Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean ( Vicia faba ) grown under elevated [CO 2 ] and different water supply

Summary Photosynthetic stimulation by elevated [CO 2 ] (e[CO 2 ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N 2 ‐fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO 2 ]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO 2 ] 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 2 ] and subjected to well‐watered or drought treatments, and then exposed to 13 C pulse‐labelling using custom‐built chambers to track the fate of new photosynthates. Drought decreased 13 C uptake and nodule sink strength, and this effect was even greater under e[CO 2 ], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N 2 fixation, and increased accumulation of new photosynthates ( 13 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 2 ], and legumes may only be able to achieve greater C gain if nodule activity is maintained.