Effect of defoliation stress on 2-hydroxy cinnamic acid content at different growth stages in Melilotus albus

Melilotus albus (Medik.) is a pasture legume with potential value in Australian farming systems, especially in salt-affected areas. However, the use of this species has been limited because of concerns over high coumarin concentrations. The aim of this trial was to determine the effect of defoliation on the concentration of 2-hydroxy cinnamic acid, a coumarin precursor, at 3 stages of growth (vegetative, flowering and post-flowering) in 6 accessions. The concentration of 2-hydroxy cinnamic acid was determined by high performance liquid chromatography in leaves of plants grown in a glasshouse. Defoliation stress increased the 2-hydroxy cinnamic acid content in the leaves of all accessions studied by about 8%, from 0.89 to 0.96% of dry matter. The 2-hydroxy cinnamic acid concentrations were not significantly different between vegetative and flowering stages in any of the accessions studied, but showed a significant increase post-flowering for 4 out of the 6 accessions studied. This study indicates that grazing may increase the coumarin or 2-hydroxy cinnamic acid concentration in Melilotus albus, but in screening for suitable accessions to progress to new cultivars, it is not necessary to subject plants to a defoliation stress to rank the accessions for 2-hydroxy cinnamic acid or coumarin concentration.