Regulation of Photosynthetic Light Energy Capture
39
even at dawn (Demmig et al. 1988; Adams and Demmig-Adams, unpubl.
data). This is in contrast to the daily courses seen under favorable conditions, where the Z formed during exposure to high daytime PFDs is
gradually reepoxidized to V as the PFD decreases in the late afternoon and
the amount of Z present in the leaves in the early morning is usually quite
low.
Changes in the V + A + Z Pool Size. On the assumption that substitution of
Z and A for V (directly or indirectly) determines the upper limit of NRD
that can be reached in a leaf under excess light, this limit would ultimately
depend on the total pool size of V + A + Z. Hence, one would expect the
response to a prolonged exposure to excess light to be an increase in the size
of this pool. There is now much evidence that this is indeed the case.
All studies to date consistently show that the V + A + Z pool changes
in response to the growth light regime, both under controlled conditions
(Demmig-Adams et al. 1989c) and in natural situations (Thayer and Bjorkman 1990; Demmig-Adams and Adams 1992b) and also in response to
drought (Demmig-Adams et al. 1988) and unfavorable temperatures (Bilger
and Bjorkman 1991). Sun leaves consistently have a larger pool size than
shade leaves (up to four times). The very small V + A + Z pool size present
in extreme shade leaves, such as those of Oxalis oregana, could thus be the
cause of the low maximum capacity for NRD (Fig. 2.9).
In leaves of plants normally found in open environments, the V + A + Z
pool increases with increasing daily photon receipt during leaf development
(see Adams et al. 1992). An example of this response is given in Fig. 2.15,
which shows the carotenoid contents of cotton cotyledons growing in the
field over a wide range of different total daily photon receipts. As was found
in other species studied thus far, the V + A + Z pool size shows a very
large and continuous increase with increased photon receipt up to full daily
solar radiation while the changes in other carotenoids are much smaller.
Figure 2.16 shows that also in leaves or cotyledons that have developed
either under a low, or a high photon receipt, the V + A + Z pool size can
respond dynamically to changes in light regime. When cotton cotyledons
that had developed under the lowest light regime used in these field experiments were exposed to full daylight (Fig. 2.16, left panel), the pool size
began to increase within several hours and within 6 days approached that of
the cotyledons that had developed under full daily solar radiation. Perhaps
more surprisingly, upon shading of cotyledons that had developed under the
highest light regime (Fig. 2.16, right panel), the pool size exhibited a decline
with a similar time course and extent as that of the rise shown in Fig. 2.16,
left panel. Quantitatively similar results as shown for the cotyledons were
obtained with cotton leaves when exposed to the same changes in light
regime.
It is also noteworthy that changes in the V + A + Z pool size appear not
to respond to changes in PFD as such but rather to changes in excess PFD.
39
even at dawn (Demmig et al. 1988; Adams and Demmig-Adams, unpubl.
data). This is in contrast to the daily courses seen under favorable conditions, where the Z formed during exposure to high daytime PFDs is
gradually reepoxidized to V as the PFD decreases in the late afternoon and
the amount of Z present in the leaves in the early morning is usually quite
low.
Changes in the V + A + Z Pool Size. On the assumption that substitution of
Z and A for V (directly or indirectly) determines the upper limit of NRD
that can be reached in a leaf under excess light, this limit would ultimately
depend on the total pool size of V + A + Z. Hence, one would expect the
response to a prolonged exposure to excess light to be an increase in the size
of this pool. There is now much evidence that this is indeed the case.
All studies to date consistently show that the V + A + Z pool changes
in response to the growth light regime, both under controlled conditions
(Demmig-Adams et al. 1989c) and in natural situations (Thayer and Bjorkman 1990; Demmig-Adams and Adams 1992b) and also in response to
drought (Demmig-Adams et al. 1988) and unfavorable temperatures (Bilger
and Bjorkman 1991). Sun leaves consistently have a larger pool size than
shade leaves (up to four times). The very small V + A + Z pool size present
in extreme shade leaves, such as those of Oxalis oregana, could thus be the
cause of the low maximum capacity for NRD (Fig. 2.9).
In leaves of plants normally found in open environments, the V + A + Z
pool increases with increasing daily photon receipt during leaf development
(see Adams et al. 1992). An example of this response is given in Fig. 2.15,
which shows the carotenoid contents of cotton cotyledons growing in the
field over a wide range of different total daily photon receipts. As was found
in other species studied thus far, the V + A + Z pool size shows a very
large and continuous increase with increased photon receipt up to full daily
solar radiation while the changes in other carotenoids are much smaller.
Figure 2.16 shows that also in leaves or cotyledons that have developed
either under a low, or a high photon receipt, the V + A + Z pool size can
respond dynamically to changes in light regime. When cotton cotyledons
that had developed under the lowest light regime used in these field experiments were exposed to full daylight (Fig. 2.16, left panel), the pool size
began to increase within several hours and within 6 days approached that of
the cotyledons that had developed under full daily solar radiation. Perhaps
more surprisingly, upon shading of cotyledons that had developed under the
highest light regime (Fig. 2.16, right panel), the pool size exhibited a decline
with a similar time course and extent as that of the rise shown in Fig. 2.16,
left panel. Quantitatively similar results as shown for the cotyledons were
obtained with cotton leaves when exposed to the same changes in light
regime.
It is also noteworthy that changes in the V + A + Z pool size appear not
to respond to changes in PFD as such but rather to changes in excess PFD.