Cucumber (Cucumis sativus) and radish (Raphanus sativus) were grown under several levels of water stress in a growth chamber programmed with a day-night rhythm to simulate the diurnal course of temperature and irradiation. In addition to white light, the seedlings received UV-B radiation equivalent to either ambient levels in the early spring (control) or levels predicted to reach the earth's surface with approximately 12% ozone depletion (enhanced UV-B) at the summer solstice at 49 degrees N latitude. Cotyledon fresh weight and leaf area were reduced in radish by water stress and enhanced UV-B. Cucumber cotyledons were more sensitive to UV-B radiation than radish. This higher sensitivity might be due to reduced levels of protective flavonoid compounds in cucumber. Protein content of radish cotyledons was unaffected by either treatment. This study indicates that cucumber is one of the most UV-B sensitive crop species so far identified with respect to reduced growth and flavonoid content.