556
E.-D. Schulze and M.M. Caldwell
Chap. 3, this Vol.). A remarkable array of mechanisms regulate the interception and dissipation of energy, both short- and long-term. What had not
been clear from gas exchange measurements is becoming very obvious,
namely that the overexcitation of the light-harvesting system must be
avoided, or damage will result (Bjorkman and Demmig-Adams, Chap. 2,
this Vol.). Thus, plants have evolved structural mechanisms, such as leaf
movements or changes in reflectance to reduce absorption of light, and
physiochemical mechanisms to dissipate excess absorbed energy in the form
of heat or fluorescence. However, despite these mechanisms, photochemical
damage is in fact a very common event (Trebst, Chap: 1, this Vol.) and
primarily located in a single protein of the light-harvesting complex, the Dl
protein. This protein has an unexpectedly high turnover rate because of the
direct damage by light. The sensitivity of this protein is highly dependent on
environmental conditions. Any stress, such as air pollution and mineral
deticiency, will enhance Dl protein turnover. It is also very dependent on
the nitrogen and carbohydrate status of the plant (Sch~ifer 1993). If the
light-dependent repair mechanism fails, photo system II may be completely
inactivated, and in a chain reaction this can lead to the sudden and complete
death of the assimilatory organ. Trebst (Chap. 1) indicates that needle loss
of conifers, which is a major symptom of forest decline in Europe (see also
Heber et al., Chap. 14), could be related to this molecular event, a fact
which could never have been discovered by gas exchange or fluorescence
measurements alone.
It appears that plant growth in full sunlight is associated with the ability
to avoid photo inhibition and yet to make use of the photon flux density for
carbon gain (Bjorkman 1981). Shade plants encounter a similar problem,
which can be even more serious as they experience highly variable light
conditions between shade light and sunflecks (Pearcy and Ptitsch, Chap. 17).
These plants must make use of the photon flux during light flecks in order to
avoid starvation, yet they are adapted to deep shade as their average light
climate and, thus, they are more susceptible to photoinhibition than plants
occurring in full sunlight. Pearcy and Ptitsch (Chap. 18) explain the balance
between these risks of starvation by lack of photosynthates and photoinhibition during periods of high illumination by minimizing the induction
period of photosynthesis.
For obvious reasons gas exchange research has concentrated on photosynthesis and much less on respiration. We presently lack important information on the regulation of respiratory processes (Amthor, Chap. 4).
Respiration, photo respiration , and photosynthesis interact since these processes share intermediates such as ATP, reductants, and carbon skeletons,
and products of the individual processes are substrates or cofactors for the
others. Thus, all three processes are well coordinated and regulated at the
whole-plant level. But it is still unclear how this is achieved. It appears
that accumulation of starch "protects" carbohydrates from being respired
(Schulze and Schulze, Chap. 6), and this may promote growth in a day-night
E.-D. Schulze and M.M. Caldwell
Chap. 3, this Vol.). A remarkable array of mechanisms regulate the interception and dissipation of energy, both short- and long-term. What had not
been clear from gas exchange measurements is becoming very obvious,
namely that the overexcitation of the light-harvesting system must be
avoided, or damage will result (Bjorkman and Demmig-Adams, Chap. 2,
this Vol.). Thus, plants have evolved structural mechanisms, such as leaf
movements or changes in reflectance to reduce absorption of light, and
physiochemical mechanisms to dissipate excess absorbed energy in the form
of heat or fluorescence. However, despite these mechanisms, photochemical
damage is in fact a very common event (Trebst, Chap: 1, this Vol.) and
primarily located in a single protein of the light-harvesting complex, the Dl
protein. This protein has an unexpectedly high turnover rate because of the
direct damage by light. The sensitivity of this protein is highly dependent on
environmental conditions. Any stress, such as air pollution and mineral
deticiency, will enhance Dl protein turnover. It is also very dependent on
the nitrogen and carbohydrate status of the plant (Sch~ifer 1993). If the
light-dependent repair mechanism fails, photo system II may be completely
inactivated, and in a chain reaction this can lead to the sudden and complete
death of the assimilatory organ. Trebst (Chap. 1) indicates that needle loss
of conifers, which is a major symptom of forest decline in Europe (see also
Heber et al., Chap. 14), could be related to this molecular event, a fact
which could never have been discovered by gas exchange or fluorescence
measurements alone.
It appears that plant growth in full sunlight is associated with the ability
to avoid photo inhibition and yet to make use of the photon flux density for
carbon gain (Bjorkman 1981). Shade plants encounter a similar problem,
which can be even more serious as they experience highly variable light
conditions between shade light and sunflecks (Pearcy and Ptitsch, Chap. 17).
These plants must make use of the photon flux during light flecks in order to
avoid starvation, yet they are adapted to deep shade as their average light
climate and, thus, they are more susceptible to photoinhibition than plants
occurring in full sunlight. Pearcy and Ptitsch (Chap. 18) explain the balance
between these risks of starvation by lack of photosynthates and photoinhibition during periods of high illumination by minimizing the induction
period of photosynthesis.
For obvious reasons gas exchange research has concentrated on photosynthesis and much less on respiration. We presently lack important information on the regulation of respiratory processes (Amthor, Chap. 4).
Respiration, photo respiration , and photosynthesis interact since these processes share intermediates such as ATP, reductants, and carbon skeletons,
and products of the individual processes are substrates or cofactors for the
others. Thus, all three processes are well coordinated and regulated at the
whole-plant level. But it is still unclear how this is achieved. It appears
that accumulation of starch "protects" carbohydrates from being respired
(Schulze and Schulze, Chap. 6), and this may promote growth in a day-night