Kinetics of caffeoylquinic acid formation in N-deficient sunflower as a test of the growth–defense trade-off framework

The mechanistic aspects of resource availability on carbon allocation to growth or defense of plants has been widely discussed. This study tests the growth-defense trade-off framework by comparing rates of carbon assimilation and secondary metabolite production in novel time scales in N-limited sunflower. Upon exposure to N deficiency, increased accumulation of caffeoylquinic acid derivatives in leaf and root tissue was detected, which, however, represented only a small fraction of the assimilated carbon. Furthermore, there was no increased production of lignin under N limitation. Instead, the 'excess' of assimilated carbon not used for leaf expansion was largely allocated to the roots for vegetative processes. Lastly, an active steering of caffeoylquinic acid biosynthesis was indicated by increased expression of hydroxycinnamoyl CoA:quinate hydroxycinnamoyl transferase 3. Despite a relatively late reduction of the N concentration in the plants, it could not be definitively resolved to what extent changes in the physiological C/N balance may have influenced caffeoylquinic acid formation. Nevertheless, there is no definitive support for the mass-action-based accumulation of secondary metabolites suggested by a traditional view of the growth-defense trade-off framework. One may assume that the correlation of resource availability and defense investment has been shaped by complex evolutionary processes and is coordinated by tightly regulated biochemical networks, although it may be triggered by carbon/nutrient imbalance at the cellular scale.