Suitability of non-destructive sensors for monitoring physiological and biochemical responses of tomato leaves and fruits to abiotic stresses

Abstract

The major aim of the present work was to proof the suitability of chlorophyll fluorescence-based indices to non-destructively monitor and predict changes in the content of plant compounds in response to abiotic stresses. Simultaneously, another objective was to generate knowledge about the potential of tomato leaves from commercial production systems for the extraction of industrially relevant secondary metabolites, particularly rutin and solanesol, which are known to accumulate under abiotic stress conditions. For this purpose, tomato plants were grown under moderate abiotic stress treatments to induce the accumulation of the compounds of interest without lowering fruit yield as primary aim of commercial production. Plants’ stress responses were monitored non-destructively with a hand-held multiparametric fluorescence sensor. First, a nitrogen and a general nutrient deficiency was applied to achieve changes in leaf compounds of different development stages. To prove the suitability of the fluorescence-based sensor to track changes in leaf compounds, physiological responses, which were recorded non-destructively, were compared to the biochemical parameters determined by means of HPLC analyses. Second, we hypothesized that supplementary light-emitting-diodes (LED) in the blue and red regions promote the accumulation of rutin in tomato leaves. Here again, fluorescence-based recordings and laboratory HPLC analyses were undertaken on young and mature leaves and the relationships between both were evaluated. Third and last, we examined the suitability of the multiparametric sensors to monitor and estimate concentrations of tomato fruit maturity compounds during ripening under a mild nitrogen and water deficiency in the greenhouse. The fluorescence-based indices were compared to the well-established reflection-based ripening index a*/b*. In summary, the following results were achieved in each chapter of this thesis:<br /> 1.The fluorescence-based indices SFR_R, NBI_G, FLAV and ANTH_RG were suitable to monitor differences in young leaves between plants grown under control conditions (full standard nutrient solution), nitrogen deficiency (standard nutrient solution without N-containing compounds) and a general nutrient deficiency (tap water). However, no differences were observed in mature leaves. The FLAV index, representing a parameter for polyphenol assessment, was not reliable to estimate rutin or solanesol concentrations non-destructively in tomato leaves. Nitrogen deficiency as well as a general nutrient deficiency led to an accumulation of rutin in young leaves, whereas solanesol concentration was higher in fully developed mature leaves. Fruits showing symptoms of blossom-end-rot were seen more frequently in plants exposed to a general nutrient deficiency. <br /> 2.Tomato plants exposed to supplementary LED light (red and blue in a ratio of 4:1) showed stress-related differences for the chlorophyll fluorescence-based indices SFR_R and FLAV in leaves of all investigated development stages, while the indices NBI_G and ANTH_RG were limited to more mature leaves. Supplementary LED light induced higher rutin concentrations mainly in young leaves and partly in mature leaves. Correlation analyses between FLAV index and rutin concentrations could not show a precise relationship between both parameters but clusters according to the leaf age and the time point of harvest. <br /> 3.Fluorescence-based indices and the reflection index a*/b* are suitable to monitor an acceleration of tomato fruit ripening for fruits grown under water deficit. Further, chlorophyll concentration was estimated non-destructively since the coefficient of determination between the SFR_R and the chlorophyll concentration determined analytically was very high with r² = 0.84. Moreover, the single signal FRF_G and the lycopene concentration showed a coefficient of determination of r² = 0.81 even if a precise differentiation between the maturity stages two (breaker) to four (pink) was not observed. Compared with fluorescence-based indices, the relation between the reflection index and pigment concentration was lower for chlorophyll (r ²= 0.74) and higher for lycopene (r² = 0.94). The chlorophyll decrease was the driving force affecting all fluorescence signals. In consequence, an estimation of other maturity/quality compounds with chlorophyll fluorescence-based recordings, such as flavonoids with FLAV or FLAV_UV, is not appropriate.