24
O. Bjorkman and B. Demmig-Adams
South America, numerous species along gradients of decreasing precipitation
either increase their leaf pubescence in response to increasing aridity or are
replaced by closely related more pubescent species (Ehleringer et al. 1981).
A disadvantage associated with an increased leaf pubescence is that it
leads to a reduction in photosynthetic rate at times when light is limiting,
and once the highly reflective pubescence has developed, it is irreversible
(Ehleringer 1982). Moreover, it appears that when an active apical meristem
has been stressed, all subsequently produced leaves develop pubescence to a
degree that matches that of the "direst" conditions to which the plant has
been exposed (Ehleringer 1982).
In contrast to the situation in Encelia, increases in leaf reflectance are
evidently largely reversible where they are partly caused by a deposit of salt
crystals on the leaf surface, as is the case in many halophytic species of
the genus Atriplex or in salt-secreting mangrove species such as Aegialitis
annulata. In Death Valley, Atriplex hymenelytra leaves produced during the
winter months reflect less than 30% of the photosynthetically active radiation, whereas summer leaves reflect as much as 60%. The reflectance of
these summer leaves decreased to that of winter leaves within a few days
after the leaves were allowed to rehydrate by covering the plants with
reflective plastic bags. The rehydration caused dilution of the salt with no
change in salt content per leaf dry weight (Mooney et al. 1977).
2.3.3 Chloroplast Movements
It is well established that the position and orientation of the chloroplasts
within each cell depends on the PFD incident on the leaf (Haupt and
Scheuerlein 1990). At low PFDs, the chloroplasts are arranged to provide
maximum light interception. Typically, they are assembled perpendicular to
the light direction. As the PFD increases, they gradually line up along the
vertical cell walls, parallel to the light direction, thereby allowing more light
to be transmitted through the leaf. The rapidity of these movements depends
on the extent of change in PFD; it is also strongly temperature dependent.
A typical half-time for the response is 5-10 min (Brugnoli and Bjorkman
1992). The action spectrum is similar to that for leaf movements and the
photoreceptor is therefore presumably also similar.
The maximum change in absorptance caused by chloroplast movements
among the species studied so far was obtained in Oxalis oregana (Fig. 2.6).
The change integrated over the photosynthetic range is 19-20%. Changes in
the order of 10-15% were obtained in shade leaves of several other species
but were considerably smaller in most of the other species (Inoue and
Shibata 1974; Brugnoli and Bjorkman 1992). Even in Oxalis the decrease in
light interception caused by chloroplast movement upon transition from low
to high light is modest in comparison with those caused by the leaf move-
O. Bjorkman and B. Demmig-Adams
South America, numerous species along gradients of decreasing precipitation
either increase their leaf pubescence in response to increasing aridity or are
replaced by closely related more pubescent species (Ehleringer et al. 1981).
A disadvantage associated with an increased leaf pubescence is that it
leads to a reduction in photosynthetic rate at times when light is limiting,
and once the highly reflective pubescence has developed, it is irreversible
(Ehleringer 1982). Moreover, it appears that when an active apical meristem
has been stressed, all subsequently produced leaves develop pubescence to a
degree that matches that of the "direst" conditions to which the plant has
been exposed (Ehleringer 1982).
In contrast to the situation in Encelia, increases in leaf reflectance are
evidently largely reversible where they are partly caused by a deposit of salt
crystals on the leaf surface, as is the case in many halophytic species of
the genus Atriplex or in salt-secreting mangrove species such as Aegialitis
annulata. In Death Valley, Atriplex hymenelytra leaves produced during the
winter months reflect less than 30% of the photosynthetically active radiation, whereas summer leaves reflect as much as 60%. The reflectance of
these summer leaves decreased to that of winter leaves within a few days
after the leaves were allowed to rehydrate by covering the plants with
reflective plastic bags. The rehydration caused dilution of the salt with no
change in salt content per leaf dry weight (Mooney et al. 1977).
2.3.3 Chloroplast Movements
It is well established that the position and orientation of the chloroplasts
within each cell depends on the PFD incident on the leaf (Haupt and
Scheuerlein 1990). At low PFDs, the chloroplasts are arranged to provide
maximum light interception. Typically, they are assembled perpendicular to
the light direction. As the PFD increases, they gradually line up along the
vertical cell walls, parallel to the light direction, thereby allowing more light
to be transmitted through the leaf. The rapidity of these movements depends
on the extent of change in PFD; it is also strongly temperature dependent.
A typical half-time for the response is 5-10 min (Brugnoli and Bjorkman
1992). The action spectrum is similar to that for leaf movements and the
photoreceptor is therefore presumably also similar.
The maximum change in absorptance caused by chloroplast movements
among the species studied so far was obtained in Oxalis oregana (Fig. 2.6).
The change integrated over the photosynthetic range is 19-20%. Changes in
the order of 10-15% were obtained in shade leaves of several other species
but were considerably smaller in most of the other species (Inoue and
Shibata 1974; Brugnoli and Bjorkman 1992). Even in Oxalis the decrease in
light interception caused by chloroplast movement upon transition from low
to high light is modest in comparison with those caused by the leaf move-