Content of total phenolics and phenolic acids in tomato (Lycopersicon esculentum Mill.) fruits as influenced by cultivar and solar UV radiation

Two cultivars of fresh market tomato [Oregon Spring (OS) and Red Sun (RS)] were each grown in two high tunnels at Beltsville, MD covered with a contrasting material of similar thickness (0.152 mm) and durability (4-year polyethylene). One covering material (Tyco Tufflite IV) transmitted ambient solar UV radiation from 290 to 400 nm (designated+UV) while the other material (Dura-Film Super 4) blocked UV wavelengths below 380 nm (designated-UV). Both films transmitted comparable amounts of photosynthetically active radiation (PAR) from 400 to 700 nm. Ripe tomato fruits comparable in size and development were collected at maturity from plants of the two cultivars grown in each high tunnel under the contrasting covering materials. Four lots of tomatoes of each cultivar and each UV treatment were assayed for total phenolic (TP) content by a colorimetric Folin-Ciocalteu (FC) assay and for content of individual phenolic acids by a high performance liquid chromatography (HPLC)-diode array detection (DAD) procedure. The phenolic acids extracted from the base hydrolyzed fraction were identified as caffeic acid, p-coumaric acid, and ferulic acid. Caffeic acid was the predominant phenolic acid in both tomato cultivars (OS and RS) grown in the two high tunnels. The total concentration of these three phenolic acids measured by HPLC was approximately 20% higher under +UV than under -UV treatment; this was true for both cultivars. A similar trend in quantities of TP yield was also observed when tomato extracts were assayed by a FC method. These results indicate that the phenolic content of tomato fruits is significantly affected by the spectral quality of solar UV radiation. Since phenolic compounds are known to play a key role as antioxidants in human nutrition, subtle differences in phenolic composition between the two high tunnels as a result of differences in the UV transmission properties of these different covering materials may be of considerable importance.