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R.W. Pearcy and W.A. Ptitsch
to Rubisco. For example, there appeared to be very little limitation imposed
by stomatal conductance in Adenocaulon leaves in the field because of the
high humidities.
An important feature of both Rubisco and stomatal conductance responses to lightflecks is their hysteretic behavior. The decreases in both
stomatal conductance and Rubisco are slower than the increases. Moreover,
for short «10 min) sunflecks, stomatal opening continues for up to 20 min
afterwards. Thus, the occurrence of a sunfleck primes the leaf so that it is
better able to use subsequent sunflecks. Tinoco-Ojanguren and Pearcy
(1992) found that use of series of short (1-16s) lightflecks was significantly
enhanced by the occurrence 20 min earlier of a 4-min lightfleck. This enhancement was related to the higher conductances following the 4-min
lightfleck. Since sunflecks often occur in clusters, this priming effect helps
to minimize the induction limitations during subsequent sunflecks in the
clusters.
Under natural sunfleck regimes, the induction requirement of photosynthesis, post-illumination CO2 fixation and the steady-state photosynthetic
characteristics all interact to determine the carbon gain. Throughout much
of the day, leaves are unlikely to be fully induced, which will limit the
utilization of sunflecks. Daily courses of stomatal conductance show a
continuous modulation, increasing during sunfleck periods and decreasing in
low light periods (Bjorkman et al. 1972; Pearcy 1987), but rarely, if ever,
reaching steady-state values. Similar studies of Rubisco activity have not
been conducted with understory plants, but simulations of its behavior
(Pearcy and Gross, unpubl.) suggest that it, too, would be continuously
modulated at intermediate activity levels. Post-illumination CO2 fixation
may offset these induction limitations to a certain extent, but its effect will
depend on the frequency of short « 1 0 s) sunflecks where its contribution
can be a large fraction of the total CO2 assimilation due to a sunfleck.
17.4 Photosynthesis in Natural Sunfleck Pegimes
Despite their potentially large contribution to the carbon gain of understory
plants, few studies have been carried out in the field to assess the importance of the spatial and temporal variation of sunflecks or of the mechanisms
that regulate their use. Pfitsch and Pearcy (1989a) measured the daily course
of gas exchange of Adenocaulon leaves at six microsites on different days.
These days were all similar, with the fog and overcast that is common in the
morning dissipating by 0830 to 0930 h and the remainder of the day being
clear. Thus differences among sites probably reflect more the spatial variation
in sunflecks than any seasonal or day-to-day variation. The microsites were
selected to be typical of those where Adenocaulon occurred and to give a
range of sunfleck activity.
R.W. Pearcy and W.A. Ptitsch
to Rubisco. For example, there appeared to be very little limitation imposed
by stomatal conductance in Adenocaulon leaves in the field because of the
high humidities.
An important feature of both Rubisco and stomatal conductance responses to lightflecks is their hysteretic behavior. The decreases in both
stomatal conductance and Rubisco are slower than the increases. Moreover,
for short «10 min) sunflecks, stomatal opening continues for up to 20 min
afterwards. Thus, the occurrence of a sunfleck primes the leaf so that it is
better able to use subsequent sunflecks. Tinoco-Ojanguren and Pearcy
(1992) found that use of series of short (1-16s) lightflecks was significantly
enhanced by the occurrence 20 min earlier of a 4-min lightfleck. This enhancement was related to the higher conductances following the 4-min
lightfleck. Since sunflecks often occur in clusters, this priming effect helps
to minimize the induction limitations during subsequent sunflecks in the
clusters.
Under natural sunfleck regimes, the induction requirement of photosynthesis, post-illumination CO2 fixation and the steady-state photosynthetic
characteristics all interact to determine the carbon gain. Throughout much
of the day, leaves are unlikely to be fully induced, which will limit the
utilization of sunflecks. Daily courses of stomatal conductance show a
continuous modulation, increasing during sunfleck periods and decreasing in
low light periods (Bjorkman et al. 1972; Pearcy 1987), but rarely, if ever,
reaching steady-state values. Similar studies of Rubisco activity have not
been conducted with understory plants, but simulations of its behavior
(Pearcy and Gross, unpubl.) suggest that it, too, would be continuously
modulated at intermediate activity levels. Post-illumination CO2 fixation
may offset these induction limitations to a certain extent, but its effect will
depend on the frequency of short « 1 0 s) sunflecks where its contribution
can be a large fraction of the total CO2 assimilation due to a sunfleck.
17.4 Photosynthesis in Natural Sunfleck Pegimes
Despite their potentially large contribution to the carbon gain of understory
plants, few studies have been carried out in the field to assess the importance of the spatial and temporal variation of sunflecks or of the mechanisms
that regulate their use. Pfitsch and Pearcy (1989a) measured the daily course
of gas exchange of Adenocaulon leaves at six microsites on different days.
These days were all similar, with the fog and overcast that is common in the
morning dissipating by 0830 to 0930 h and the remainder of the day being
clear. Thus differences among sites probably reflect more the spatial variation
in sunflecks than any seasonal or day-to-day variation. The microsites were
selected to be typical of those where Adenocaulon occurred and to give a
range of sunfleck activity.