Higher Plant Respiration and Its Relationships to Photosynthesis
87
Work to date relating respiration to previous photosynthesis has been
with herbs and with seedlings and small individuals of woody plants, whereas
the bulk of global higher plant photosynthesis and respiration occurs in large
trees. Temporal relationships between whole-tree photosynthesis and respiration may be loose because of the greater average distance of transport
from sources to sinks and slower turnover time of some carbohydrate storage
pools. Knowledge of those relationships, however, is scant. Also, differences
between on/off and sinusoidal (natural) light treatments have not been well
studied with respect to the links between photosynthesis and respiration
occurring over a few hours, with most quantitative research employing
on/off light treatments. Temporal patterns of leaf metabolite levels differ
with sinusoidal and on/off light regimes (Fondy et al. 1989; Servaites et al.
1989a,b) and respiration could respond to those differences.
4.5.1 Light Level
One consequence of long-term high light is fast leaf respiration (Bjorkman
1981). This is independent of photosynthetic capacity (Sims and Pearcy
1991) and may reflect a response to daytime net photosynthesis. Actual CO2
assimilation (use of machinery) rather than capacity for assimilation (amount
of machinery) could be the link to maintenance processes and therefore leaf
maintenance respiration. Respiratory acclimation to a change in prevailing
light can occur within a few days and disrupts the relationship between
mature leaf respiration rate and nitrogen content (Sims and Pearcy 1991),
showing that a leaf maintenance coefficient based on nitrogen or protein
level (m(N); see Appendix) is a function of prevailing light as well. Also, the
true growth yield (Y G(C); see Appendix) of leaves can be inversely related to
light level during growth (Williams et al. 1989).
In addition to the relationship between daytime light levels and subsequent respiration caused by different photosynthesis rates, Heichel (1970)
found that leaf respiration of the C4 species Zea mays was positively related
to previous light level independent of net CO2 assimilation. Measurements
were made in CO2-free air so CO2 was assimilated only at the rate at which
it was released in respiration and other decarboxylation processes. Heichel
(1970) concluded that "light ... was required to produce a substrate which
was subsequently used in ... respiration," but it is also plausible that nonCOr assimilating light-driven reactions related to, e.g., nitrogen metabolism,
were supported in part by respiration and that such metabolism (including
related maintenance processes) continued into the dark period. Notably,
blue light can stimulate respiration by unknown mechanisms in the absence
of photosynthesis and this can persist into a following dark period (Kowallik
1982). Both past and present light levels affect respiration rate.
87
Work to date relating respiration to previous photosynthesis has been
with herbs and with seedlings and small individuals of woody plants, whereas
the bulk of global higher plant photosynthesis and respiration occurs in large
trees. Temporal relationships between whole-tree photosynthesis and respiration may be loose because of the greater average distance of transport
from sources to sinks and slower turnover time of some carbohydrate storage
pools. Knowledge of those relationships, however, is scant. Also, differences
between on/off and sinusoidal (natural) light treatments have not been well
studied with respect to the links between photosynthesis and respiration
occurring over a few hours, with most quantitative research employing
on/off light treatments. Temporal patterns of leaf metabolite levels differ
with sinusoidal and on/off light regimes (Fondy et al. 1989; Servaites et al.
1989a,b) and respiration could respond to those differences.
4.5.1 Light Level
One consequence of long-term high light is fast leaf respiration (Bjorkman
1981). This is independent of photosynthetic capacity (Sims and Pearcy
1991) and may reflect a response to daytime net photosynthesis. Actual CO2
assimilation (use of machinery) rather than capacity for assimilation (amount
of machinery) could be the link to maintenance processes and therefore leaf
maintenance respiration. Respiratory acclimation to a change in prevailing
light can occur within a few days and disrupts the relationship between
mature leaf respiration rate and nitrogen content (Sims and Pearcy 1991),
showing that a leaf maintenance coefficient based on nitrogen or protein
level (m(N); see Appendix) is a function of prevailing light as well. Also, the
true growth yield (Y G(C); see Appendix) of leaves can be inversely related to
light level during growth (Williams et al. 1989).
In addition to the relationship between daytime light levels and subsequent respiration caused by different photosynthesis rates, Heichel (1970)
found that leaf respiration of the C4 species Zea mays was positively related
to previous light level independent of net CO2 assimilation. Measurements
were made in CO2-free air so CO2 was assimilated only at the rate at which
it was released in respiration and other decarboxylation processes. Heichel
(1970) concluded that "light ... was required to produce a substrate which
was subsequently used in ... respiration," but it is also plausible that nonCOr assimilating light-driven reactions related to, e.g., nitrogen metabolism,
were supported in part by respiration and that such metabolism (including
related maintenance processes) continued into the dark period. Notably,
blue light can stimulate respiration by unknown mechanisms in the absence
of photosynthesis and this can persist into a following dark period (Kowallik
1982). Both past and present light levels affect respiration rate.