Gyula Váradi

Defensive strategies against high light stress in wild and D1 protein mutant biotypes of Erigeron canadensis

The Ser264AEGly substitution on the D1 protein is accompanied by a higher photosensitivity of the mutant plant. This may be due to an increased D1 protein turnover and/or to a lower xanthophyll cycle activity in vivo. The relative importance of these two photoprotective mechanisms in wild and D1 protein mutant biotypes of Erigeron canadensis L. was established by using dithiothreitol and streptomycin. Moreover, the interconversion of violaxan-thin to zeaxanthin via antheraxanthin was studied in isolated thylakoids and in intact leaves treated with paraquat. Streptomycin caused a more severe decrease in the optimal quantum yield (Fv/Fm) of PS II and a large increase in the initial fluorescence yield (Fo) in the mutant compared to the wild biotype. In the fluorescence-quenching parameters of the wild-type leaves, dithiothreitol caused alterations similar to those observed in the mutant plant without dithiothreitol. A lowered activity of the xanthophyll cycle was detected in the mutant biotype compared to the wild-type in vivo. However, under in vitro, conditions which were optimal for violaxanthin de-epoxidation, or when paraquat was used on intact leaves to accelerate the electron transport, violaxanthin could readily be converted to zeaxanthin even in the mutant plants. This demonstrates that neither the decrease in the enzymatic activity of violaxanthin de-epoxidase nor the low availability of violaxanthin is responsible for the low zeaxanthin formation under in vivo conditions. It is presumed that, in vivo, the D1 protein mutation results in slower electron transport, a smaller DpH and lower zeaxanthin formation, and thereby in alterations in the defensive strategies against high light illumination.

Xanthophyll Cycle Patterns and in vivo Photoinhibition in Herbicide-Resistant Biotypes of Conyza canadensis

Summary The xanthophyll cycle and in vivo photoinhibition were investigated in the herbicide-susceptible (S), paraquat-resistant (PQ-R), atrazine-resistant (Atr-R) and paraquat-atrazine co-resistant (PQAtr-R) biotypes of Conyza canadensis using both low-(LLG) and high-light-grown (HLG) plants. HPLC revealed markedly reduced levels of zeaxanthin under photoinhibitory conditions in atrazine-resistant (Atr-R and PQAtr-R) biotypes as compared with the S and PQ-R biotypes. The variable chlorophyll fluorescence (F v ) decrease and constant fluorescence (F o ) increase revealed an enhanced susceptibility to in vivo photoinhibition in Atr-R and PQAtr-R biotypes. LLG plants were more susceptible than HLG plants to in vivo photoinhibitory treatment. It is suggested that the increased susceptibility to photoinhibition in Atr-R and PQAtr-R biotypes of C. canadensis is a consequence not only of D1 protein mutation, but also possibly of a lower rate of violaxanthin to zeaxanthin de-epoxidation.

Damage of Grapevine Leaf Photosynthesis by UV-B (280–320 nm) Radiation in Two Cultivars of Vitis Vinifera

Recent increase in solar UV-B (280–320 nm) radiation on the surface of the Earth is a potential damage for all living organisms. In plants, an important site of damage is the photosynthetic electron transport, mainly Photosystem (PS) II, the pigment-protein complex embedded in the thylakoid membrane of chloroplasts. PS II electron transport is impaired by UV-B irradiation and the D1 and D2 protein subunits of PS II reaction centre are degraded as a consequence of this functional damage [1, 2].