“…Symbiotic N 2 fixation (SNF) is a function particularly sensitive to DS. Thus, both infection, nodule development, and nodule function are impaired by DS [ 6 , 21 , 22 ], and in nodulated soybean plants, DS was shown to inhibit the SNF process before photosynthesis [ 23 ]. Several hypotheses, which are not mutually exclusive, have been proposed to explain the decline in SNF during drought [ 6 , 24 ]: (1) the local carbon-based feedback inhibition of SNF linked to the inhibition of sucrose synthase and subsequent sucrose accumulation and organic acid depletion in nodules [ 25 , 26 , 27 ]; (2) local nitrogen-based feedback regulation linked to the restriction on the export of nitrogen compounds in nodules and their inhibitory action against N 2 fixation [ 24 , 28 , 29 , 30 ]; (3) nitrogen-based systemic regulation linked to the inhibition of SNF by phloem-delivered nitrogen compounds [ 31 , 32 ]; and (4) a decline in oxygen (O 2 ) availability for bacteroid respiration linked to an increase in nodular O 2 diffusion resistance [ 23 , 33 ].…”