthate [rather than reserve
carbohydrate] is the primary source of energy for N2 fixation" in such plants
(Ryle et al. 1985). On the whole, herb root and nodule respiration responds
positively to shoot photosynthesis; this is not surprising. In keeping with
the previous theme, translocation of sugars to roots will tend to stimulate
processes using sugars and respiratory products, and root activity itself is
likely to facilitate import of photosynthate. In nature, respiration in individual roots or groups of roots is likely to vary from the average r~te of the
whole root system due to local differences in soil conditions and root activity
and ontogeny (Holthausen and Caldwell 1980).
4.7 Conclusions
Photosynthesis, photo respiration , and respiration most likely interact in
photosynthetic cells during the day because the three processes share intermediates. Both photorespiration and respiration release CO2 in mitochondria
and probably compete for access to the respiratory chain. Photosynthesis
and respiration can each supply ATP, reductant, and carbon skeletons to
the cytosol. Moreover, products of the individual processes are substrates
and cofactors of the other processes, e.g., CO2 released by respiration
and photo respiration is a photosynthetic substrate and sugars produced by
photosynthesis are respiratory substrates. Respiration and photosynthesis
can interact via source-sink relationships, too; an increase in sink activity
can stimulate photosynthesis whereas a decline in sink activity may slow
photosynthesis (Herold 1980) with respiration often coupled to sink activity.
Respiration and photosynthesis are also related because respiration contributes to the construction of photosynthetic machinery.
There is a more or less fixed ratio of daily whole-plant carbon gain in
photosynthesis (less photorespiration), carbon use in growth, and carbon
l.S. Amthor
to support nutrient acquisition for the whole plant and growth and maintenance of the root. In herbs, a change in photosynthesis caused by higher
or lower light results in a respective increased or decrease in root respiration
within a few hours (Hansen 1977; Massimino et al. 1981). Carbohydrate reserves allow some temporal delays in the linkage. At constant temperature,
root respiration may be most rapid during the day (Huck et al. 1962). The
relationship between photosynthesis and root respiration is paralleled by
changes in nutrient uptake (Huck et al. 1962; Massimino et al. 1981; Gastal
and Saugier 1989) and perhaps nitrate reduction (Hansen 1980) and root
growth. Daily root respiration may be linearly related to translocation of
sugar to roots (Hatrick and Bowling 1973), which is likely to depend to daytime photosynthesis. Respiration rate of nodulated Trifolium repens roots is
sensitive to changes in shoot photosynthesis in as short a time as 10 min,
which may reflect the time taken to translocate photosynthate from leaves
to roots and indicates that "current photosynthate [rather than reserve
carbohydrate] is the primary source of energy for N2 fixation" in such plants
(Ryle et al. 1985). On the whole, herb root and nodule respiration responds
positively to shoot photosynthesis; this is not surprising. In keeping with
the previous theme, translocation of sugars to roots will tend to stimulate
processes using sugars and respiratory products, and root activity itself is
likely to facilitate import of photosynthate. In nature, respiration in individual roots or groups of roots is likely to vary from the average r~te of the
whole root system due to local differences in soil conditions and root activity
and ontogeny (Holthausen and Caldwell 1980).
4.7 Conclusions
Photosynthesis, photo respiration , and respiration most likely interact in
photosynthetic cells during the day because the three processes share intermediates. Both photorespiration and respiration release CO2 in mitochondria
and probably compete for access to the respiratory chain. Photosynthesis
and respiration can each supply ATP, reductant, and carbon skeletons to
the cytosol. Moreover, products of the individual processes are substrates
and cofactors of the other processes, e.g., CO2 released by respiration
and photo respiration is a photosynthetic substrate and sugars produced by
photosynthesis are respiratory substrates. Respiration and photosynthesis
can interact via source-sink relationships, too; an increase in sink activity
can stimulate photosynthesis whereas a decline in sink activity may slow
photosynthesis (Herold 1980) with respiration often coupled to sink activity.
Respiration and photosynthesis are also related because respiration contributes to the construction of photosynthetic machinery.
There is a more or less fixed ratio of daily whole-plant carbon gain in
photosynthesis (less photorespiration), carbon use in growth, and carbon