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Journal of Experimental Botany, Vol. 52, No. 362, pp. 1805-1810, September 1, 2001
© 2001 Oxford University Press

Original Papers

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

Congming Lu1,3, Qingtao Lu1, Jianhua Zhang2 and Tingyun Kuang1

1 Photosynthesis Research Centre, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, PR China
2 Department of Biology, Hong Kong Baptist University, Kowloon, Hong Kong, PR China

Photosynthetic pigment composition and photosystem II (PSII) photochemistry were characterized during the flag leaf senescence of wheat plants grown in the field. During leaf senescence, neoxanthin and ß-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 progression of senescence, the maximal efficiency of PSII photochemistry decreased only slightly in the early morning (low light conditions), but substantially at midday (high light conditions). Actual PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centres decreased significantly both early in the morning and at midday and such decreases were much greater at midday than in the early morning. At the same time, non-photochemical quenching, zeaxanthin and antheraxanthin contents at the expense of violaxanthin increased both early in the morning and at midday, with a greater increase at midday. The results in the present study suggest that a down-regulation of PSII occurred in senescent leaves and that the xanthophyll cycle plays a role in the protection of PSII from photoinhibitory damage in senescent leaves by dissipating excess excitation energy, particularly when exposed to high light.

Key words: Chlorophyll fluorescence, pigment composition, photosystem II photochemistry, xanthophyll cycle, wheat (Triticum aestivum L.).


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