Photosynthetic parameters of switchgrass (Panicum virgatum) under low radiation: Influence of stable overexpression of Miscanthus x giganteus PPDK on responses to light and CO₂ under warm and cool growing conditions
Background: Switchgrass (Panicum virgatum) is one of the leading candidates to provide lignocellulosic biomass for biofuel production. Switchgrass is capable of relatively high productivity on marginal land or when intercropped with trees. Production of switchgrass is dependent upon light use efficiency at the canopy level. Thus, maintenance of photosynthesis at light limiting and cool conditions ought to elongate the growing season and increase productivity of switchgrass. Photosynthesis under cool conditions and low light is maintained higher in giant miscanthus (Miscanthus × giganteus) than switchgrass by retaining relatively high expression of pyruvate orthophosphate dikinase (PPDK). Our main goal was to create lines of switchgrass with upregulated PPDK and to evaluate photosynthetic responses of those lines to growth temperature under low radiation conditions. Our approach was to grow replicate plants of each transgenic event with an untransformed control in low light environments at either warm (28 °C day/24 °C night) or cool (14 °C day/12 °C night) conditions. Photosynthesis parameters of all plants were assessed with fluorescence kinetics, light response curves and carbon dioxide response curves.
Results: We created several lines of transgenic switchgrass with documented upregulation of cDNA for the PPDK gene (C4ppdk1). Photoinhibition was higher in the transgenic lines, but electron transport rates (ETR) and quantum yield of photosystem II were not inhibited by cool conditions. The higher than expected ETR under cool conditions was associated with increased non-photochemical quenching, which indicated that enzymatic reactions of photosynthesis were inhibited more by cool conditions than photochemical processes. In all except one transgenic line, most metrics of biochemical processes decreased under cool growth conditions, which resulted in significantly lower productivity under cool conditions.
Conclusions: All transgenic lines were able to balance electron transport and biochemical process at low radiation keeping apparent quantum yield constant and the light saturation point relatively low. Thus, the photosynthetic changes associated with the transgenic events could make the transgenic lines appropriate for use in low light regions such as forest intercropping systems if productivity was increased. Although one transgenic line had weakly improved photosynthesis under cool conditions in this study, improving cold temperature photosynthesis in switchgrass will require more than manipulating the expression of a single gene.