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Journal of agricultural and food chemistry 69(23), 6431-6443 (2021). doi:10.1021/acs.jafc.1c00908, Published by American Chemical Society Publications, Washington, DC [u.a.]

At the end of the annual horticultural production cycle of greenhouse-grown crops, large quantities of residual biomass are discarded. Here, we propose a new value chain to utilize horticultural leaf biomass for the extraction of secondary metabolites. To increase the secondary metabolite content of leaves, greenhouse-grown crop plants were exposed to low-cost abiotic stress treatments after the last fruit harvest. As proof of concept, we evaluated the production of the flavonoid rutin in tomato plants subjected to nitrogen deficiency. In an interdisciplinary approach, we observed the steady accumulation of rutin in young plants under nitrogen deficiency, tested the applicability of nitrogen deficiency in a commercial-like greenhouse, developed a high efficiency extraction for rutin, and evaluated the acceptance of the proposed value chain by its key actors economically. On the basis of the positive interdisciplinary evaluation, we identified opportunities and challenges for the successful establishment of horticultural leaf biomass as a novel source for secondary metabolites.

There is growing interest to monitor plant stress responses, developmental processes and concentrations of health-promoting pigments in vegetables by non-destructive measurements. One device for non-invasive monitoring of plant performance and quantification of pigment concentrations in vegetables is the portable fluorescence sensor Multiplex® (Force-A, Orsay Cedex, France). Here, plant pigment concentrations are estimated based on vegetation indices derived from multiparametric fluorescence measurements. This method is increasingly used to monitor stress-related changes in plant pigments and flavonoids in horticultural experiments, but there is little evidence for an exact quantification of these leaf compounds. In this study, we assessed the applicability, but also limitations of the Multiplex® sensor in monitoring of plant stress responses and quantification of pigment concentrations in leaves of vegetable crops. We conducted two studies: First, we investigated changes in leaf metabolism of tomato plants subjected to nitrogen deficiency, and linked fluorometric measurements using the Multiplex® with biochemical analyses of foliar pigment and flavonoid concentrations. Second, we tested if fluorometric measurements using the Multiplex® are useful for predicting pigment concentrations of leafy vegetables. Here, we correlate biochemically assessed concentrations of chlorophylls in spinach and anthocyanins in red cabbage to non-invasive quantification derived from Multiplex® data. In order to measure samples with varying pigment concentrations, spinach as well as red cabbage plants were grown in the field either supplied with 50% or 100% of the recommended nitrogen supply. For both experiments, fluorometric measurements with the Multiplex® were performed on leaves. Immediately after, leaf samples were frozen for biochemical analysis of pigment content. Preliminary results show that measurements with the Multiplex® represent the onset of stress-related changes in leaf metabolism of tomato plants subjected to nitrogen deficiency well. Nevertheless, there was high variation in the quantification of pigment content of leaves and vegetables by biochemical and fluorometric analyses. In consequence, the correlations between pigment concentrations assessed by biochemical analysis and Multiplex® measurements were rather weak. We conclude that the Multiplex® sensor can be of advantage in monitoring of plant development and stress responses, but is of limited use for non-invasive quantification of the concentration of foliar pigment concentrations in leafy vegetables.

Die Blätter vieler Nutzpflanzen, insbesondere Nachtschattengewächse, enthalten wertvolle Sekundärmetabolite, die industriell genutzt werden könnten (Taylor und Fraser, 2011). Nichtsdestotrotz sind die Gehalte in Nutzpflanzen geringer als in den üblicherweise zu Ihrer Gewinnung angebauten Pflanzen (Oksman-Caldentey und Inzé, 2004). Eine erhöhte Biosynthese von Sekundärmetaboliten wird unter anderem durch abiotischen Stress induziert (Cramer et al., 2011; Ramakrishna und Ravishankar, 2011). Das interdisziplinäre BioSC-Projekt InducTomE untersucht das Potential, die Blattbiomasse von nach der Tomatenernte verbleibenden Pflanzen im Unterglasanbau durch gezielte Stressbehandlungen zur Gewinnung hochwertiger Sekundärmetaboliten zu nutzen. Dazu wird der Einfluss gezielter Stressbehandlungen auf den Sekundärmetabolitgehalt von unter kommerziellen Bedingungen angebauten Tomatenpflanzen untersucht und geeignete Extraktionsprozesse entwickelt, um zu evaluieren, ob der vorgeschlagene Wertschöpfungsprozess ökonomisch relevant gestaltet werden kann. Dadurch könnte ein Mehrwert beim gartenbaulichen Anbau von Nutzpflanzen generiert werden, ohne dass der Fruchtertrag beeinflusst wird.