Oxidative stress increases quality of tomatoes in organic farming
Tomato fruits from organic farming accumulate more nutritional compounds, such as phenolics and vitamin C as a consequence of the stressing conditions associated with farming system
The consumption of fruit and vegetables has been associated with lower risk of chronic human health problems like cardiovascular diseases, cancer, hypertension and diabetes type two due to their high contents in dietary bioactive compounds, the so-called phytochemicals, endowed with protective properties.
It may be hypothesized that cropping systems that allow plants to undergo (moderate) stress such as organic farming result in products with higher concentrations in phytochemicals resulting of low mineral availability and, therefore of the diversion of carbon skeletons from protein synthesis.
The hypothesis that oxidative stress is involved in enhanced concentrations in phytochemicals of fruits and vegetables from organic farming has rarely been tested to our knowledge. The objective of the study by Aurelice B. Oliveira, Universidade Federal do Ceará, Depto. Bioquímica e Biologia Molecular, Fortaleza-CE, Brazil, is to contribute to fulfilling this gap by comparing not only the concentrations in compounds contributing to quality in fruits from organic and conventional farming, but also by measuring indicators of oxidative stress, namely the activities of antioxidant enzymes, superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT), the concentration in ascorbate (AsA) and cell membrane lipid peroxidation (LPO) degree. The study was conducted on tomatoes which are climacteric fruits, representing a relevant source of vitamins C and E and other phytochemicals such as carotenoids and polyphenols. Researchers focused in this study on phenolic compounds and the activity of phenylalanine ammonia-lyase (PAL) because this enzyme controls a rate-determining step of the biosynthetic pathway of phenolic compounds in plants and is well-known to be induced by environmental stress.
Growth was reduced in fruits from organic farming while titratable acidity, the soluble solids content and the concentrations in vitamin C were respectively +29%, +57% and +55% higher at the stage of commercial maturity. At that time, the total phenolic content was +139% higher than in the fruits from conventional farming which seems consistent with the more than two times higher activity of phenylalanine ammonia lyase (PAL) we observed throughout fruit development in fruits from organic farming. Cell membrane lipid peroxidation (LPO) degree was 60% higher in organic tomatoes. SOD activity was also dramatically higher in the fruits from organic farming. Taken together, our observations suggest that tomato fruits from organic farming experienced stressing conditions that resulted in oxidative stress and the accumulation of higher concentrations of soluble solids as sugars and other compounds contributing to fruit nutritional quality such as vitamin C and phenolic compounds.