Biology Reference
In-Depth Information
slowest dark reaction of the entire PS must have a turnover rate of 200 per s (Lien
and San Pietro
1975
).
Under full sunlight, which is about tenfold higher than in the shade, the turnover
rate of the limiting dark reaction would be 200*10
¼
2,000 per s. This turnover
μ
rate corresponds to a rate of O
2
evolution of about 9,000
mol of O
2
per mg Chl per
hour (h). Yet, under saturating light intensities, and other optimal conditions, the
maximal rate of O
2
evolution observed during a Hill reaction, which results in
the oxidation of H
2
O and the release of O
2
, rarely exceeds 5-10 % of the above
value. In other words, that rate is equal to the optimal rate of O
2
evolution observed
in the shade (Lien and San Pietro
1975
).
Furthermore extensive kinetic studies have demonstrated that the rate limiting
steps of the PETS do not reside in the initial photochemical reactions that take place
in the RC, but reside within the redox-carriers, i.e. the electron transport chains
connecting PSII to PSI. The discrepancy between the capacity of the photon
gathering apparatus, i.e. the antenna Chl-protein complexes and the capacity of
the rate-limiting dark reactions has been named the antenna/PS Chl mismatch
(Lien and San Pietro
1975
).
16.3.3
Impact of the Antenna/PS Chl Mismatch
The first and most important effect of the antenna/PS Chl mismatch is one of
reduced quantum conversion efficiencies at light intensities above shade levels.
The second effect relates to the photodestructive effects of the excess photons
collected by antenna Chl but not used in the initial photochemical acts. The energy
of these unused photons leads to serious photodestruction of the PETS that must be
repaired at a cost (Lien and San Pietro
1975
).
16.4 Correction of the Antenna/Photosystem
Chlorophyll Mismatch
Early on, the possible correction of the antenna/PS mismatch attracted the interest
and curiosity of the photosynthesis community. It was suggested that one way of
correcting the mismatch was to reduce the size of the PSU, which may be achieved
by growing plants with chloroplasts having less antenna and more RC Chl per unit
thylakoid area (Lien and San Pietro
1975
). Research performed in the early 1970s
failed however in its effort to alter significantly the PSU size in algal cell cultures
(Lien and San Pietro
1975
). Now, on the basis of deeper understanding of the
chemistry and biochemistry of the greening process, which was achieved during the
past 40 years, we have reason to believe that alteration of the PSU has become a
realistic possibility.
Search WWH ::
Custom Search