86
I.S. Amthor
chloroplast dihydroxyacetone-P/3-phosphoglycerate shuttle (Fig. 4.5), glycolysis, or a chloroplast malate/OAA shuttle. An a-ketoglutarate/glutamate
- malate/aspartate shuttle between mitochondria and peroxisomes is apparently not important in leaves (Kromer and Heldt 1991b).
4.5 Daytime Photosynthesis and Nighttime Respiration
Respiration in a mature leaf at night can be positively related to the previous
daytime net photosynthesis of that leaf (Ludwig et al. 1975; Azcon-Bieto
and Osmond 1983). Similarly, fruit (Satterlee and Koller 1984) or shoot and
whole-plant (Amthor 1933b) nighttime respiration may be positively related
to the amount of photosynthesis during the previous daytime. High sugar
levels resulting from rapid photosynthesis may increase energy use for compartmentation. Short-term storage of sucrose occurs in vacuoles (ap Rees
1988), and although Kaiser and Heber (1984) reported that sucrose transport
across the tonoplast in Hordeum vulgare mesophyll protoplasts was not
energy-dependent, Getz (1991) observed ATP-dependent sucrose transport
into tonoplast vesicles from Beta vulgaris root tissue. High sugar levels
might also accelerate futile cycling between triose-Ps and hexose-Ps (by
mass action or allosteric mechanisms) which is thought to enhance the sensitivity of metabolism to demands for sugars and carbon skeletons (Dancer
et al. 1990; Hatzfeld and Stitt 1990; Hatzfeld et al. 1990). Fast respiration
after rapid photosynthesis is probably in part related to metabolic costs of
translocation (phloem loading, unloading, and related processes; Amthor
1993a) and is conceivably linked to maintenance of enzymes and membranes
used during the day for carbon and nitrogen assimilation and processing. In
the long term, ample carbohydrates resulting from rapid photosynthesis
might induce growth in immature organs as outlined above (see Fig. 4.4),
leading to increased respiration to support processes such as biosynthesis
and nitrogen assimilation.
According to Azcon-Bieto et al. (1983), respiration rate and engagement
of the alternative pathway (Fig. 4.3) are enhanced when sugar levels are
high. The implication is that the alternative pathway allows rapid respiration
(bypassing respiratory control by ADP) and functions to oxidize "excess"
sugars (e.g., Steingrover 1981). Ap Rees (1988), however, has argued that
insufficient evidence exists to show that the alternative pathway aids in the
disposition of excess carbohydrates. Moreover, if sugars stimulate rapid
growth, "overproduction" of NAD(P)H may result (Penning de Vries et al.
1974). Engagement of the alternative pathway, and the rotenone-resistant
bypass of complex I, would then facilitate the regeneration of NAD(P) + in
the face of ADP limitations and expedite biosynthetic processes rather than
waste carbohydrates.
I.S. Amthor
chloroplast dihydroxyacetone-P/3-phosphoglycerate shuttle (Fig. 4.5), glycolysis, or a chloroplast malate/OAA shuttle. An a-ketoglutarate/glutamate
- malate/aspartate shuttle between mitochondria and peroxisomes is apparently not important in leaves (Kromer and Heldt 1991b).
4.5 Daytime Photosynthesis and Nighttime Respiration
Respiration in a mature leaf at night can be positively related to the previous
daytime net photosynthesis of that leaf (Ludwig et al. 1975; Azcon-Bieto
and Osmond 1983). Similarly, fruit (Satterlee and Koller 1984) or shoot and
whole-plant (Amthor 1933b) nighttime respiration may be positively related
to the amount of photosynthesis during the previous daytime. High sugar
levels resulting from rapid photosynthesis may increase energy use for compartmentation. Short-term storage of sucrose occurs in vacuoles (ap Rees
1988), and although Kaiser and Heber (1984) reported that sucrose transport
across the tonoplast in Hordeum vulgare mesophyll protoplasts was not
energy-dependent, Getz (1991) observed ATP-dependent sucrose transport
into tonoplast vesicles from Beta vulgaris root tissue. High sugar levels
might also accelerate futile cycling between triose-Ps and hexose-Ps (by
mass action or allosteric mechanisms) which is thought to enhance the sensitivity of metabolism to demands for sugars and carbon skeletons (Dancer
et al. 1990; Hatzfeld and Stitt 1990; Hatzfeld et al. 1990). Fast respiration
after rapid photosynthesis is probably in part related to metabolic costs of
translocation (phloem loading, unloading, and related processes; Amthor
1993a) and is conceivably linked to maintenance of enzymes and membranes
used during the day for carbon and nitrogen assimilation and processing. In
the long term, ample carbohydrates resulting from rapid photosynthesis
might induce growth in immature organs as outlined above (see Fig. 4.4),
leading to increased respiration to support processes such as biosynthesis
and nitrogen assimilation.
According to Azcon-Bieto et al. (1983), respiration rate and engagement
of the alternative pathway (Fig. 4.3) are enhanced when sugar levels are
high. The implication is that the alternative pathway allows rapid respiration
(bypassing respiratory control by ADP) and functions to oxidize "excess"
sugars (e.g., Steingrover 1981). Ap Rees (1988), however, has argued that
insufficient evidence exists to show that the alternative pathway aids in the
disposition of excess carbohydrates. Moreover, if sugars stimulate rapid
growth, "overproduction" of NAD(P)H may result (Penning de Vries et al.
1974). Engagement of the alternative pathway, and the rotenone-resistant
bypass of complex I, would then facilitate the regeneration of NAD(P) + in
the face of ADP limitations and expedite biosynthetic processes rather than
waste carbohydrates.