We
examined in the field the photosynthetic utilization
of fluctuating light by six neotropical rainforest
shrubs of the family Rubiaceae. They were growing
in three different light environments: forest
understory, small gaps, and clearings. Gas exchange
techniques were used to analyse photosynthetic
induction response, induction maintenance during
low-light periods, and lightfleck (simulated
sunfleck) use efficiency (LUE). Total daily
photon flux density (PFD) reaching the plants
during the wet season was 37 times higher in
clearings than in the understory, with small
gaps exhibiting intermediate values. Sunflecks
were more frequent, but shorter and of lower
intensity in the understory than in clearings.
However, sunflecks contributed one-third of
the daily PFD in the understory. Maximum rates
of net photosynthesis, carboxylation capacity,
electron transport, and maximum stomatal conductance
were lower in understory species than in species
growing in small gaps or clearings, while the
reverse was true for the curvature factor of
the light response of photosynthesis. No significant
differences were found in the apparent quantum
yield. The rise of net photosynthesis during
induction after transfer from low to high light
varied from a hyperbolic shape to a sigmoidal
increase. Rates of photosynthetic induction
exhibited a negative exponential relationship
with stomatal conductance in the shade prior
to the increase in PFD. Leaves of understory
species showed the most rapid induction and
remained induced longer once transferred to
the shade than did leaves of medium- or high-light
species. LUE decreased rapidly with increasing
lightfleck duration and was affected by the
induction state of the leaf. Fully induced leaves
exhibited LUEs up to 300% for 1-s lightflecks,
while LUE was below 100% for 1-80 s lightflecks
in uninduced leaves. Both induced and uninduced
leaves of understory species exhibited higher
LUE than those of species growing in small gaps
or clearings. However, most differences disappeared
for lightflecks 10 s long or longer. Thus, understory
species, which grew in a highly dynamic light
environment, had better capacities for utilization
of rapidly fluctuating light than species from
habitats with higher light availability.
|