Xanthophyll cycle, light energy dissipation and electron transport in transgenic tobacco with reduced carbon assimilation capacity

The effects of reduced CO₂ assimilation capacity on the leaf pigment composition and the dissipation of light energy were studied using transgenic tobacco (Nicotiana tabacum L. cv. W38). Two plant types were used: anti-SSu plants with reduced amounts of Rubisco and anti-GAPDH plants with reduced activity of chloroplast glyceraldehyde 3-phosphate dehydrogenase. A moderate reduction in the photosynthetic capacity increased the de-epoxidation state of the xanthophyll-cycle pigments. In contrast, there was no large effect on the leaf pigment composition and the ratio of the xanthophyll cycle pigments to chlorophyll, and total carotenoids increased only in the most severe transgenic plants. The light induction of photosynthesis, fluorescence quenching and de-epoxidation of the xanthophyll cycle pigments were also followed in wild-type and anti-SSu plants. Anti-SSu plants maintained high nonphotochemical quenching and increased xanthophyll de-epoxidation in the light but the reduction state of Q(A) remained high. For both wild-type and anti-SSu plants, the electron transport rate estimated from chlorophyll a fluorescence appeared to be much higher than that required to support the observed rate of CO₂ assimilation and photorespiration during the early phase of photosynthetic induction. However, the two estimates converged with the onset of steady-state photosynthesis.