The Consequences of Sunftecks for Photosynthesis
349
the chamber so that sunflecks were blocked but diffuse light was received.
Daily assimilation on this day (day F) was 14mmolm- 2 d- 1 . The carbon
gain resulting from utilization of sunflecks ranged from about 3 mmol m- 2 d- 1
on the day with the least sunflecks (day C) to over 30 mmol m -2 day-l (day
E). These values represent 20 to 65% of the total carbon gain.
In addition to the daily course shown in Fig. 17.1, steady-state light
response curves for assimilation were determined for each leaf on the next
day using an artificial light source. These curves made it possible to accurately fit a simple model of the steady-state photosynthetic light response
for each leaf, which would take as an input the light data shown in Fig.
17.1 and calculate photosynthesis at 1-s intervals. The model assumes an
instantaneous response to the new light input and thus ignores induction
limitations or enhancements due to post-illumination CO2 fixation. Therefore, comparisons between the measured and simulated daily carbon gains
reveal the effects of the dynamic responses that are important to sunfleck
utilization.
Figure 17.2 shows the measured and modeled daily assimilation totals as a
function of the daily PFD. Both gave nearly identical totals on day F when
the leaf received no sunflecks and on day A when most of the assimilation
occurred during a single period of sunfleck activity dominated by one long
sunfleck. On the other days, the measured assimilation totals were 15 to
30% lower than the predicted values. On these days, there was no apparent
relationship between the nature of the sunfleck activity and the difference
between the measured and predicted values. Both the measured and predicted values on day A were low relative to the trend of daily assimilation and
daily PFD for other measurements, probably because the sunflecks had
much higher PFD on this day as compared to the others.
The lower measured than predicted carbon gain is evidence that induction
is a significant limitation to sunfleck utilization by Adenocaulon in its natural
environment. Post-illumination CO2 fixation may partially offset some of the
induction limitation, but it is clear that it is much less than needed to
overcome it. Although the vast majority of sunflecks are brief (Fig. 17.3,
top), most of the PFD is supplied in longer sunflecks (Fig. 17.3, bottom) for
which post-illumination CO2 fixation makes only a small contribution to the
total. Moreover, many of the brief sunflecks have too Iowa PFD to cause
significant post illumination CO2 fixation. The situation may, however, be
quite different in aspen canopies where post-illumination CO 2 fixation
resulting from the many brief but high PFD sunflecks may be sufficient to
overcome any induction limitations (Roden and Pearcy 1992).
17.5 The Significance of Sunflecks to annual Carbon Gain
Cloudiness and other factors make it difficult to extrapolate directly from
the contribution of sunflecks on clear days to a contribution on an annual
349
the chamber so that sunflecks were blocked but diffuse light was received.
Daily assimilation on this day (day F) was 14mmolm- 2 d- 1 . The carbon
gain resulting from utilization of sunflecks ranged from about 3 mmol m- 2 d- 1
on the day with the least sunflecks (day C) to over 30 mmol m -2 day-l (day
E). These values represent 20 to 65% of the total carbon gain.
In addition to the daily course shown in Fig. 17.1, steady-state light
response curves for assimilation were determined for each leaf on the next
day using an artificial light source. These curves made it possible to accurately fit a simple model of the steady-state photosynthetic light response
for each leaf, which would take as an input the light data shown in Fig.
17.1 and calculate photosynthesis at 1-s intervals. The model assumes an
instantaneous response to the new light input and thus ignores induction
limitations or enhancements due to post-illumination CO2 fixation. Therefore, comparisons between the measured and simulated daily carbon gains
reveal the effects of the dynamic responses that are important to sunfleck
utilization.
Figure 17.2 shows the measured and modeled daily assimilation totals as a
function of the daily PFD. Both gave nearly identical totals on day F when
the leaf received no sunflecks and on day A when most of the assimilation
occurred during a single period of sunfleck activity dominated by one long
sunfleck. On the other days, the measured assimilation totals were 15 to
30% lower than the predicted values. On these days, there was no apparent
relationship between the nature of the sunfleck activity and the difference
between the measured and predicted values. Both the measured and predicted values on day A were low relative to the trend of daily assimilation and
daily PFD for other measurements, probably because the sunflecks had
much higher PFD on this day as compared to the others.
The lower measured than predicted carbon gain is evidence that induction
is a significant limitation to sunfleck utilization by Adenocaulon in its natural
environment. Post-illumination CO2 fixation may partially offset some of the
induction limitation, but it is clear that it is much less than needed to
overcome it. Although the vast majority of sunflecks are brief (Fig. 17.3,
top), most of the PFD is supplied in longer sunflecks (Fig. 17.3, bottom) for
which post-illumination CO2 fixation makes only a small contribution to the
total. Moreover, many of the brief sunflecks have too Iowa PFD to cause
significant post illumination CO2 fixation. The situation may, however, be
quite different in aspen canopies where post-illumination CO 2 fixation
resulting from the many brief but high PFD sunflecks may be sufficient to
overcome any induction limitations (Roden and Pearcy 1992).
17.5 The Significance of Sunflecks to annual Carbon Gain
Cloudiness and other factors make it difficult to extrapolate directly from
the contribution of sunflecks on clear days to a contribution on an annual