The biochemical and physiological peculiarities of the photosynthetic apparatus and its response to drought in transgenic sunflower plants of the VK-121 line (T3 generation) harboring a double-stranded RNA suppressor of the proline dehydrogenase (ProDH) gene were studied in comparison with the wild type line VK-121. Under optimal water conditions, the transgenic plants did not differ from the wild type in total chlorophyll content, quantum efficiency of PSII, or CO2 assimilation and transpiration rates; however, they exhibited significantly higher activities of the antioxidant enzymes superoxide dismutase (SOD) and ascorbate peroxidase (APX) in chloroplasts. Soil drought caused a smaller decline in chlorophyll content, maximum quantum efficiency of PSII photochemistry (Fv/Fm), effective quantum yield of PSII in the light (fPSII), as well as respiration and transpiration rates in the transgenic plants, although the net CO2 assimilation rate was equally inhibited in both lines. At the same time, the plants with the double-stranded RNA suppressor of ProDH also demonstrated a smaller reduction in CO2 assimilation after an abrupt increase in leaf temperature. Under drought conditions, non-photochemical quenching (NPQ) of absorbed light energy was lower in transgenic VK-121 T3 plants than in the wild type VK-121 line. This was accompanied by an increase in the degree of xanthophyll cycle pigment de-epoxidation in the wild type line, while remaining constant in the leaves of the transgenic plants. Drought induced a stronger increase in SOD and APX activity in the leaves of the wild type line compared to the transgenic line. The results indicate that the VK-121 T3 transgenic sunflower plants with partial suppression of the proline dehydrogenase gene exhibit enhanced tolerance of the photosynthetic apparatus compared to the wild type VK-121 line, owing to the pronounced activity of protective and regulatory mechanisms under soil drought.
Keywords: Helianthus annuus L., sunflower, genetic transformation, double-stranded RNA-suppressor of proline dehydrogenase gene, photosynthesis, superoxide dismutase, ascorbate peroxidase, drought
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