20
O. Bjorkman and B. Demmig-Adams
in leaf water deficit, this enables the plant to respond rapidly to periods of
peak radiation load and then to recover when the radiation load is low and
water relations improve.
Active Leaf Movements. Leaves of many species belonging to different
taxonomic groups have the ability to actively change their orientation during
the course of the day, thereby changing the angle of the leaf relative to that
of the solar beam (Ehleringer and Forseth 1980). The physiology of these
movements has recently been reviewed by Koller (1990). Leaves capable of
such active movements are equipped with a pulvinus attaching the leaf blade
to the petiole or stem. The movements are light-driven and under the
control of a blue-light-absorbing pigment system. The identity of this photoreceptor pigment has not been established but the action spectrum for the
response indicates that it is a flavoprotein or, possibly, a carotenoid. The
leaf or leaflets unfold or fold by simultaneous expansion/contraction in
opposite sectors of their pulvinar motor tissue, resulting from massive fluxes
of (mainly potassium) ions and water.
Especially rapid and effective active leaf movements are present in shade
species of the genus Oxalis and a striking example is provided by Oxalis
oregana, a species native to the floor of redwood forests. In this habitat,
the prevailing daytime PFD can be as low as 31lmol photons m- 2 s-l,
but occasional sunflecks can suddenly increase the PFD to as much as
1800Ilmolm-2s-1. These leaves have a very low capacity for light-saturated
photosynthesis (3-5Ilmol CO2 m -2 S-l) and reach 90% of light saturation at
a PFD of about 120 Ilmol m - 2 S -1. Each of the three leaflets is attached to
the petiole by a pulvinus, which in this species is responsible for both the
photoreceptor and the motor functions. In the absence of sunflecks, each
leaflet moves such that it continuously faces the brightest area of the canopy
above, thereby maximizing light interception. However, when a bright
sunfleck strikes the pulvinus and the incident PFD rises from, e.g., 4 to
1600Ilmolm-2s-1, the leaflet rapidly folds down (Fig. 2.2). After a lag of
about 10 s the leaf angle changes at a rate of 20° min -1, and within 6 min the
leaflets reach a position essentially parallel to the solar beam. The leaflets
remain in this position until the sunfleck disappears, then slowly return to
their original orientation.
The fraction of incident radiation intercepted by a leaf is given by the
cosine of the leaf angle in relation to the light beam. Hence the light
interception changed from over 90% to less than 10% during exposure to a
sunfleck in the experiment shown in Fig. 2.2. Subsequent studies under
controlled conditions showed that the steady-state position of the leaflets is
closely tuned to the PFD over a wide range (Fig. 2.3). Clearly, each
pulvinus adjusts the leaflet angle such that light interception is sufficient to
saturate photosynthesis while avoiding supersaturation.
As in the Oxalidaceae, active leaf movements are common in species in
the families Malvaceae and Fabaceae. Unlike Oxalis oregana, many of these
O. Bjorkman and B. Demmig-Adams
in leaf water deficit, this enables the plant to respond rapidly to periods of
peak radiation load and then to recover when the radiation load is low and
water relations improve.
Active Leaf Movements. Leaves of many species belonging to different
taxonomic groups have the ability to actively change their orientation during
the course of the day, thereby changing the angle of the leaf relative to that
of the solar beam (Ehleringer and Forseth 1980). The physiology of these
movements has recently been reviewed by Koller (1990). Leaves capable of
such active movements are equipped with a pulvinus attaching the leaf blade
to the petiole or stem. The movements are light-driven and under the
control of a blue-light-absorbing pigment system. The identity of this photoreceptor pigment has not been established but the action spectrum for the
response indicates that it is a flavoprotein or, possibly, a carotenoid. The
leaf or leaflets unfold or fold by simultaneous expansion/contraction in
opposite sectors of their pulvinar motor tissue, resulting from massive fluxes
of (mainly potassium) ions and water.
Especially rapid and effective active leaf movements are present in shade
species of the genus Oxalis and a striking example is provided by Oxalis
oregana, a species native to the floor of redwood forests. In this habitat,
the prevailing daytime PFD can be as low as 31lmol photons m- 2 s-l,
but occasional sunflecks can suddenly increase the PFD to as much as
1800Ilmolm-2s-1. These leaves have a very low capacity for light-saturated
photosynthesis (3-5Ilmol CO2 m -2 S-l) and reach 90% of light saturation at
a PFD of about 120 Ilmol m - 2 S -1. Each of the three leaflets is attached to
the petiole by a pulvinus, which in this species is responsible for both the
photoreceptor and the motor functions. In the absence of sunflecks, each
leaflet moves such that it continuously faces the brightest area of the canopy
above, thereby maximizing light interception. However, when a bright
sunfleck strikes the pulvinus and the incident PFD rises from, e.g., 4 to
1600Ilmolm-2s-1, the leaflet rapidly folds down (Fig. 2.2). After a lag of
about 10 s the leaf angle changes at a rate of 20° min -1, and within 6 min the
leaflets reach a position essentially parallel to the solar beam. The leaflets
remain in this position until the sunfleck disappears, then slowly return to
their original orientation.
The fraction of incident radiation intercepted by a leaf is given by the
cosine of the leaf angle in relation to the light beam. Hence the light
interception changed from over 90% to less than 10% during exposure to a
sunfleck in the experiment shown in Fig. 2.2. Subsequent studies under
controlled conditions showed that the steady-state position of the leaflets is
closely tuned to the PFD over a wide range (Fig. 2.3). Clearly, each
pulvinus adjusts the leaflet angle such that light interception is sufficient to
saturate photosynthesis while avoiding supersaturation.
As in the Oxalidaceae, active leaf movements are common in species in
the families Malvaceae and Fabaceae. Unlike Oxalis oregana, many of these