Abstract
Nitrogen fixation (NF) in soybean (Glycine max L. Merr.) is highly sensitive to soil drying. This sensitivity has been related to an accumulation of nitrogen compounds, either in shoots or in nodules, and a nodular carbon flux shortage under drought. To assess the relative importance of carbon and nitrogen status on NF regulation, the responses to the early stages of drought were monitored with two soybean cultivars with known contrasting tolerance to drought. In the sensitive cultivar ('Biloxi'), NF inhibition occurred earlier and was more dramatic than in the tolerant cultivar ('Jackson'). The carbon flux to bacteroids was also more affected in 'Biloxi' than in 'Jackson', due to an earlier inhibition of sucrose synthase activity and a larger decrease of malate concentration in the former. Drought provoked ureide accumulation in nodules of both cultivars, but this accumulation was higher and occurred earlier in 'Biloxi'. However, at this early stage of drought, there was no accumulation of ureides in the leaves of either cultivar. These results indicate that a combination of both reduced carbon flux and nitrogen accumulation in nodules, but not in shoots, is involved in the inhibition of NF in soybean under early drought.
Highlights
Drought-related inhibition of nitrogen fixation (NF) seriously limits legume yield in many arid and semiarid regions of the world
The main carbon source transported from shoots into nodules is Suc, which may be hydrolyzed by either Suc synthase (SS) or alkaline invertase (AI), and studies with rug4 mutants of pea (Pisum sativum) have shown that SS activity is essential for nodule functioning (Gordon et al, 1999)
There is no available information about nodule carbon fluxes in soybean cultivars showing contrasting tolerance to drought and how these carbon fluxes are correlated with the nitrogen metabolism response at the nodule level
Summary
Drought-related inhibition of nitrogen fixation (NF) seriously limits legume yield in many arid and semiarid regions of the world. Serraj et al (2001) suggested that ureide inhibition of nodular activity could occur as either a direct feedback within the nodule or an indirect feedback originated from shoots The latter has been suggested to depend on ureide catabolism pathways (Sinclair et al, 2003). ‘Biloxi’ is sensitive to soil drying (Serraj and Sinclair, 1996b) and under drought there is a higher accumulation of ureides in shoots compared to ‘Jackson’ (Serraj and Sinclair, 1996b; Purcell et al, 1998) This differential accumulation of ureides has been associated with distinct ureide catabolism pathways in leaves (Vadez and Sinclair, 2002), supporting the idea of a systemic regulation of NF. The aim of this study is to characterize the carbon and nitrogen nodule metabolism of these two soybean cultivars at early drought stages to ascertain the relevance of nodule carbon metabolism in NF tolerance to drought
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