In sugar beet (Beta vulgaris L.) iron deficiency
decreased not only the photosynthetic rate but also the actual photosystem II
efficiency at steady-state photosynthesis. In moderate iron deficiency, the
decrease in actual photosystem II efficiency under illumination was related to
closure of photosystem II reaction centers, whereas in severe iron deficiency
it was associated to decreases of intrinsic photosystem II efficiency. The
O2 evolution, on an absorbed light basis, decreased more
than the actual photosystem II efficiency, suggesting the presence of a
significant fraction of electron transport to molecular oxygen or the
existence of some form of cyclic electron flow. Iron-deficient leaves reduced
the excess of light absorbed that cannot be used in photosynthesis not only by
decreasing absorptance, but also by dissipating a large part of the light
absorbed by the photosystem II antenna. This mechanism, that protects the
photosystem II reaction centers through the enhancement of energy dissipation,
was related to the de-epoxidation of violaxanthin (V) to antheraxanthin (A)
and zeaxanthin (Z) in iron-deficient leaves. These data provide additional
support for a role of Z+A in photoprotection under conditions of excess
photosynthetic light absorption.
The photosystem II-associated Cah3 in Chlamydomonas enhances the O2 evolution rate by proton removal
