88
l.S. Amthor
4.5.2 CO2 Concentration
Ludwig et al. (1975) observed that the relationship between Lycopersicon
esculentum leaf nighttime respiration and previous daytime net photosynthesis was stronger when photosynthesis was varied by changing light level
(C02 held constant) compared to changing CO2 level (light held constant)
with onloff light treatments. In other instances too, elevated daytime CO2
level leads to a decline in the ratio of nighttime respiration to daytime
photosynthesis (e.g., Gifford et al. 1985; Du Cloux et al. 1987; Dutton et al.
1988; Gaudillere and Mousseau 1989), but the ratio may eventually return
to that of plants under ambient CO2 (Grodzinski 1992; Morin et al. 1992).
Charles-Edwards and Ludwig (1975) attributed this to high levels of "glycollate pathway products" (versus starch) with low CO2 and that the glycolate
pathway intermediates, e.g., glycine, were important substrates for CO2
releasing reactions during the night. Photorespiratory intermediates do
appear to contribute to CO2 efflux for the first ca. 20-30min of a dark
period following a period of constant light in Triticum aestivum leaves
(Azcon-Bieto and Osmond 1983), but the differential response of respiration
to previous photosynthesis across CO2 levels is more far-reaching.
It is not surprising that growth and respiration can respond markedly to a
change in light. Diurnal and seasonal cycles of light include large amplitudes,
and spatial variation in light levels within plant communities is often high.
Plants are adapted to such spatial and temporal variations in light, and
several photo receptors are known to exist. Conversely, natural temporal
and spatial variations in CO2 are small. Indeed, no CO2 sensor is known to
exist in higher plants, although carbamate formation on Rubisco might serve
such a function (see Lorimer 1983).
Elevated CO2 increases the ratio of starch to sucrose, with surcose production being linked to growth and other processes using respiratory products, but with starch accumulating in a less coordinated way (Morin et al.
1992). (Wullschleger et al. (1992a) observed decreased leaf sucrose in spite
of enhanced photosynthesis due to elevated CO2 in two field-grown tree
species.) Some of the increase in carbon accumulation and decline in
the ratio of nighttime respiration to daytime net photosynthesis (increase
in growth efficiency, Y(C); see Appendix) with elevated CO2 is due to
increased short-term storage pool size (Rowland-Bamford et al. 1990) rather
than only increased efficiency of growth or respiration. A corollary is that
24-h net CO2 exchange is not a reliable measure of growth because the size
of the short-term storage pools can vary considerably from day to day
depending on environmental and developmental circumstances (Mold au and
Karolin 1977; McCree 1986).
Long-term elevated daytime CO2 often results in a decrease in specific
(dry mass basis) respiration rate even though photosynthesis increases
(Amthor 1991; Drake and Leadley 1991). On a nitrogen or protein basis,
however, effects of CO2 history on respiration are less striking or non-
l.S. Amthor
4.5.2 CO2 Concentration
Ludwig et al. (1975) observed that the relationship between Lycopersicon
esculentum leaf nighttime respiration and previous daytime net photosynthesis was stronger when photosynthesis was varied by changing light level
(C02 held constant) compared to changing CO2 level (light held constant)
with onloff light treatments. In other instances too, elevated daytime CO2
level leads to a decline in the ratio of nighttime respiration to daytime
photosynthesis (e.g., Gifford et al. 1985; Du Cloux et al. 1987; Dutton et al.
1988; Gaudillere and Mousseau 1989), but the ratio may eventually return
to that of plants under ambient CO2 (Grodzinski 1992; Morin et al. 1992).
Charles-Edwards and Ludwig (1975) attributed this to high levels of "glycollate pathway products" (versus starch) with low CO2 and that the glycolate
pathway intermediates, e.g., glycine, were important substrates for CO2
releasing reactions during the night. Photorespiratory intermediates do
appear to contribute to CO2 efflux for the first ca. 20-30min of a dark
period following a period of constant light in Triticum aestivum leaves
(Azcon-Bieto and Osmond 1983), but the differential response of respiration
to previous photosynthesis across CO2 levels is more far-reaching.
It is not surprising that growth and respiration can respond markedly to a
change in light. Diurnal and seasonal cycles of light include large amplitudes,
and spatial variation in light levels within plant communities is often high.
Plants are adapted to such spatial and temporal variations in light, and
several photo receptors are known to exist. Conversely, natural temporal
and spatial variations in CO2 are small. Indeed, no CO2 sensor is known to
exist in higher plants, although carbamate formation on Rubisco might serve
such a function (see Lorimer 1983).
Elevated CO2 increases the ratio of starch to sucrose, with surcose production being linked to growth and other processes using respiratory products, but with starch accumulating in a less coordinated way (Morin et al.
1992). (Wullschleger et al. (1992a) observed decreased leaf sucrose in spite
of enhanced photosynthesis due to elevated CO2 in two field-grown tree
species.) Some of the increase in carbon accumulation and decline in
the ratio of nighttime respiration to daytime net photosynthesis (increase
in growth efficiency, Y(C); see Appendix) with elevated CO2 is due to
increased short-term storage pool size (Rowland-Bamford et al. 1990) rather
than only increased efficiency of growth or respiration. A corollary is that
24-h net CO2 exchange is not a reliable measure of growth because the size
of the short-term storage pools can vary considerably from day to day
depending on environmental and developmental circumstances (Mold au and
Karolin 1977; McCree 1986).
Long-term elevated daytime CO2 often results in a decrease in specific
(dry mass basis) respiration rate even though photosynthesis increases
(Amthor 1991; Drake and Leadley 1991). On a nitrogen or protein basis,
however, effects of CO2 history on respiration are less striking or non-