Evaluating the Combined Effects of Elevated [CO2] and Accelerated NPQ Relaxation on Yield, Physiology and Transcription in Soybean.

Improving photosynthesis is proposed as a means of enhancing crop productivity. To assess whether simultaneous increases in light use-efficiency and carbon assimilation are additive, we investigated the impact of elevated CO2 on the performance of a transgenic soybean line engineered for faster relaxation of photoprotection by overexpression of three genes: violaxanthin de-epoxidase, photosystem II subunit S and zeaxanthin epoxidase (VPZ-34A). Plants were grown in field plots with and without elevated CO2. Physiological and transcriptomic measurements were collected during vegetative and reproductive stages and compared to final biomass and seed production. VPZ-34A showed improved light use efficiency during vegetative growth, but no increase in carbon assimilation, and no increase in seed or biomass-related traits under ambient or elevated CO2 concentrations. A small number of genes were differentially expressed in VPZ-34A, including downregulation of a mitochondrial thioredoxin (Trx o1) and up-regulation of two AP2 transcription factors, which may suggest alterations in redox balance and seed development. In conclusion, the improvements in light use efficiency by modulating photoprotection did not have an additive effect with elevated CO2 concentrations and did not stimulate soybean yield in the year measured. Our data highlight the complex relationship between photoprotection, carbon assimilation and yield, and provide avenues for future research.