Phenolic Content of Hydroponically Cultivated and Grafted Tomato Varieties

Published: 25-02-2020| Version 1 | DOI: 10.17632/w3m8wr6344.1
Contributors:
Jamie Greathouse,
Shelby Henning,
Mette Soendergaard

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. 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.

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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. Total phenolic content was determined by 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). A gallic acid standard curve (0.03, 0.10, 0.12, 0.24, and 0.30 uM) was used to calculate of gallic acid equivalents (GAE; μmol/g tomato dry weight). 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.