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 compos ition and the dissipation of light energy were studied using transgenic tob acco (Nicotiana tabacum L. cv. W38). Two plant types were used: anti-SSu pl ants with reduced amounts of Rubisco and anti-GAPDH plants with reduced act ivity of chloroplast glyceraldehyde 3-phosphate dehydrogenase. A moderate r eduction in the photosynthetic capacity increased the de-epoxidation state of the xanthophyll-cycle pigments. In contrast, there was no large effect o n the leaf pigment composition and the ratio of the xanthophyll cycle pigme nts to chlorophyll, and total carotenoids increased only in the most severe transgenic plants. The light induction of photosynthesis, fluorescence que nching and de-epoxidation of the xanthophyll cycle pigments were also follo wed in wild-type and anti-SSu plants. Anti-SSu plants maintained high nonph otochemical 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 fluor escence appeared to be much higher than that required to support the observ ed rate of CO2 assimilation and photorespiration during the early phase of photosynthetic induction. However, the two estimates converged with the ons et of steady-state photosynthesis.