Characterization of photosynthetic pigment composition, photosystem II photochemistry and thermal energy dissipation during leaf senescence of wheat plants grown in the field

Abstract

Photosynthesis, photosynthetic pigment composition and photosystem II (PSII) photochemistry during leaf senescence of wheat plants grown in the field were characterized. During leaf senescence, neoxanthin and b -carotene decreased concomitantly with chlorophyll, whereas lutein and xanthophyll cycle pigments were less affected, leading to increases in lutein/chlorophyll and xanthophyll cycle pigments/chlorophyll ratios. The chlorophyll a/b ratio also increased. With the progressing of senescence, maximal efficiency of PSII photochemstry decreased only slightly in the early morning but substantially at midday. Actual PSII efficiency, photochemical quenching, efficiency of excitation capture by open PSII centers, and the I-P phase of fluorescence induction curves decreased significantly both in the early morning and at midday and such decreases were much greater at midday than early in the morning, while the O-I phase of fluorescence induction curves remained unchanged. At the same time, non-photochemical quenching, thermal dissipation and de-epoxidation status of the xanthophyll cycle increased both early in the morning and at midday, with a greater increase at midday. Such increases indicated that the xanthophyll cycle played a role in photoprotection of PSII in senescent leaves by dissipating excess excitation energy and that D pH could be the main factor controlling thermal dissipation. Taking into consideration the substantial loss of photosynthetic capacity during senescence, 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.