Comparison of high-light effects with and without methyl viologen indicate barley chlorina mutants exhibit contrasting sensitivities depending on the specific nature of the chlorina mutation: Comparison of wild type, chlorophyll-b-less clo f₂ and light-sensitive chlorophyll-b-deficient clo f₁₀₄ mutants

This study compared the response to methyl viologen (MV)-induced photooxidation in wild-type barley (wt), and both its chlorina f₁₀₄-nuclear gene mutant (that restricts Chl a and b synthesis) and its f₂-nuclear gene mutant (that inhibits all Chl b synthesis). Without MV, the f₂ mutant showed the highest sensitivity to high light, with F_v/F_m being reduced by 80% after 80 min of irradiation. There was little difference in response to high light without MV between f₁₀₄ and wt. After vacuum infiltration with 100 μM MV and exposure to high light, f₁₀₄ exhibited the highest sensitivity while f₂ was the most tolerant to the photooxidation effects. 77K fluorescence spectral analysis indicated that PSII of f₁₀₄ was especially damaged, as evidenced by the appearance of a new Chl a emission band around 700 nm at the expense of the F₆₈₅ and F₆₉₅ bands from the PSII core-inner antenna. With MV, chlorophyll degraded more rapidly in f₁₀₄ than in either f₂ or wt. During MV treatment, zeaxanthin content increased significantly during the initial period of exposure (0-20 min) in all strains, but decreased sharply in f₁₀₄ after longer exposure time (20-80 min). β-Carotene, on a chlorophyll basis, was not much changed under high light without MV, but with MV it decreased significantly, mostly in f₁₀₄, intermediately in f₂ and least in wt. We conclude that the light-sensitive chlorosis phenotype of f₁₀₄ is exacerbated by MV-induced photooxidation.