Chlorophyll fluorescence quenching, zeaxanthin formation and light scattering in intact leaves of triazine-resistant and triazine-susceptible Chenopodium album plants

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The triazine-resistant biotype of Chenopodium album has an impaired activity of photosystem II. In vivo, this leads to a higher sensitivity to photoinhibition, especially when the plants have been grown at a high irradiance. The activities of several protecting energy dissipation pathways were compared in triazine-resistant and triazine-susceptible biotypes which were grown at low and high irradiances. It was found that the resistant plants have more light-induced zeaxanthin formation and a larger change in light scattering than does the susceptible biotype. The differences were more pronounced when the plants were grown at a high irradiance. Stern-Volmer plots indicated that high light-grown plants show a linear relationship between non-photochemical quenching q N and zeaxanthin content, and also between q N and change in light scattering. In low light-grown plants, the zeaxanthin content and the change in light scattering are not linearly related to q N and there is a strong increase in the value of q N already at low levels of zeaxanthin content and change in light scattering. The latter effect is smaller in the resistant biotype. Photorespiration is found to be more important as an energy dissipation pathway in resistant plants compared with susceptible plants. It is concluded that the increased sensitivity to photoinhibition of resistant plants is not caused by a lower activity of the photoprotective pathways, including qN, zeaxanthin formation and photorespiration. Instead, it is suggested that the shade-type characteristics of the chloroplasts of the resistant plants are responsible for the greater sensitivity to photoinhibition.