Is the C:N ratio a reliable indicator of C allocation to primary and defence-related metabolisms in tomato?

Plant growth and defence are both fuelled by compounds synthesized from a common pool of carbon and nitrogen, implying the existence of a competition for carbon and nitrogen allocation to both metabolisms. The ratio of carbon to nitrogen (C:N) of an organ is often regarded as a convenient indicator of growth and quality. The purpose of this work was to assess whether or not it is possible to extend its use to characterize the trade-off between growth and defence processes. Therefore, we calculated C:N ratios in the pool of resources and in the total plant, and correlated them to the concentrations of diverse compounds of the primary and secondary metabolisms in young tomatoes. Plants were grown hydroponically at N availabilities either limiting (0.1 mM) or not (7 mM) for growth in two glasshouses maintained either under ambient or enriched (700 vpm) air CO(2). These conditions yielded a large array of C:N in fully developed leaves, developing leaves, stem and roots, sampled 27, 35 and 47 days after sowing. Growth parameters and tissue concentrations of primary metabolites (carbohydrates, starch), defence-related compounds (polyphenols, glycoalkaloids), lignin, nitrate, ammonium, C and N were analyzed. Net CO(2) exchange rate was also measured at the last sampling date. Low N limited plant growth more than photosynthesis. The C:N in the resource pool was far higher than the total C:N. Starch was the most responsive compound, attaining high concentration under high C:N, whereas lignin remained stable. Chlorogenic acid, rutin, kaempferol-rutinoside and tomatine concentrations correlated positively to C:N. The same patterns were observed for most organs and molecules, except soluble carbohydrates in fully developed leaves whose concentration was not influenced. Among the organs, developing leaves showed the highest concentrations of secondary compounds and were the most responsive to C:N variations. Neither the biochemical nature of the compounds (C-based or N- containing metabolites) nor the calculation mode of C:N, influenced the patterns observed. Within the range of N availabilities considered (up to N limitation but not deficiency), the C:N can be considered as a good indicator of the secondary compounds concentrations in organs, especially for those involved in the chemical defence.