Correlation between Photoinhibition Sensitivity and the Rates and Relative Extents of Xanthophyll Cycle De-epoxidation in Chlorina Mutants of Barley (Hordeum vulgare L.)

We compared photoinhibition sensitivity to high irradiance (HI) in wild-type barley (wt) and both its chlorina f₁₀₄-nuclear gene mutant, that restricts chlorophyll (Chl) a and Chl b synthesis, and its f₂-nuclear gene mutant, that inhibits all Chl b synthesis. Both F_v/F_m and _PS2 decreased more significantly in f₂ than f₁₀₄ and wt with duration of HI exposure. Chl degraded more rapidly in the f₂ than in either f₁₀₄ or wt. Most sensitivity to photoinhibition was exhibited for f₂, whereas there was little difference in response to HI between the f₁₀₄ and wt. The highest de-epoxidation (DES) value at every time point of exposure to HI was measured for f₂, whereas the wt had the lowest value among the three strains. There were two lifetime components resolved for the conversion of violaxanthin (V) to zeaxanthin plus antheraxanthin (Z + A). The most rapid lifetime was around 6 min and the slower lifetime was >140 min, in both the mutants and wt. However, the wt and f₁₀₄ both displayed larger amplitudes of both de-epoxidation lifetimes than f₂. The difference between the final de-epoxidation state (DES = [Z + A]/[V + A + Z]) in the light compared to the dark expressed as DES for wt, f₁₀₄, and f₂ was 0.630, 0.623, and 0.420, respectively. The slow lifetime component and overall larger DES in the wt and f₁₀₄ correlated with more photoprotection, as indicated by relatively higher F_v/F_m and _PS2, compared to the f₂. Hence the photoprotection against photoinhibition has no relationship with the absolute DES value, but there is a strong relationship with de-epoxidation rate and relative extent or DES.