Antioxidant Capacity of Hydroponically Cultivated and Grafted Tomato Varieties

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.