Low energies of near-ultraviolet radiation (300-400 nanometers), applied simultaneously with visible radiation to Nicotiana tabacum L. during daily illumination periods, increased levels of chlorogenic acid isomers, total soluble phenolics, alkaloids, and soluble sugars in expanding leaf lamina compared with controls that had near-ultraviolet filtered out. However, total nitrogen concentrations decreased. The responses to near-ultraviolet were interrelated with intensity of visible light. The presence of near-ultraviolet (which accounted for less than 4% of the total light energy) along with visible light resulted in component concentration differences similar to those caused by much greater increases of visible light without near-ultraviolet.Plants in their natural habitats are exposed to relatively low intensities of near-ultraviolet radiation from sunlight, which includes wavelengths down to about 300 nm (20). The amount of this radiation varies with latitude, altitude, time of day, season, and atmospheric conditions.Lower concentrations of soluble phenolics were found in lamina from tobacco (Nicotiana tabacum L.) plants grown in a greenhouse as compared with those in the field (11). Greenhouse plants received sunlight through glass, which filters out part of the near-UV radiation (20). In addition, the activity of polyphenoloxidase, an enzyme for which several plant phenolics serve as substrates, was lower in greenhouse-grown than in field-grown tobacco leaves (2). Low energies of near-UV (300-400 nm), along with one intensity of visible radiation, resulted in about twice the concentrations of total soluble phenolics and chlorogenic acid isomers in tobacco leaves compared with those that did not receive near-UV radiation (1).The objectives of the present research were to test effects of near-UV on contents of several soluble chemical constituents of tobacco lamina important to tobacco quality and to ascertain whether effects of near-UV are independent of visible light intensity.MATERIALS AND METHODS Plant Materials. Tobacco (Nicotiana tabacum L.) seedlings were started and grown for about 8 weeks in expanded peat pellets at 28 C under 14-hr photoperiods from cool-white fluo-