Solanaceous crops: a sustainable source of valuable bioactive leaf compounds?

Modern horticulture targets both sustainable food production and non-competitive use of biomass residues for the extraction of bioactive compounds potentially of high interest for both health care and pharmaceutical sectors. Annually, huge amounts of green biomass accrue in sweet pepper and tomato cultivation. These materials contain many different secondary metabolites such as phenolics and isoprenoids. According to their healthy and medicinal properties these plant-based compounds are of high interest for the pharmaceutical, cosmetic and dietary sector. From the plant biochemical perspective, the accumulation of secondary metabolites is a measure of plants to cope with abiotic stress conditions. As well-known example, flavonoids accumulate in leaves due to severe nutritional imbalances, unusually high or extended light exposure, drought or salt stress. Thus, despite manipulation of the growing parameters, a targeted enrichment of specific bioactive leaf compounds is feasible. The determination of highly accumulated amounts and, therefore, of optimal harvest time points can be supported by optical measurement techniques, e.g. fluorescence and hyperspectral devices assessing physiological and biochemical alterations in the plant tissues. In the context of the BioSc project InducTomE (www.biosc.de/inductomewww.biosc.de/inductome ) and the recently started BMBF-project TaReCa, we are focusing on the accumulation of flavonoids and isoprenoids during additional lighting and salt stress in sweet pepper and tomato. One aim is to monitor the accumulation of the bioactive compounds using fluorescence and spectral based non-destructive indices as well as standard HPLC analysis. Our first results in tomato show that the content of rutin increased in young leaves under LED light and sodium vapor lamps, while the content of solanesol was significantly enhanced in mature leaves under salt stress cultivation. Metabolic alterations were also triggered in sweet pepper under different light and salt conditions. The changes of plant metabolism were well indicated by the index of epidermal flavonols (FLAV) under light impact and the blue-to-red-fluorescence ratio (BFRR_UV) under salt influence. To ensure the utilization of solanaceous biomass residues, further effective treatments will be identified for triggering the enrichment of bioactive compounds under commercial-like greenhouse production conditions.