further, which might be associated with the leaf water loss
observed later. Therefore, the present study allowed us to study the
response of plants to severe water deficit and how this species
affords protection to the photosynthetic apparatus when water
loss cannot be avoided. This, together with the fact that studies
were performed under high light conditions, allowed us to study
what is the response of this species when there is a strong
photoprotective demand associated with a severe water loss.
Together with a-tocopherol, the xanthophylls, and especially
zeaxanthin have been ascribed a protective function in thylakoids.
The high light- and drought-adapted plants display large pools of
a-tocopherol and xanthophylls and maximal conversion to
zeaxanthin [18,24–27]. Almost all violaxanthin was de-epoxidized
to zeaxanthin at midday in IR plants (DPS of ca. 0.94), thus
indicating a high demand for dissipation of excess excitation
energy in leaves. When the photoprotective demand was
additionally increased by short-term water deficit, a nocturnal
increase in the DPS was observed, which was associated with
smaller Fv/Fm ratios. These results suggest that water stress led to
increased susceptibility to photoinhibition, which might be
attributed to a photoprotective process rather than to a photodamage
process. Such a photoprotection was associated with an
enhanced formation of zeaxanthin via de-epoxidation of violaxanthin
at midday, but not with increased accumulation of
zeaxanthin at night. Indeed, it has been suggested that under
lasting stress conditions and in some plant species, the flexible
dissipation of excess energy is replaced by a highly effective, but
less flexible continuous engagement of zeaxanthin in dissipation
that does not require a DpH. Under these conditions, the