Phenolic Content 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 lycopene content 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. Phenolic content of methanol extracts was determined using the Folin-Ciocalteu assay. In brief, 20 uL tomato extract was incubated with 10 uL 2 N Folin-Ciocalteu reagent, 100 uL ddH2O, and 120 uL 12.5% sodium carbonate, and incubated for 30 min. Phenolic content was then measured spectrophotometrically at 750 nm (Spectra Max 250 Microplate Reader, Molecular Devices, San Jose, CA). Gallic acid was used as a phenolic standard for calculations of gallic acid equivalents (GAE; μmol/g tomato dry weight).
The phenolic content of Big Beef, Black Krim, and Green Zebra grafted onto WT, Arnold, and Supernatural was 13.57±3.14, 14.34±2.40, 15.80±2.77 (Big Beef), 14.40±2.72, 15.29±2.93, 13.59±2.58 (Black Krim), and 16.97±4.03, 10.95±4.22, 15.84±2.81 (Green Zebra; GAE; mean±std), respectively. Green Zebra grafted onto Arnold exhibited significantly lower (p<0.01) phenolic levels compared to self-grafted Green Zebra.