Photosynthesis, the xanthophyll cycle, light energy dissipation and
down-regulation of photosystem II (PSII) in senescent leaves of wheat plants
grown in the field were investigated. With the progress of senescence, maximal
efficiency of PSII photochemistry decreased only slightly early in the morning
but substantially at midday. Actual PSII efficiency, photochemical quenching,
efficiency of excitation capture by open PSII centres, and the I–P phase
of fluorescence induction curves decreased significantly and such decreases
were much more evident at midday than in the morning. At the same time,
non-photochemical quenching, thermal dissipation and de-epoxidation status of
the xanthophyll cycle increased, with much greater increases at midday than in
the morning. These results suggest that the xanthophyll cycle played a role in
photoprotection of PSII in senescent leaves by dissipating excess excitation
energy. Taking into account the substantial decrease in photosynthetic
capacity in senescent leaves, our data seem to support the view that the
decrease in actual PSII efficiency in senescent leaves may represent a
mechanism to down-regulate photosynthetic electron transport to match the
decreased CO2 assimilation capacity and avoid
photodamage of PSII from excess excitation energy.
