Phytonutrients of organic tomatoes: soil fertility and/or plant defense

Luke Nordell Gustafson

There is growing evidence that organically grown crops contain higher levels of phytonutrients than their conventional counterparts. This study investigates the relative effects of organic and inorganic nutrient treatments and herbivory on tomato phytonutrient concentrations. Organically managed soils generally have higher organic nitrogen to inorganic nitrogen ratios, and research indicates that organically grown apples, strawberries and tomatoes often have higher antioxidant activity than those grown conventionally. Because organic producers have fewer available pest control options, organic crops may be exposed to more insect pests. Herbivory by insects induces production of plant defense compounds which can act as feeding deterrents to generalist insects. Many of these compounds also act as antioxidants. In this study, tomato plants (Solanum lycopersicum L. ‘Oregon Spring’) were grown in a randomized complete-block experimental design in a greenhouse under either organic or inorganic fertility management. To determine the effect of herbivory on fruit quality, green peach aphids (Myzus persicae) were introduced to half of the plants within each nutrient treatment. To contain the aphids, all plants were grown in mesh exclusion covers. Fruit were harvested at varying stages of ripeness, and measurements were taken of percent soluble solids, mass, and diameter. Frozen ripe fruit samples were analyzed for total phenolics (TP), lycopene, Trolox equivalent antioxidant capacity (TEAC) and ascorbic acid. Red-ripe tomato fruit from organically fertilized plants had statistically higher concentrations of soluble solids, TP, lycopene, lipophilic and total TEAC and ascorbic acid. Red-ripe fruit in the organic fertility treatment had lower average mass. Concentrations of leaf TP were not affected by fertility treatment, but organic tomatoes supported lower aphid densities. Leaf tissue in the organic treatment contained higher concentrations of calcium, potassium, magnesium and sulfur, while in the conventional treatment total carbon, total nitrogen and phosphorous concentrations were higher. There were no treatment differences in red-ripe fruit mineral concentrations.

Phytonutrients of organic tomatoes: soil fertility and/or plant defense

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