Lycopene 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. Freeze-dried tomato powder (100 mg) was extracted by 1 mL hexane/acetone/ethanol (2:1:1, v/v/v) under shaking for 30 min at room temperature, after which 0.2 mL ddH2O was added and vigorously mixed. The polar and non-polar phases were separated by centrifugation at 3000 xg for 10 min, after which the non-polar phase containing lycopene was collected. The remaining plant material was extracted once more using 1 mL hexane/acetone/ethanol (2:1:1, v/v/v) as described above. The content of lycopene was determined by measuring the absorbance spectrophotometrically at 503 nm (εlycopene =1.72 × 105 M-1cm-1). Comparing self-grafted Big Beef, Black Krim, and Green Zebra showed lycopene contents of 9.97±2.02, 15.55±1.97, and 3.37±0.39 mg/kg dry weight (mean±std), respectively. The results were statistically significantly different between the three varieties reflecting the natural pigmentation in each. When comparing the effect of grafting onto non-WT rootstocks, surprisingly, Big Beef grafted onto Supernatural demonstrated significantly higher lycopene content (17.25±0.24 mg/kg dry weight) compared to both WT (9.97±2.02 mg/kg dry weight) and Arnold (8.09±1.39 mg/kg dry weight) rootstocks. Similar effects were not observed for the grafted heirloom varieties, for which the lycopene content was determined to be 15.55±1.97, 16.29±2.93, 19.14±3.21 mg/kg dry weight for Black Krim grafted onto WT, Arnold, and Supernatural, respectively, and 3.37±0.39, 3.52±0.15, 3.03±0.03 mg/kg dry weight for Green Zebra grafted onto WT, Arnold, and Supernatural, respectively.