Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean (Vicia faba) grown under elevated [CO ] and different water supply.

Photosynthetic stimulation by elevated [CO ] (e[CO ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N -fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO ]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO ] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO ] and subjected to well-watered or drought treatments, and then exposed to C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased C uptake and nodule sink strength, and this effect was even greater under e[CO ], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N fixation, and increased accumulation of new photosynthates ( C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO ], and legumes may only be able to achieve greater C gain if nodule activity is maintained.