356
R.W. Pearcy and W.A. Ptitsch
stresses that limit growth at the same time as additional carbon is
gained.
An example the stress created by a sunfleck is shown in Fig. 17.6.
Sunflecks increase the leaf temperature and hence the transpiration rate.
Consequently, water potentials decrease rapidly, reaching the turgor loss
point, before recovering more slowly after the sunfleck passes. Visible leaf
wilting as evidenced by a change in reflectance and curling of the leaf
margins occurred during prolonged (>10 min) sunflecks. Both stomatal
conductance and light-saturated assimilation rate decrease by 30 to 50%
during sunflecks, presumably because of the wilting, but recovery as measured by g and light-saturated A was 80-90% complete within 1.5 h after
the sunfleck. The only exceptions were during long, intense sunflecks on
a warm day when leaf temperatures exceeded 37°C. Recovery of photosynthetic rates was less than 50% 2 h after the sunfleck and only 75%
complete on the next day in this case.
17.7 Conclusions
It is clear from both the daily gas exchange measurements and the seasonal
estimates from carbon isotope discrimination that sunflecks are of considerable importance to the carbon gain of Adenocaulon plants. The 30 to
65% of daily carbon gain that was due to utilization of sunflecks is similar to
values reported for understory tree seedlings in Australian and Hawaiian
tropical forests (Pearcy and Calkin 1983; Pearcy 1987) but is considerably
higher than the 6 to 19% previously estimated for tree seedlings and understory plants in deciduous forests (Schulze 1972; Weber et al. 1985). Two
factors may account for the difference. First, diffuse light levels were considerably higher in the deciduous forest, decreasing the relative importance
of assimilation due to sunflecks. Second, deciduous forest species had lower
photosynthetic capacities per unit leaf area than Adenocaulon or the tropical
forest species studied, which also reduces the contribution by sunflecks. The
exception reported by Schulze was Deschampsia ftexuosa, which had higher
photosynthetic capacities and depended on sunflecks for 27% of its assimilation. The reasons for the generally lower photosynthetic capacities despite
a higher overall light level in the deciduous forest are unclear. It may possibly indicate an additional limitation by nutrients or some other factor.
Alternatively, it may be that selection or acclimation has favored increased
efficiency of utilization of diffuse light in the deciduous forest plants, where
it is relatively more important as compared to tropical and redwood forest
plants, where sunflecks are a more important component of the available
light. In the latter, higher photosynthetic capacities might be advantageous
in allowing better utilization of sunflecks. Better use of diffuse light could
R.W. Pearcy and W.A. Ptitsch
stresses that limit growth at the same time as additional carbon is
gained.
An example the stress created by a sunfleck is shown in Fig. 17.6.
Sunflecks increase the leaf temperature and hence the transpiration rate.
Consequently, water potentials decrease rapidly, reaching the turgor loss
point, before recovering more slowly after the sunfleck passes. Visible leaf
wilting as evidenced by a change in reflectance and curling of the leaf
margins occurred during prolonged (>10 min) sunflecks. Both stomatal
conductance and light-saturated assimilation rate decrease by 30 to 50%
during sunflecks, presumably because of the wilting, but recovery as measured by g and light-saturated A was 80-90% complete within 1.5 h after
the sunfleck. The only exceptions were during long, intense sunflecks on
a warm day when leaf temperatures exceeded 37°C. Recovery of photosynthetic rates was less than 50% 2 h after the sunfleck and only 75%
complete on the next day in this case.
17.7 Conclusions
It is clear from both the daily gas exchange measurements and the seasonal
estimates from carbon isotope discrimination that sunflecks are of considerable importance to the carbon gain of Adenocaulon plants. The 30 to
65% of daily carbon gain that was due to utilization of sunflecks is similar to
values reported for understory tree seedlings in Australian and Hawaiian
tropical forests (Pearcy and Calkin 1983; Pearcy 1987) but is considerably
higher than the 6 to 19% previously estimated for tree seedlings and understory plants in deciduous forests (Schulze 1972; Weber et al. 1985). Two
factors may account for the difference. First, diffuse light levels were considerably higher in the deciduous forest, decreasing the relative importance
of assimilation due to sunflecks. Second, deciduous forest species had lower
photosynthetic capacities per unit leaf area than Adenocaulon or the tropical
forest species studied, which also reduces the contribution by sunflecks. The
exception reported by Schulze was Deschampsia ftexuosa, which had higher
photosynthetic capacities and depended on sunflecks for 27% of its assimilation. The reasons for the generally lower photosynthetic capacities despite
a higher overall light level in the deciduous forest are unclear. It may possibly indicate an additional limitation by nutrients or some other factor.
Alternatively, it may be that selection or acclimation has favored increased
efficiency of utilization of diffuse light in the deciduous forest plants, where
it is relatively more important as compared to tropical and redwood forest
plants, where sunflecks are a more important component of the available
light. In the latter, higher photosynthetic capacities might be advantageous
in allowing better utilization of sunflecks. Better use of diffuse light could