17 The Consequences of Sunflecks for
Photosynthesis and Growth of
Forest Understory Plants
R.W. Pearcy and W.A. Pfitsch
17.1 Introduction
Plants in forest understories are subjected to light environments consisting
of a very low background of diffuse light that is punctuated by often much
brighter sunflecks lasting from a few seconds to several minutes. These
sunflecks, although usually present for less than 10% of the time, typically
contribute 10 to 80% of the photosynthetically active photon flux density
(PFD) (Chazdon 1988). Therefore much of the photosynthesis of understory
plants may occur under transiently changing light conditions that characterize sunflecks. The environmental and physiological controls on photosynthesis during transient light changes are not necessarily the same as those
that determine photosynthetic performance under steady-state conditions.
The shade-plant syndrome of understory plants has been widely studied
but mostly in terms of the controls on steady-state photosynthetic characteristics. Until recently, relatively little attention has been given to the
mechanisms regulating the use of sunflecks.
Essentially all of the components of the photosynthetic apparatus of a
leaf respond either directly or indirectly to a change in light, but on very
different time scales. Steps in charge separation and electron transport
occur virtually instantaneously. However, increases in metabolite concentrations necessary for maximal photosynthetic rates require long~r times.
Finally, slow changes in activation state of enzymes and stomatal opening
result in a slow increase in photosynthesis towards a final steady-state
level. To further complicate matters, metabolite concentrations also exhibit
slow changes due to the light activation of the enzymes that regulate the
flux through the pools. Since light fluctuations can occur on a time scale
much faster than stomatal opening or enzyme activation, the response to
any given light change will in part be a function of the previous light
environment.
In this chapter, we will discuss the spatial and temporal variation in
photosynthesis in relation to sunflecks in a forest understory. We will also
present results of studies designed to understand the role of this variation in
the carbon balance and growth in the forest understory. Most of work was
completed utilizing Adenoeaulon bieolor, (Asteraceae) a common plant in
Photosynthesis and Growth of
Forest Understory Plants
R.W. Pearcy and W.A. Pfitsch
17.1 Introduction
Plants in forest understories are subjected to light environments consisting
of a very low background of diffuse light that is punctuated by often much
brighter sunflecks lasting from a few seconds to several minutes. These
sunflecks, although usually present for less than 10% of the time, typically
contribute 10 to 80% of the photosynthetically active photon flux density
(PFD) (Chazdon 1988). Therefore much of the photosynthesis of understory
plants may occur under transiently changing light conditions that characterize sunflecks. The environmental and physiological controls on photosynthesis during transient light changes are not necessarily the same as those
that determine photosynthetic performance under steady-state conditions.
The shade-plant syndrome of understory plants has been widely studied
but mostly in terms of the controls on steady-state photosynthetic characteristics. Until recently, relatively little attention has been given to the
mechanisms regulating the use of sunflecks.
Essentially all of the components of the photosynthetic apparatus of a
leaf respond either directly or indirectly to a change in light, but on very
different time scales. Steps in charge separation and electron transport
occur virtually instantaneously. However, increases in metabolite concentrations necessary for maximal photosynthetic rates require long~r times.
Finally, slow changes in activation state of enzymes and stomatal opening
result in a slow increase in photosynthesis towards a final steady-state
level. To further complicate matters, metabolite concentrations also exhibit
slow changes due to the light activation of the enzymes that regulate the
flux through the pools. Since light fluctuations can occur on a time scale
much faster than stomatal opening or enzyme activation, the response to
any given light change will in part be a function of the previous light
environment.
In this chapter, we will discuss the spatial and temporal variation in
photosynthesis in relation to sunflecks in a forest understory. We will also
present results of studies designed to understand the role of this variation in
the carbon balance and growth in the forest understory. Most of work was
completed utilizing Adenoeaulon bieolor, (Asteraceae) a common plant in