Drought effects on Helianthus annuus and Glycine max metabolites: from phloem to root exudates

Numerous compounds are exuded by roots that play a central role in microbial decomposition and stabilization of soil carbon. The release of root exudates is sensitive to drought, but it is unclear how compound-specific exudates are related to drought-induced changes in plant metabolism. We investigated drought effects on root exudate quality and quantity for sunflower (Helianthus annuus) and soybean (Glycine max). We analyzed metabolites in phloem and root biomass extracts, to investigate whether root exudation is controlled by above- or below-ground processes. Sunflower and soybean showed different drought responses. Sunflower increased rates of exudation after rewetting (+330% in C) but the composition of metabolites remained unchanged compared to the control (constant moisture). Soybean did not change rates but the composition of metabolites changed with increased concentrations of osmolites (proline and pinitol). For specific groups, positive relationships were observed between exudates and phloem (amino-acids and organic acids) and between exudates and root biomass (sugars). Our results indicate that drought can induce different responses in plant metabolism causing changes in the quantity or composition of root exudates following rewetting. Furthermore, our results suggest that metabolism in shoots can influence exudation of organic acids and amino acids, while roots have a stronger control over exudation of sugars.