Antioxidant Capacity of Hydroponically Cultivated and Grafted Tomato Varieties
Description
Heirloom tomato varieties are in demand by consumers due to high antioxidant levels. However, these varieties are difficult to produce and are prone to disease and low yield. To overcome these problems, heirloom tomatoes may be grafted onto disease-resistant rootstocks and cultivated in hydroponic systems. However, it is unknown if the antioxidant capacity of hydroponically grown tomatoes is affected by grafting. Heirloom (Black Krim and Green Zebra) and standard (Big Beef) varieties were grafted onto wild type (WT) or productive rootstocks (Arnold and Supernatural). Tomatoes were harvested at maturity, freeze-dried, ground into a powder, and stored at -20ºC until further analysis. Antioxidant capacity of methanol extracts was evaluated by the 2,2’-azino-di[3-ethylbenzthiazoline sulfonsyr]sulphonic acid (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays. For the ABTS assay, the antioxidant capacity was measured by incubating 10 uL tomato extract was with 95 uL 3.5 mM ABTS• for 30 s at room temperature, and the absorbance at 734 nm was measured spectrophotometrically (Spectra Max 250 Microplate Reader, Molecular Devices, San Jose, CA). Trolox was used as an antioxidant standard for calculations of trolox equivalents (TE; μmol/g tomato dry weight). The antioxidant capacity of Big Beef, Black Krim, and Green Zebra grafted onto WT, Arnold, and Supernatural was 12.18±0.82, 12.34±0.65, 12.26±0.79 (Big Beef), 12.35±0.83, 11.99±1.31, 11.60±1.54 (Black Krim), and 11.42±1.12, 11.76±1.93, 11.71±0.83 (Green Zebra; TE; mean±std), respectively. For the DPPH assay, 10 μM DPPH in 90% aqueous methanol was measured spectrophotometrically at 517 nm (Spectra Max 250 Microplate Reader, Molecular Devices, San Jose, CA) to ensure that the absorbance was between 0.510-0.540. To determine the antioxidant capacity, 10 uL tomato extract was mixed with 195 uL 10 μM DPPH in 90% aqueous methanol and incubated in the dark for 15 min. The decrease in absorbance at 517 nm was then measured. Trolox was used as an antioxidant standard for calculations of TE. The antioxidant capacity of Big Beef, Black Krim, and Green Zebra grafted onto WT, Arnold, and Supernatural was 7.88±0.72, 7.62±0.87, 7.99±0.68, (Big Beef), 7.59±0.78, 8.11±0.54, 7.70±0.88 (Black Krim), and 8.00±0.53, 7.66±0.78, 8.19±0.64 (Green Zebra; TE; mean±std). The results further showed that none of the tomato varieties exhibited statistically significantly different antioxidant capacity.
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Steps to reproduce
Scions of two heirloom (Black Krim and Green Zebra), one commercial standard (Big Beef), and rootstocks (wild-type; WT, Arnold or Supernatural) were produced by planting one seed per cell into 72-cell trays filled with peat-based growing mix (ProMix BS, with Biofungicide, Premier Tech Horticulture, Cromwell, MN) at the Western Illinois University School of Agriculture Greenhouse facility, Macomb, IL. Plants were splice-grafted as previously described (Guan, 2016). Each scion/stock combination was prepared for evaluation. The hydroculture system utilized for post-grafted growth and production was a containerized recirculating system. Two tomato plants were transplanted per 11 L hydroponic greenhouse pot (Bato troughs; Hort Americas, Bedford, TX) containing coarse perlite (Deerfield Supplies, Elkton, Ky) for the remainder of the trial. A two-part complete hydroponic fertilizer (CropKing; Lodi, OH) consisting of a complete fertilizer (4.4N-13.0P-34.0K; HydroGro Vine Crops) supplemented with greenhouse-grade calcium nitrate (15.5N-0.0P-0.0K; YaraLiva Calcinit) fertilizer mixed per manufacturer instructions. The fertilizer solution was monitored daily and adjusted if necessary to maintain at an electrical conductivity of 2000 µS cm-1 and a pH of 5.5 and replaced at 14 d intervals. Plants were exposed to a 12:12 h light-dark cycle and irrigation scheduling was set for 30 s every 30 m during the lighted portion of the growing cycle. Tomatoes were harvested at maturity, freeze-dried, ground into a powder, and stored at -20ºC until further analysis. For the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) assay, the ABTS radical (ABTS•) was made by incubating equal volumes of 7 mM ABTS and 2.45 mM potassium persulfate for 12-18 h in the dark. To measure the antioxidant capacity, 10 uL tomato extract was incubated with 95 uL 3.5 mM ABTS• for 30 s at room temperature, and the absorbance at 734 nm was measured spectrophotometrically (Spectra Max 250 Microplate Reader, Molecular Devices, San Jose, CA). A standard curve using 0.09, 0.18, 0.36, 0.72, and 1.44 nM trolox in 90% methanol was used to calculate trolox equivalents (TE; μmol/g tomato dry weight). Prior to each 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, 10 μM DPPH in 90% aqueous methanol was measured spectrophotometrically at 517 nm (Spectra Max 250 Microplate Reader, Molecular Devices, San Jose, CA) to ensure that the absorbance was between 0.510-0.540. To determine the antioxidant capacity, 10 uL tomato extract was mixed with 195 uL 10 μM DPPH in 90% aqueous methanol and incubated in the dark for 15 min. The decrease in absorbance at 517 nm was then measured. A standard curve using 0.09, 0.18, 0.36, 0.72, and 1.44 nM trolox in 90% methanol was used to calculate TE. One-way ANOVA using a Tukey’s test for multiple comparisons was performed to analyze statistical significance using Prism Graphpad Software (8.30). A P-value of ≤0.05 was considered significant.