Lycopene Content 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 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.
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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. 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). The data was analyzed using one-way ANOVA with a Tukey's test for multiple comparisons using GraphPad Prism (vs. 8.3.0).