The relationship between root exudation properties and root morphological traits of cucumber grown under different nitrogen supplies and atmospheric CO2 concentrations

Aims Nitrogen supply and atmospheric CO2 concentration ([CO2]) could influence root exudates directly by altering compound concentrations in roots and indirectly by regulating root morphology. This study assessed these direct and indirect effects on cucumber root exudation. Methods Cucumber roots with various morphological traits were obtained in different combinations of nitrogen supplies and [CO2] treatments. Then, the correlations between ten compounds in root exudates and their concentrations in root extracts as well as root morphological traits were evaluated. Results In case of root exudates, the amounts of sugars were more closely correlated to the root surface area, whereas organic acids and amino acids were more closely associated with the number of root tips. Moreover, fructose, glucose, sucrose and oxalic acid in root exudates were correlated to their concentrations in root extracts, whereas there was little correlation between root exudates and extracts for malic acid, citric acid or four amino acids. Conclusions Sugars were probably released from the whole roots by passive or facilitated diffusion, so both the direct and indirect effects were important. Organic acids and amino acids were mainly secreted from the root apices by active transport, thus the indirect effect was more important.