The Consequences of Sunftecks for Photosynthesis
345
17.3 Mechanisms Regulating the Utilization of Sunftecks
The lack of repeatability and predictability of natural light regimes means
that most studies of the physiological regulation of sunfleck use have utilized
lamp and shutter systems to simulate the temporal nature of sunflecks in
a spatially uniform and repeatable manner. These simulated sunflecks
have been termed lightflecks (Pearcy et al. 1985). The requirement for fastresponse gas-exchange systems and computer-based data acquistion systems
have also tended to restrict most studies to laboratories. However, newer
systems and portable computers mean that fast-response systems suitable for
field use are now more readily constructed. Pfitsch and Pearcy (1989b), for
example, examined the response to lightflecks of leaves on Adenocaulon
plants in situ and found responses to be generally similar to those measured
for other species in the laboratory.
Using systems of the type briefly discussed above, considerable progress
has been made in the last 5-10 years in understanding the controls on
sunfleck utilization (see review by Pearcy 1990). These controls are now
known to be of two basic types: the slow responses to light changes that
comprise the induction requirement of photosynthesis and the much faster
changes in assimilation that relate to increases and decreases in metabolite
levels within the chloroplast. The induction requirement largely sets the
capacity of the system to respond to a light increase. The changes in
metabolite pools determine the rate of increase and decrease of photosynthesis occurring in response to a sunfleck itself. Metabolites built up during a
lightfleck can be used to support continued CO2 fixation for a short period
after the lightfleck (post-illumination CO2 fixation), substantially enhancing
the total carbon gain in some circumstances (Pearcy et al. 1985; Chazdon
and Pearcy 1986b).
The induction requirement photosynthesis in understory plants is now
known to be a function of light-dependent stomatal opening and of light
regulation of the primary COz-fixing enzyme, ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco) and the light-activated enzymes involved
with RuBP regeneration (Seemann et al. 1988; Woodrow and Mott 1989).
During induction, activation of RuBP regenerating system may limit for
the first minute or so (Sassenrath-Cole and Pearcy 1992) while later, the
increase in Rubisco activity and stomatal conductance become the primary
factors controlling the rate of increase in photosynthetic capacity (Kirschbaum
and Pearcy 1988). The increase in Rubisco activity is generally complete
within 5-lOmin, whereas stomatal conductance may continue to increase
for 30 to 60 min. The relative role of these two limitations depends on the
initial conductance established in the low light period prior to the beginning
of induction. However, the high humidities characteristic of understories
often cause stomatal conductance to be high, shifting more of the limitation
345
17.3 Mechanisms Regulating the Utilization of Sunftecks
The lack of repeatability and predictability of natural light regimes means
that most studies of the physiological regulation of sunfleck use have utilized
lamp and shutter systems to simulate the temporal nature of sunflecks in
a spatially uniform and repeatable manner. These simulated sunflecks
have been termed lightflecks (Pearcy et al. 1985). The requirement for fastresponse gas-exchange systems and computer-based data acquistion systems
have also tended to restrict most studies to laboratories. However, newer
systems and portable computers mean that fast-response systems suitable for
field use are now more readily constructed. Pfitsch and Pearcy (1989b), for
example, examined the response to lightflecks of leaves on Adenocaulon
plants in situ and found responses to be generally similar to those measured
for other species in the laboratory.
Using systems of the type briefly discussed above, considerable progress
has been made in the last 5-10 years in understanding the controls on
sunfleck utilization (see review by Pearcy 1990). These controls are now
known to be of two basic types: the slow responses to light changes that
comprise the induction requirement of photosynthesis and the much faster
changes in assimilation that relate to increases and decreases in metabolite
levels within the chloroplast. The induction requirement largely sets the
capacity of the system to respond to a light increase. The changes in
metabolite pools determine the rate of increase and decrease of photosynthesis occurring in response to a sunfleck itself. Metabolites built up during a
lightfleck can be used to support continued CO2 fixation for a short period
after the lightfleck (post-illumination CO2 fixation), substantially enhancing
the total carbon gain in some circumstances (Pearcy et al. 1985; Chazdon
and Pearcy 1986b).
The induction requirement photosynthesis in understory plants is now
known to be a function of light-dependent stomatal opening and of light
regulation of the primary COz-fixing enzyme, ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco) and the light-activated enzymes involved
with RuBP regeneration (Seemann et al. 1988; Woodrow and Mott 1989).
During induction, activation of RuBP regenerating system may limit for
the first minute or so (Sassenrath-Cole and Pearcy 1992) while later, the
increase in Rubisco activity and stomatal conductance become the primary
factors controlling the rate of increase in photosynthetic capacity (Kirschbaum
and Pearcy 1988). The increase in Rubisco activity is generally complete
within 5-lOmin, whereas stomatal conductance may continue to increase
for 30 to 60 min. The relative role of these two limitations depends on the
initial conductance established in the low light period prior to the beginning
of induction. However, the high humidities characteristic of understories
often cause stomatal conductance to be high, shifting more of the limitation